genic monoamines has been an approach for creating new drugs. So far inhibitors of monoamine oxidase (MAO) and decarboxylase of aromatic L-amino acids ...
Chapter 12
Inhibitors of monoamine oxidase and decarboxylase • of aromatic amino acids By
A. PLE'fSCHER, K.:F. GEY and W. P. BURKARD With 13 Figures
I. Introduction In recent years it has become evident that pharmacological interference with the metabolism of monoamines might be of therapeutic value in certain diseases. Inhibition of the enzymes responsible for the formation and degradation of biogenic monoamines has been an approach for creating new drugs. So far inhibitors of monoamine oxidase (MAO) and decarboxylase of aromatic L-amino acids (DCA), enzymes involved in the degradation and formation respectively of monoamines, have got therapeutic importance. This development is closely connected with the research on the metabolism of aromatic amines which progresses rapidly. The first potent inhibitor of MAO used in human therapy was iproniazid. This derivative of hydrazine had originally been developed for the treatment of tuberculosis, but on the basis of clinical observations by KLINE (LOOMER et al. 1958; SAUNDERS et al. 1957) as well as by CESARMAN (CESARMANN 1957) it was introduced in 1957 for the therapy of mental depression and angina pectoris. Contrary to other psychotropic drugs, a biochemical mechanism of iproniazid was already known since ZELLER (ZELLER and BARSKY 1952; ZELLER et al. 1952 a; ZELLER et al. 1952b; ZELLER et al. 1952c) had shown in 1952 that the drug potently inhibited MAO. These observations constituted the basis for a rapid development in the field of MAO inhibitors. At present, several MAO inhibitors, hydrazines and nonhydrazines, are in therapeutic use. With the progressing knowledge of the metabolism of monoamines, attempts have also been made to inhibit their formation. The first inhibitor of DCA introduced in 1962 for the therapy of arterial hypertension was IX-methyl-3,4dihydroxyphenylalanine (IX-methyldopa). This compound was shown in 1954 by SOURKES (SOURKES 1954a) to inhibit DCA. This IX-alkylated amino acid stimulated further research, although its main effect in hypertension may not be due to inhibition of DCA. Other more potent and specific inhibitors have been developed; none has, however, been introduced in human therapy. A survey of a fast developing field, such as inhibitors of l\1AO and DCA, is, of course, rather problematic. The present review is mainly concerned with those inhibitors of MAO and DCA which have a potent effect in vivo. Thereby attempts are made to correlate biochemical and pharmacodynamic effects l . 1 The literature was reviewed systematically until October 1962 and partially until July 1964.
Handb. d. expo Pharmakol.. Erg.·Werk Bd. XIX
V. Erspamer (ed.), 5-Hydroxytryptamine and Related Indolealkylamines © Springer-Verlag, Berlin · Heidelberg 1966
38
594
Monoamine oxidase
II. Monoamine oxidase l A. Biochemistry of the enzyme Oxidative deamination of primary amines in vivo, in situ, and in vitro has been reported about 80 years ago (EwINS and LAIDLAW 1910/11, 1913; GUGGENHEIM and LOFFLER 1916; MINKOWSKY 1883; Mosso 1890; SCHMIEDEBERG 1881). An enzyme responsible for this reaction was first described as tyramine oxidase in 1928 (M. L. C. BERNHEIM 1931; HARE 1928) and subsequently found to correspond to an enzyme oxidizing aliphatic amines and epinephrine (BHAGVAT et aL 1939; BLASCHKO et aL 1937a, 1937b, 1937c; H. I. KOHN 1937; PUGH and QUASTEL 1937a, 1937b; RICHTER 1937). In order to differentiate this enzyme from diamine oxidase (DAO) (EC 1.4.3.6), the term monoamine oxidase (MAO) (ZELLER 1938) has been used. Several reviews on MAO have been published previously (BLASCHKO 1952, 1963; DAVISON 1958c; PLETSCHER et aL 1960c; ZELLER 1942).
1. Reaction formula Amine oxidases transform the terminal amino-alkyl group of primary or secondary amines into a carbonyl group according to the general equation: (1)
The net balance of this formula has been proved with numerous amines (AEBI 1962; AEBI et aL 1962b; M. L. C. BERNHEIM 1931; CREASEY 1956; FELLMAN 1959a; GOLDSTEIN et aL 1960c; HARE 1928; KOBAYASHI and SCHAYER 1955; H. I. KOHN 1937; KOPIN 1960; KOPIN and AXELROD 1960; LEEPER et aL 1958; LUSCHINSKY and SINGHER 1948; PHILPOT 1937; PUGH and QUASTEL 1937b; RICHTER 1937; WEISSBACH et aL 1957d; ZELLER 1940a). Since H 2 0 2 is decomposed (e.g. by catalase) (CREASEY 1956) to H 20 and '/2 O 2 , the following overall reaction results: (II)
With regard to formula I, amine oxidases belong to the two-electrone transfer oxidases (H. S. MASON 1957a, 1957b): (III) Accordingly, amines are probably transformed into imino compounds which undergo immediate hydrolysis (M. L. C. BERNHEDI 1931; BLASCHKO 1952; RICHTER 1937; T. E. SMITH et aL 1962):
+ O -+ R-CH=N+H + H 0 N+H 2 + H 0 -+ R-CHO + N+H4
R-CH2-N+Ha R-CH =
2
2
2
2
2
(IV) (V)
2. Differentiation Amine oxidases can only in part be classified according to their substrates. Thus, monoamine oxidases deaminate aliphatic and aromatic monoamines as well as some long-chain alkylene diamines, derivatives of ethylene diamine, histamine, etc. On the other hand, diamine oxidases deaminate, besides their typical substrates, i.e. the short-chained diamines, also some monoamines (BLANKSMA 1962; BLASCHKO 1952, 1963; BLASCHKO and DUTHIE 1945b; BLASCHKO and HAWKINS 1950; BLASCHKO and HrMMS 1955; FOUTS 1954; FOUTS et al. 1957; KOBAYASHI 1957; MANN and SMITHIES 1955a, 1955b; SATAKE et aL 1953; ZELLER et al. 1956b, 1957a). Because of this lack of substrate specificity, both classes of amine oxidases can be better distinguished by their inhibitors. Monoamine oxidases are relatively resistant to some carbonyl reagents (e.g. semicarbazide, isoniazid, aminoguanidine) and to cyanide, but are inhibited by octanoL Diamine oxidases are inhibited by the carbonyl reagents mentioned above, but not by octanol (BLASCHKO 1956; BLASCHKO et al. 1959; BURKARD et aL 1962a; DAVISON 1958c; LAZANAS and ZELLER 1958; ZELLER 1956a, 1959; ZELLER et al. 1940, 1958b, 1959). The group of monamine oxidases consists of the typical monoamine oxidase (MAO) (to which this review mainly refers) and of amphetamine oxidase, whereas the group of diamine oxidases contains the typical diamine oxidase (DAO), spermine or benzylamine oxidase of blood plasma, methylamine oxidase, and mezcaline oxidase (ZELLER et al. 1959). 1 Official name according to the Enzyme Commission (Int. Union Biochem. 1961): Monoamine: O 2 oxidoreductase (deaminating); EC 1.4.3.4.
Tissue distribution
595
3. Substrates Substrates of the classical MAO have been found among primary and secondary alkyl-, aralkyl-, indolyl-, and imidazolyl-amines (ALLES and HEEGARDS 1943; BARLOW et al. 1955; BARSKY 1958; F. BERNHEDI and M. L. C. BERNHEIM 1938; BEYER 1943; BEYER and MORRISON 1945; BHAGVAT et al. 1939; BLAXKS~IA 1962; BLASCHKO 1952, 1963; BLASCHKO and CHRUSCIEL 1959; BLASCHKO and DUTHIE 1945a, b; BLASCHKO and HAWKINS 1950; BLASCHKO et al. 1937a; ERSP.UIER et al. 1960a; HEDI 1950a; KAxAoKA et al. 1961; KISHI et a1. 1957; H. 1. KOHN 1937; PUGH and QUASTEL 1937a, 1937b; RANDALL 1946; RAXDALL and BAGDOX 1958; SNYDER and OBERST 1946; \VERLE and ROEWER 1952; ZELLER 1963a; ZELLER ct a1. 1958b, 1959). The best substrate hitherto known is meta-iodobenzylamine (ZELLER 1963a, b). The relative rate of dcamination of some physiological amines by MAO may be approximated as follows (ALLES and HEEGARDS 1943; F. BERNHEDr and .:II. L. C. BERNHEnI 1938; BLASCHKO 1952; BLASCHKO and PHILPOT 1953; BLASCHKO et a1. 1937a; DAYISON 1958b; FREYBURGER ct a1. 1952; GonER et a1. 19;'53; HEEGARD and ALLES 1943; HOPE and S~IITH 1960; KALIMAX 1961; KOBAYASHI 1957; KOBAYASHI and SCHAYER 1955; H. 1. KOHN 1937; LINDELL and 'VESTLIXG 1957; LUSCHINSKY and SINGHER 1948; ;\L-IcNN and QUASTEL 1940; PUGH and QUASTEL 1937a, 1937b; RANDALL 1946; RICHTER 1937; SJOERDS3IA et al. 1955; THO:\-IPSON and TICKXER 1951; VANE 1959; 'VEINER 1960; \VEISSBACH et a1. 1957 d, 1960b; ZELLER et a1. 1955a, 1956, 1958b, 1959): High (100~()6%) 3-hydroxytyramine (dopamine), tyramine . •vledium (65~34%) 3-methoxytyramine, tryptamine, 5-hydroxytryptamine (5-HT), isoamylamine (and similar alkylamines). Low « 33 ~o) metanephrine (3-0-methylepinephrinc), normetanephrine (3-0-methylnorepinephrine), epinephrine, norepinephrine (NE), phenylethanolamine, octopamine (4hydroxyphenylethanolamine), phenylethylamine, kynuramine, N-methylhistamine and in some species also histamine. This classification is, however, limited by many exceptions, since the extent of deamination varies greatly according to the source of the enzyme (organ, species) and to the method of tissue preparation (ALLES and HEEGARD 1943; BHAGVAT et a1. 1939; BLASCHKO and PHILPOT 1953; BLASCHKO et a1. 1937a; HOPE and S}IITH 1960; LUSCHINSKY and SINGHER 1948; SJOERDSMA et a1. 1955; \VEINER 1960). With regard to the human brain, for instance, dialyzed total homogenate shows a relati,-e order of substrate deamination similar to that giyen aboye, whereas mitochondria oxidize isoamylamine to the greatest extent (\VEINER 1960). Tertiary monoamines (e.g. hordenine) are poorly deaminated, quarternary amines not at all. Oxidation of secondary and tertiary monoamines results in the formation of alkylamine instead of ammonia (ALLES and HEEGARDS 1943; F. BERNHEDI and ]\I. L. C. BERNHEDI 1938; BEYER 1943; BLASCHKO et a1. 1937a; ERSPAMER et a1. 1960a; EWINS and LAIDLAW 1910/11; H. 1. KOHN 1937; RANDALL 1946; RICHTER 1937; SNYDER et a1. 1946). Stereospecificity of various MAO preparations has been demonstrated using epinephrine, phenylethanolamine, and tyramine asymmetrically labelled with deuterium. Thereby, the natural R- respectively D( -)-isomeres are preferentially oxidized (BELLEAU and BURBA 1960; BELLEAU and MORAX 1962, 1963; BELLEAU eta1. 1960,1961; BIEL etal. 1959a; BLASCHKO and PRATESI 1959; BLASCHKO et al. 1937b; PRATESI and BLASCHKO 1959; ZELLER et al. 1957 c).
4. Tissue distribution .:IIAO occurs widely in mammalian tissues, which show approximately the following activity of the enzyme (AR}IIN et a1. 1953; BARLOW 1961; BARSKY 1958; :F. BERNHEIM and M. L. C. BER~fHEI:\1 1945; BERNHEDIER et al. 1962; BIRIUiAUSER 1940, 1941; BLASCHKO 1952, 1963; BLASCHKO et a1. 1937a, 1937b; BOGDANSKI et a1. 1957; BURN and ROBINSON 1952; DAnSON et a1. 1957; DAVISOX and SAXDLER 1956; Epps 1945; GANROT et al. 1962b; GIAR}IAN and DAY 1959; HAGEN and WEIXER 1959; HOLTZ et a1. 1938b; HOPE and SMI'l'H 1960; IISALO 1962; IrSALO and PEKKARINEN 1954; KALlMAN 1961; KOELLE 1959; KOELLE and VALK 1954; KRISHNA et al. 1961; LANGE}IANN 1944, 1951; LAXGE}IANN et a1. 1943; LEVINE and SJOERDSl\IA 1962b, 1963a; LOVENBERG et al. 1962a; LUSCHINSKY and SINGHER 1948; D. R. MAXWELL etal. 1961; OTSUKA and KOBAYASHI 1964; PAASONEN 1961a; PAASONEN and KIYALO 1962; PAASONEX e1, a1. 1964; PEKKARINEN et al. 1958; PHILPOT 1937; PLETSCHER et al. 1960c; PUGH and QUASTEL 1937a; RICHTER and TINGEY 1939; J. ROBINSON 1952; SCHAPIRA 1945; SJOERDSlI[A et al. 1955; SKILLEN et al. 1962; STOCK and \VESTERMANX 1962a; STRO}IBLAD 1959a, 1959b; THQ)IPSON 1952; THQ)IPSON and TICKNER 1949, 1951; {;"DENFRIEND et al. 1958b; VANE 1959; WAALKES and COBURN 1958; WEINER 1959; WERLE and ::VLEXXICKEN 1938; WERLE and PECIDIANN 1948/49; WERLE and ROEWER 1952; WEST 1958a; ZELLER and JOEL 1941; ZELLER et al. 1956 b): High (approx. 1600---400 [Ll 02/g tissue/h): liver, sympathetic ganglia, salivary glands, kidney. 38*
596
Biochemistry of the enzyme
Moderate (approx. 4OO-75!J.l 02/g/h): large arteries, intestine, spleen, lung, skeletal muscle, uterus, brain, iris·ciliary body, optic nerve, adrenal medulla, placenta. Low « 75!Jol 02/g/h): heart, extraocular muscle, retina, cornea, lens, pituitary, thyreoidea, pancreas, adipose tissue, testis, blood. MAO activity of tumors has been found equal to that of the corresponding normal tissue or lower (BIRKH.XUSER 1940; BULBRING 1953; DAVISON and SANDLER 1956; HAGEN 1959; LANGEMANN 1944; SARKAR et al. 1960b; STARR et al. 1962). Marked species differences exist concerning the absolute and relative activity of MAO in various tissues. Men seem to differ from animals by a much higher MAO activity in heart and jejunal mucosa and a relatively lower activity in sympathetic ganglia (GANROT et al. 1962b; LEVINE and SJOERDSMA 1962b; WEINER 1961). MAO activity within one organ may vary markedly according to the anatomical structure (BLASCHKO 1952; DAVISON 1958c; LANGEMANN 19(4). The brain, for instance, shows high activity in the hypothalamus, thalamus, several parts of the limbic system, the caudate nucleus, mesencephalon, pons, and about three times lower values in the cortex, subcortex and cerebellum. Furthermore, MAO is present in nerve cells and capillary walls, but not in glial cells or nerve fibers. This has been shown biochemically (APRISON et al. 1964; BERN· HEIMER and HORN"l.'KlEWICZ 1962; BIRKIIAUSER 1940, 1941; BOGDANSKI et al. 1957; GANROT et al. 1962b; LEVINE and SJOERDSMA 1962a; WEINER 1961) and by histochemical techniques (AmOKA and TANIMUKAI 1957; ARvy 1960, 1961; BLASCHKO and HELLMANN 1953; EDER 1957; FRANCIS 1953; GLENNER et al. 1957; KOELLE and VALK 1954; MWHOTTE and DEVAHIA 1958; MUSTAKALLIO et al. 1961; SHIMUZU et al. 1959; B. SMITH 1962; YASUDA 1962; WATARI 1958; WATTENBERG 1959; WOHLRAB 1960, 1961). Within the cell, MAO is predominantly (up to 75%) bound to the particulate fraction, mainly to Initochondria, whereas the remainder is contained in the microsomal fraction (BARSKY 1958; BAUDHIN et al. 1964; COTZIAS and DOLE 1951b; DUVE et al. 1960; HAWKINS 1952a; LORES ARN.AIZ et al. 1962; OSWALD and STRITTMATTER 1963; WEINER 1959, 1961; ZELLER et al. 1955a). In certain tissues (e.g. guinea pig liver), some MAO has also been found in the supernatant (BLASCHKO and DUTmE 1945a; WEISSBACH et al. 1957d), but this might result from lytic changes of the Initochondria (OSWALD and STRITTMATTER 1963). The particulate bound MAO seems to be a true mitochondrial enzyme and not related to synaptic membranes or vesicles of nerve endings (LORES ARN.AIZ et al. 1962).
5. Purification and properties The enzyme shows great stability at a physiological pH if bound to mitochondria (ALLES and HEEGARDS 1943; BLASCHKO et al. 1937b; DAVISON 1958c; GIORDANO et al. 1960a; WESTLEY and SEIDEN 1962). The latter can be solubilized to some extent (BARSKY 1958; BLASCHKO 1952; COQ and BARON 1964; COTZIAS et al. 1954; GORKIN et al. 1963; KOBAYASm and SCHAYER 1955; SAKOMOTO et al. 1963; SOURKES and LAGNADO 1957a, 1957b; SOURKES et al. 1955; VEREVKINA et al. 1964; WERLE and HENNIG 1960; WESTLEY and SEIDEN 1962; ZELLER et al. 1955a, 1958b; ZILE and LARDY 1959). Complete disorganization of the mitochondria, however, inactivates MAO. Therefore, purification has not increased the activity of the Initochondrial enzyme by more than a factor 20 to 300 (ALLES and HEEGARDS 1943; BARBATO and ABOOD 1963; BLASCHKO et al. 1937b; BORN et al. 1958; GANROT and ROSENGREN 1962; GORKIN 1959; HARE 1928; E. S. HARRIs 1960; SAKOMOTO et al. 1963; SEIDEN and WESTLEY 1962; WEISSBACH et al. 1957d; WESTLEY and SEIDEN 1962). It has not been ascertained whether MAO is homogenous or heterogenous. The fact that deamination is not additive if several substrates are present simulataneously (BLASCHKO 1952; BLASCHKO et aI. 1937a; KOBAYASm and SCHAYER 1955; H. I. KOHN 1937; PuGH and QUASTEL 1937b) may be an argument in favour of homogeneity. Marked species- and organ-dependent differences in the substrate pattern of MAO as well as other kinetic observations, however, indicate that MAO can be separated into various types each with preference for special substrates (homologuous enzymes or isoenzymes). Thereby, it has not been distinguished wether various specific monoamine oxidases are associated in different proportions or whether different protein moieties cause various degrees of sterlc hindrance of the hydrocarbon residue of certain substrates. (ALLES and HEEGARD 1943; BARBATO and ABOOD 1963; BARLOW 1961; F. BERNHEIM and M. L. C. BERNHEIM 1938; BLANKSMA 1962; CHODERA et al. 1964; GoRKIN 1963, 1964b; HAGEN 1959; HAGEN and WEINER 1959; HARDEGG 1961, HARDEGG and HEILBRONN 1961; HOPE and SMITH 1960; LONG 1962a, 1962b; McCARMAN 1961 ; OSWALD and STRITTMATTER 1963; RANDALL 1946; SARKAR and ZELLER 1961; S.ARK.AR etal.l960b; SATAKE 1955; WEINER 1960; WERLE and ROEWER 1952; ZELLER 1961, 1963a, 1963b, 1963d; ZELLER and FOUTS 1963). Recently, fractionation of liver Initochondria succeeded in a partial separation of at least two different monoamine oxidases (GORKIN 1963, pers. corum.).
Active center and reaction mechanism
597
The activity of MAO increases with oxygen tension (AEBI et a1. 1962b; F. BERNHEIM and M. L. C. BERNHEIM 1938, 1945; IrSALO and PEKKARINEN 1958; H. I. KOHN 1937; NoVICK 1961 a, 1961 b; PmLPoT 1937) and depends also on the functional state of mitochondria. Thus, swollen mitochondria of liver with impairment of pyruvate oxidation and of oxidative phosphorylation were found to have a higher activity of the enzyme than intact mitochondria (AEBI 1962; AEBI et a1. 1963). This may be related to the finding that high concentrations of 5'(pyro)-triphosphate of adenosine (ATP) or adenosinemonophosphate inhibit MAO (STUTTGEN et a1. 1961). Optimal substrate concentrations are of the order of 10-2 to 10-3 M (BLASCHKO et a1. 1937a; COTZIAS and DOLE 1951a; HOPE and S:\IITH 1960; KOBAYASm and SCHAYER 1955; H.I. KOHN 1937; LAGNADO and SOURKES 1956b; WEISSBACH et a1. 1957d). With a MrCHAELIS-MENTON constant of about 10-3 for tyramine and 5-HT (DAVISON 1957; A. L. GREEN 1964; OSWALD and STRITTClIATTER 1963; ZELLER 1963a), )1AO has a moderate substrate affinity. The optimal pH varies markedly according to the source, purity and pretreatment of the preparation as well as to the substrate (ALLES and HEEGARD 1943; M. L. C. BERNHEIM 1931; BLASCHKO et a!. 1937b; HARE 1928; E. S. HARRIS 1960; HOPE and S~I:rTH 1960; WERLE and }IENNICKEX 1938; YorDDI and SOURKES 1964). 'Yashed mitochondria, for instance, show maximal deamination of tyramine at pH = 7.2-7.3 (l\L-I-LAFAYA-BAPTISTA et a1. 1957), but of 5-HT at pH = 8.1 or even higher (HOPE and S~IITH 1960; HORITA 1962a; 'VEISSBACH et a!. 1957 d). The redox potential of MAO seems to range between -0.23 and -;-0.195 V (PmLPoT 1937; PLETSCHER et a1. 1960c). MAO requires free sulfhydril groups (AGIN 1959; BARBATO and ABOOD 1963; FRIEDENWALD and HERmIANN 1942; LAGNADO and SOURKES 1956b, 1956c; MANUKHIN 1958; OKU;\IURA 1960; SINGER and BARBON 1945), whereas dialyzable cofactors could not be detected (BLASCHKO et a!. 1937b; PUGH and QUASTEL 1937b). The hypothesis that MAO is a flavoprotein remains to be proven, but deficiency of riboflavine might decrease the de novo synthesis of this enzyme (ALLEGRETTI and VUKADINOVIC 1950; BELLEAU and MORAN 1963; BERNSOHN and LOZ_UTYTE 1958; HAWKINS 1952b; LAGNADO 1958; LAGNADO and SOURKES 1956a, 1956c, 1958; SOURKES 1958; SOURKES and D'IoRIO 1963; WISEMAN and SOURKES 1961; WISEMAN-DISTLER and SOURKES 1963; YOUDDI and SOURKES 1964). The inyolvement of pyridoxal-5'-phosphate is unlikely (BERNSOHN and LOZAITYTE 1958; LAGNt.DO 1958; PHILLIPS et a!. 1962). MAO might be a copper enzyme, because chelating agents (e.g. ethylenediaminetetraacetic acid, 8-hydroxyquinoline, o-phenanthroline) have been found by some investigators to induce an inhibition partially reversible by bivalent cations. The latter may be without action or even inhibit the enzyme (BARBATO and ABOOD 1963; GORKIN 1959; A. L. GREEN 1964; LAGNADO and SOURKES 1956b; OGAWA and KUROSAWA 1963; OKUl\1URA 1960). MAO resists inactivation by cyanide, arsenite, CO, hydrazine, semicarbazide, isoniazide, hydroxylamine, monoiodoacetate, thiourea. fluoride, malonate, etc. (BARSKY 1958; M. L. C. BERNHEDI 1931; BLASCHKO 1952; BLASCHKO et a1. 1937b; CREASEY 1956; M. L. C. HARE 1928; E. S. HARRIS 1960; H. I. KOHN 1937; OKU11URA 1960; PmLPoT 1937; PUGH and QUASTEL 1937b; WERLE and MENNICKEN 1938).
6. Active center and reaction mechanism Comparisons of optically active substrates (BELLEAU and MORAN 1963; see also above) and inhibitors (BERNSTEIN et a!. 1959; BIEL et a!. 1959a; BLASCHKO and PRATESI 1959; BLASCHKO and STROi\IBLAD 1960; GRANA and LILLA 1959; HOPE and SMITH 1960; OZAKI et a!. 1960; PLETSCHER and GEY 1958b; PRATESI and BLASCHKO 1959; R. E. TEDEscm et a!. 1959b) indicate that MAO shows absolute R-stereospecificity. It might be assumed that at first the amino group of the substrate and its IX-carbon (respectiyely the polarizable C=Nbond) is firmly attached to the active site of MAO. If the properties of the bound compound allow an electrophilic attack on the amino group, an IX-CH bond is presumably weakened. In consequence, an IX-hydrogen might form a long and weak bond with a special acceptor of the enzyme which subsequently splits off this IX-hydrogen in the form of a proton or radical. The second IX-hydrogen is certainly of importance for oxidation of the substrate, but presumably not for its binding, since substitution (e.g. alkylation in amphetamine or phenylcypromine) produces inhibitors which are not deaminated by MAO. The p-hydrogen is also of importance for the oxidation of the substrate, possibly because in the transition state the IX,p-carboncarbon bond of the substrate acquires a double bond character. Thereby, the IX- and p-carbons might approach the trigonal state (BARSKY 1958; BARSKY et a!. 1959; BELLEAU and BURBA 1960; BELLEAU and MORAN 1962,1963; BELLEAU et a!. 1960,1961; DAVISON 1957; FOUTS et al. 1957; MCGRATH and HORITA 1962; SARKAR et a!. 1960a; T. E. S:\IITH et a1. 1962; ZELLER 1963d; ZELLER and SARKAR 1960, 1962; ZELLER et a!. 1957a, 1958a, 1959, 1960, 1962;
598
Biochemistry of the enzyme
ZmKLE et al. 1962). The structure of the hydrocarbon residue of substrates (and inhibitors) seems also to determine whether an amine is either attached to the active center without subsequent deamination (dystopic complex) or attached as well as degradated (eutopic complex) (ZELLER 1963a, 1963b). Such changes in structure might cause steric hindrance, i.e. affect the interaction of the hydrocarbon residue with nonspecific sites of the active center. Thus, the hydrocarbon residue of substrates (and inhibitors) might be bound to the enzyme by VAN DER WAALS forces (SARKAR et al. 1960a; ZELLER 1960; ZELLER and SARKAR 1962; ZELLER et al. 1960; ZIRKLE et al. 1962). Thereby, these forces possibly operate maximally if the substrates (and inhibitors respectively) were able to approach planarity when attached to the complementary surface of the MAO center (ZmKLE et al. 1962).
7. Determination Direct measurement of MAO activity in vitro is carried out in total homogenates or preferentially in the mitochondrial fraction (BLASCHKO 1952; CREASEY 1956; DAVISON 1958c; GEY and PLETSCHER 1961 b; GIORDANO et al. 1960a). Various principles may be used (PLETSCHER et al. 1960c; SOEP 1962):
a) Disappearance of substrate by biological (GADDUM 1959; HOLTZ et al. 1938b; WERLE and MENNICKEN 1938), spectrophotometrical (HORITA 1958b, 1959; NACHMIAS 1960; SJOERDSMA et al. 1955; SOEP 1962; SOURKES et al. 1955; UDENFRIEND et al. 1958a; WEISSBACH et al. 1960b; ZILE and LARDY 1959), and spectrophotofluorometrical methods (BOGDANSKI et al. 1956; HORITA 1959; KUNTZMAN et al. 1961 b; OZAKI et al. 1960; SJOERDSMA et al. 1955; WEISSBACH et al. 1960b; ZILE and LARDY 1959). b) Reduction oj eleetrone acceptors (e.g. tetrazolium salts) due to amine dehydrogenation or aldehyde oxidation (A. L. GREEN and HAUGHTON 1960; LAGNADO and SOURKES 1956a, 1956c; PLETSCHER et al. 1960c; SOURKES 1958; SOURKES and LAGNADO 1957a, 1957b; WEISSBACH et al. 1957c).
c) Oxygen consumption provided that unspecific oxygen uptake is prevented (BARSKY 1958; M. L. C. BERNHEIM 1931; BEYER 1943; BLASCHKO 1952; BLASCHKO et al. 1937a; COTZIAS and GREENOUGH 1958,1959; CREASEY 1956; DAVISON 1957, 1958c; GEY and PLETSCHER 1961 b; GIORDANO et a1.1960a; A.L.GREEN 1962; HARD EGG and HEILBRONN 1961; HORITA 1962a; KOBAYASHI and SCHAYER 1955; H.I. KOHN 1937; LUSCHINSKY and SINGHER 1948; PUGH and QUASTEL 1937b; UDENFRIEND et al. 1953; ZELLER et al. 1955a). d) Ammonia formation (COTZIAS and DOLE 1951 a, 1951 b, 1952; COTZIAS and GREENOUGH 1958, 1959; CONWAY 1957; H. I. KOHN 1937; LUSCHINSKY and SINGHER 1948; RICHTER 1937; SOEP 1962; ZELLER 1940b; ZELLER et al. 1962). e) Peroxyde formation (AEBI 1962; AEBI et al. 1962a, 1962b; M. L. C. HARE 1928; HElM 1950b; H. I. KOHN 1937; PHILPOT 1937; PUGH and QUASTEL 1937b; ZELLER 1940a, 1941b). j) Formation of aldehyde directly by spectrophotometry (BRusovA et al. 1963; GORKIN et al. 1963, 1964a; TABoR et al. 1955; ZELLER 1964), extraction (MCCAMAN 1961; OTSUKA and KOBAYASHI 1964), or trapping of the aldehyde (A. L. GREEN 1962; A. L. GREEN and HAUGHTON 1960, 1961; I. J. STERN et al. 1961); indirectly by reduction of diphosphopyridine nucleotide (DPN) in the presence of aldehyde dehydrogenase (LOVENBERG et al. 1961, 1962a; WEISSBACH et al. 1957d). g) Accumulation of a carboxylic acid (e.g. indolacetic acid) subsequent to aldehyde formation (e.g. from tryptamine) (LEVINE and SJOERDSMA 1962b, 1963b; LOVENBERG et al. 1961, 1962a; WURTMAN and AXELROD 1963a). h) Formation of dark pigments e.g. from 5-HT (SWETT et al. 1963; WYKES et al. 1959; ZELLER et al. 1962). The methods mostly used are manometry as well as extraction and spectrophotofluorometric determination of 5-HT. An elegant procedure consists in direct spectrophotometric
Physiological role
599
measurement of kynuramine deamination (SOEP 1962; \VEISSBACH et al. 1960b). Each method has principal or technical advantages and disadvantages; in consequence, measurements with more than one procedure are desirable. Indirect estimation of decreased MAO activity in vivo may be carried out according to the following principles: - Enhancement of the monoamine increase in several tissues (cspecially in brain) subsequent to injection of the corresponding aromatic amino acid, e.g. the 5-HT accumulation induced by 5-hydroxytryptophan (;,i-HTP) (GEY and PLETSCHER 1960a; HESS et al 1959c; UDE~FRIEND et al. 1957 a, 1957 d); - Enhancement of monoamine accumulation in extraeerebral tissues (e.g. heart) following injection of monoamines metabolized mainly by :\lAO, e.g. ;--;-HT, tryptamine (HESS et al. 1959b; PLETSCHER and GEY 1961a; PLETSCHER et a!. 1960a, 19GOc; \YEISSBACH et al. 1959, 19G1a) ; - IncreaRed urinary elimination of exogenous or endogenous monoHmines preferentially metabolized by :\IAO, e.g. tyramine, tryptamine, ete. (JEPSO~ et a!. 19GO; LEYIXE and S.JOERDS)[A 1963a, 1963b; OATES and ZALTZ}L\5 19;--;9; RES~ICK et a!. 1960b; SCHAYER 19i):3b; SCHAYER et a!. 1954; SJOERDS)[A et al. 1959a, 1959c, 19;'i9d, 19GOa; TABOR et al. 19.54; D. H. TEDESCHI et al. 1959b; l:DE~FRIE~D 1959a); - Increase of endogenous monoamines in tissues (especially brain) and urine, e.g. phenylethylamine, meta-tyramine, 3-hydroxyethanolamine, tryptamine, 5-HT (CREDNER et al. Hl62; E. J. DA YIS and Ropp 19GO; JEPSOX et al. 19GO; O_~TES and Z.~LTZ)IA~ 1959; PLETSCHER et al. 19GOc; SJOERDS}IA et al. 1959b, 1959c, 1959d; S. SPECTOR et al. 1958b; UDE~FRIE~D et al. 1957d); - Decrease of metabolites of endogenous monoamines in tissues (brain, etc.) and urine, e.g. of 5-hydroxyindoleacetic acid and phenolcarbonic acids (J .•\. ANDERSON et al. 1958; CARLSSO~ and HILLARP 1962; CORNE and GRAH.UI 1957; HESS et al. 195ge; JEPSON et a!. 19GO; LEYI~E and SJOERDS}IA 1963b; :\IcIsAAc and PAGE 1959; XAK.H 1958; Roos 19G2; ROSE~GREN 1960b; SJOERDS)IA et al. 1958, 1959a, 19GOa; STl-D~ITZ 1959a; TABACHNIK and RUBIN 1959; TABOR et al. 1960; D. H. TEDESCHI et a!. 1959b; TICKNER 1951; \VEISSBACH ct al. 1958).
8. Physiological rok :\IAO seems to inactivate the monoamines released irom a pool with a slo,v turnover which presumably represents the major part of the stored and thus of the total monoamines in tissues, such as heart and brain. Monoamines belonging to a pool with a fast turnover, howc\"er, undergo O-methylation prior to oxidative deamination. This is also true for exogenous c'ltecholamines which are mainly metabolized in the liver .•\Iternative routes for the metabolism of monoamines (inciudiiig O-methylated catecholamines) are the conjugation with sulfuric or glucuronic acid and the formation of X-deri,-atins (AXELROD 1959b; CARLS SOX and WALDECK 19G3; CHIDSEY and HARRISO~ 19G3; CROUT et al. 19GO, 19G1; DAVISON 1958c; DAY and GREEX 19G2b; GEY and PLETSCHER 19GOc;~. D. GOLDBERG and SHIDE}lIAN 1962; H. GREEN 19G2; HERTTIKG and AXELROD 1961; HERTTIKG and LABROSSE 1962; HERTTING et a!. 19G2; HORITA 19G2a; IISALO 1962; KEGLEnC et al. 1959; KOPI~ 1964; KOPIN and GORDON 19G3; KOPIN et a!. 19G1, 1962; LABROSSE et al. 1961; MCGOODALL et al. 1959; :\IcIs.Hc and PAGE 1959; }IAS.DII et a!. 19G2; PLETSCHER et al. 19GOc; POTTER and AXELROD 19G3; ROSENGREN 19GOb; SCHAEPDRYVER and KIRSHNER 1960, 19G1a; SHORE 1962; UDENFRIEND 1959b; WEINER 19M; WHITBY et a!. 19(1). The products of oxidative deamination, e.g. the aldehydes and the corresponding alcohols and carboxylic acids, may have physiological effects of their o""n. H 2 0 2 , also formed in this reaction, is either decomposed by catalase or used for oxidations, e.g. of formftte (AEBI et al. 19G2a). The role of ammonia split off by :VIAO has not been investigated. -"LAO ftctivity, at least as measured in yitro, is very high in comparison to the normal content and turnover of monoamines in the organs (GEY and PLETSCHER 19G1 b; RICHTER et al. 1941). Therefore, it seems doubtful whether the activity of MAO eon troIs the totftl monoamine content under physiological conditions. This doubt is supported by the following facts: The monoamine content in brain and other tissues does not parallel the activity of :\IAO but rather that of other enzymes, e.g. DCA (BERTLER and ROSENGREN 1959b; BOGD.\NSKI et a!. 1957; DALGLIESH and DUTTON 1947; K.~RKI et al. 19G2; KLINlDIAX et al. 19G4; KCNTZ)[AN et al. 19G1 b; SANDLER and \YEST 1958; \VEST 19;--;8 a) ; monoamincs accumulate in brain and other tissues only after extensive (at least 85 00) -"lAO inhibition (CHESSIN et a!. 1959; DCBNICK et al. 19G2b; GEY and PLETSCHER 1959a, 1961 b, 1961 e; H. GREEX and ERICKSON 19G2; \YURT~IA~ and AXELROD 1963b) (Fig. 4). Variations of MAO activity have been found in different organs, e.g. with age (BIRKH;\USER 1940; BURKARD et a!. 1961; Epps 1945; GEY et al .19G4; NACHMIAS 19GO; XOVICK
600
Monoamine oxidase inhibitors
1961a; SCHWEPPE et al. 1951; ZELLER 1962; ZELLER et al. 1940), with regard to different sexes after thyroxine administration (NOVICK 1961 a, 1961 b; SKILLEN at al. 1961; ZELLER 1959; ZILE 1960; ZILE and LARDY 1959), by deficiency of thiamine (BAL and DREWES 1961; MELTZER 1961 a), or pyridoxine (PmLLrPs et al. 1962); by sloop' depreviation (NIKAIDO 1961), thyreoidectomy (ZELLER 1959), sexual steroids (PAVLIN and ZUPANCIC 1956; WURTMAN and AXELROD 1963b), anaphylactic shock (ARAI 1963), toxemia of pregnancy (LINDBERG and WESTLING 1962; LUSCHINSKY 1950; SANDLER and COVENEY 1962)_ It has not been ascertained whether these variations are related to still unknown activation/inhibition mechanisms or to changes in enzyme synthesis. The latter is possible since the half-life time of the enzyme appears to be less than 3 days (ZELLER 1964).
B. Monoamine oxidase inhibitors 1 1. Chemical classification MAO is inhibited by numerous compounds which belong to various chemical classes and show different pharmacological actions, e.g. central stimulation, sedation, anesthetic, anticonvulsive, and antihistaminic properties. Most inhibitors act only in vitro; compounds active in vivo, however, must not necessarily affect the enzyme in vitro. The following substances have been shown to inhibit MAO: Alcolwls and aldehydes, e.g. octanol, thymol, but also ethanol and acetaldehyde (BLASCHKO et a1. 1937a, 1937b, 1937c; COTZIAS and GREENOUGH 1960; FOUTS et a1. 1957; HEIM 1947, 1950b; HOLTZ et al. 1938a; MAYNARD and SCHENKER 1962; ROSENFELD 1960; TOWNE 1964; USDIN and USDIN 1961; WARTBURG and AEBI 1960; WERLE and MENNICKEN 1938; ZELLER 1959; ZELLER et al. 1958b). Alcohols are typical MAO inhibitors not affecting DAO. rx-Alkylated aralkylamines, e.g. amphetamine, ephedrine (BHAGVAT et al. 1939; BLASCHKO 1938, 1940; BLASCHKO and STROMBLAD 1960; B. G. BROWN and HEY 1956; EULER and HELLNER-BJORKMAN 1955; FELDSTEIN et a1. 1959; FELLOWS and BERNHEIM 1950; FUJIMURA and OHATA 1958; A. L. GREEN 1964; HAUSCHILD 1941; HEEGARD and ALLES 1943; HEIM 1947; IISALO 1962; MAASS and NIMMO 1959; MANN and QUASTEL 1939, 1940; MCCOUBREY 1959; ORZECHOWSKI 1941; OZAKI et al. 1960; PARMAR and NICKERSON 1961; PHILPOT 1940; PLETSCHER and GEY 1959; PRATESI and BLASCHKO 1959; RICHTER and TINGEY 1939; SNYDER and OBERST 1946; SYMUL 1947,1957; VINCENT and SEGONZAC 1960), rx-alkylated indolyl alkylamines, e.g. rx-methyltryptamine and its N-methylderivative, rx-ethyltryptamine, and tertiary N-alkyl derivatives, e.g. hordenine and N-dimethylaminoethylindol (BARLOW 1961; FRETER et a1. 1958; GOVIER et al. 1953; GREIG and GIBBONS 1962; GREIGet a1.1959, 1961a, 1961 b; HEsTERet a1.1964; HUSZTI and BORSY 1964; METER eta1.1960; PFEIFER eta1.1961; SATORY et a1. 1961; D. H. TEDESCHI et a1. 1962; VANE 1959; VINCENT and SEGONZAC 1960; WOOLLEY and EDELMAN 1958). These compounds presumably have a high affinity for MAO, but they can hardly be deaminated because of the rx-alkyl residue (BEYER 1943, 1946; BEYER and MORRISON 1945; BEYER and SHAPIRO 1945; BLASCHKO and HAWKINS 1950; BLASCHKO et a1. 1937a; K. K. CHEN et a1. 1929; S. C. HARRIS et al. 1947; HEEGARD and ALLES 1943; RICHTER 1938; SNYDER et a1. 1946; SORIANO 1958). In vivo, short and weak MAO inhibition has been demonstrated for rx-alkylated aralkylamines (PLETSCHER and GEY 1959) and rx-alkylated indolylalkylamines mainly by indirect procedures (GEY and PLETSCHER 1962; GREIG and GIBBONS 1962; GREIG et a1. 1959, 1961a, 1961 b; HESTER et a1. 1964; LESSIN 1959a). It is, however, doubtful whether MAO inhibition is of importance for the pharmacological action of these compounds (GEY and PLETSCHER 1962; GRANA and 1 This topic has been reviewed several times (BIEL et al. 1964; BLASCHKO 1963; PLETSCHER et al. 1960c; PSCHEIDT 1964; SOURKES and D'!oRIO 1963; ZELLER 1963d; ZELLER and FOUTS 1963; ZIRKLE and KAISER 1964).
Chemical classification
1101
LILLA 1959; GREIG and GIBBONS 1962; MCGEER et al. 1960; PARKES et al. 1962a, 1962b; SCHOOT et al. 1962). These a-alkylated compounds are rapidly excreted in the urine; their metabolism includes hydroxylation, conjugation, and deamination (AXELROD 1954, 1959c; BEYER and SKINNER 11140; EBERTS 1961; EBERTS and DANIELS 1962; PINSON ct al. 1961b, 1962a; SZARA 1961). Aryl ethers of choline, e.g. para-tolyl choline ether (BARSKY 1958; B. G. BROWN and HEY 1952,1956; CORNE and GRAHAM 1957; KOBAYASHI and SCHAYER 1955; OZAKI et al. 1960; SCHAYER et al. 1955; USDIN and USDIN 1961). In vitro they cause moderate competitive inhibition of MAO. In vivo they inhibit the enzyme in intestine (DAVISON 1957; SCHAYER et al. 1954), but not in brain and liver (CORNE and GRAHAM 1957; EULER and HELLXER-BJORK:\IAX 1955; PLETSCHER and GEY 1959); they also reduce the oxidation of some exogenous monoamines (GRIESEMER and ~WELLS 1956; SCHAYER 1953b; SCHAYER et al. 1954, 1955). Harmane derivatives, e.g. harmaline (OZAKI et at 1960; PLETSCHER and GEY 1959; PLETSCHER et al. 1959a; SJOERDSMA et al. 1959a; TAYLOR et al. 1960a; TUNG 1960b; TUNG et al. 1960a; UDENFRIEND et al. 1958b; VINCENT and SEGONZAC 1960). These are strong and reversible MAO inhibitors in vitro (TuXG et al. 1960a). In vivo they cause MAO inhibition of short duration, i.e. of a few hours (PLETSCHER et al. 1959a; UDENFRIEND et al. 1958b). Hydrogenation to tetrahydroharmine, dealkylation (e.g. to harmol) or mcthylation of the pyridine ring reduces :\IAO inhibition. Harmaline is metabolized to harmine, harmalol, and harminic acid (FLURY 1911). X-Alkyl tertiary and quarternary nitrogen derivatives and carbonium compoullds, c.g. neostigmine, pyridine-aldoxime dodecyliodidc and bretylium. They may induce strong, in some cases irreversible :NIAO inhibition in vitro, but no or onl:' moderate inhibition in vivo (DVORNIK et al. 1963; KUNTZMAN and JACOBSOX 1963; MELTZER 1961a, 1961 b, 1962; PANT et al. 1964a, 1964b; USDIN and USDIN 1961; VIXCEXT et al. 1962). Aralkylguanidines, mono- and polyamidines (e.g. pentamidine), stilbamidines, biguanides, mono- and dithiourea derivatives (AXELROD 1957; F. BERNHEIM 1943; BLASCHKO and DUTHIE 1945a, 1945b; BLASCHKO and HIMMS 1955; CHRUSCIEL 1962; DAVISON 1958b; DVORXIK et al. 1963; FASTIER and HAWKINS 1951; GESSA et al. 1962c; KADZIELAWA 1962; KUNTZMAN and JACOBSON 1963a, 1963b; ~L MEYER 1960; SCHULER and MEYER 1955). Guanidine derivatives are strong but reversible and competitive inhibitors with \veak action in vivo (DVORNIK et al. 1963; KUNTZMAN and JACOBSON 1963a, 1963b). The other compounds have an irreversible, but moderate action in vitro. In vivo pentamidine inhibits ~IAO in liver, but not in brain (BAXTER and ROBERTS 1958; DAVISON 1958a; PLETSCHER and GEY 1959). Methylene blue (EHRINGER et al. 1961; GOVIER et al. 1946; hIAIZPMI et al. 1959; JAKUBOVIC and NECINA 1963; PHILPOT 1937, 1940; THOMPSON and TICKNER 1951; WERLE and ROEWER 1952). Its action in vitro is moderate, in vivo short and incomplete. M yristicin, a terpene-like nitrogen-free compound, seems to inhibit :NIAO moderately in vivo (TRUITT et al. 1963). Other substances. Quite a number of compounds have been described to inhibit MAO in vitro, e.g. derivatives of aminoalkane (BLASCHKO 1952; HEEGARD and ALLES 1943), dialkylamino alkyl derh·atives (BOSE and VIJAYVARGYA 1960; BROWN and HEY 1956; ERNSTIXG et al. 1962; HEB! 1950 a, 1950 b; IMAIzDn et al. 1959; ORZECHOWSKI 1941; OZAKI et at 1960; PHILPOT 1940; PLETSCHER and GEY 1959; POLONOVSKI et al. 1953; SCHAUMANN and KURBJUWEIT 1961; SCHULER and WIEDEMANN 1935; SJOERDSMA et al. 1959a; TICKNER 1951; USDIN and
Monoamine oxidase inhibitors
602
USDIN 1961; VITEK and RYSANEK 1960), urea (BHAGVAT et al. 1939; GIORDANO et al. 1962; HElM 1947; MANUKHIN 1958; PHILPOT 1937; ZELLER 1941a), urethane (BLASCHKO et al. 1937b; KOHN 1937), carbon disulfide (MAGISTRETTI and PEIRONE 1961), and possibly azides (OKUMURA 1960; WERLE and MENNICKEN 1938; WERLE and ROEWER 1952); hydroquinone and polyphenols (A. L. GREEN 1964); derivatives of pyridine (NANTKA-NAMIRSKI et al. 1963), of piperidine (SZPORNY and GOROG 1961; VINCENT and SEGONZAC 1960; WERLE and PECHMANN 1948/49), of piperazine (US DIN and USDIN 1961), of isoquinoline (USDIN and USDIN 1961), of indazole (MINATOYA 1963), of carbazol (OZAKI et al. 1960), of carboline (NANTKANAMIRSKI et al. 1963; OZAKI et al. 1960), of oxazine (MCCOLL and RICE 1960;
Table 1. MAO inhibitors with long duration of action in vivo Examples
Chemical class
a) Hydrazine derivatives
Iproniazid (Marsilid) OHa" /~~ /OH-NH-NH-OO-( N OHa ,,=/' [-~
Phenelzine( (-OH 2-OH 2 -NH-NH 2 (Nardil) -Tranylcypromine (Parnate)
b) Arylcycloalkylamines
CH 2
-/ // /(~(-OH-OH-NH2 c) N-Benzyl-N-methylpropargy lamine
----------
i
Pargyline (Eutonyl) OHa
/~/ I, (~(-OH2-N-OH2-0~OH d) 2-Methyl-3-piperidinopyrazine
"
'-==--/ "
et al. 1961; USDIN and USDIN 1961), of 1:2:4-triazol (JACKSON et al. 1962), of tetrazole (USDIN and Us DIN 1961); phenanthroline (BARBATO and ABOOD 1961), proflavine (GORKIN et al. 1964b), xanthines (CAHN 1960; VINCENT and SEGONZAC 1960; ZELLER 1941a), quinine and quinidine (lIDA 1958; VINCENT and SEGONZAC 1960), nicotine (WERLE and PECHMANN 1948/49; WERLE and PESCHEL 1949), chlortetracycline (OKUDA and OKADA 1956), alkaloids of Ergot (HElM 1947) and Rauwolfia (BOSE and VIJAYVARGYA 1960), hydrazinophthalazines (KUBIKOWSKI and WYSOKOWSKI 1963; PERRY et al. 1955; SCHULER and MEYER 1955; WERLE et al. 1955), phenothiazines and similar compounds (BOSE and VIJAYVARGYA 1960; GEY and PLETSCHER 1961 a; GORKIN 1959; JACOBI et aJ. 1961; KIVALO et al. 1961; NAKAJIMA 1959b; OZAKI et al. 1960; D. SCHWARTZ et al. 1963; USDIN and USDIN 1961), atebrine (ALLEGRETTI and VUKADINOVIC 1950; LAGNADO and SOURKES 1956a; YODIM and SOURKES 1964), and various unidentified compounds (FRIEDENWALD and HERRMANN 1942; ZELLER 1941a). As far as investigated, these substances have in general no or only little effect on MAO in vivo. The present review is mainly concerned with irreversible (long acting) MAO inhibitors which have a strong effect in vivo and which may be used in the therapy of human disease. The following four types of compounds belong into this category (Table 1). F. A. SMITH
603
Chemical classification
a) H ydrazine derivatives with the general formula
R-NH-NH-R'
In order to get activity in vitro, at least one hydrogen of the hydrazine moiety has to remain free (BARSKY et al. 1959; DAVISON 1957; MCGRATH and HORITA 1962; ZELLER and SARKAR 1962; ZELLER et al. 1955a, 1955b) and one residue (R) must be an alkyl- or aralkyl group, whereas R' may represent H, an alkyl, aralkyl, or acyl residue. Corresponding hydroxylamines are no MAO inhibitors (MAJOR and OHLY 1961). Iproniazid is the first compound of this group which has been shown by E. A. ZELLER, 1952, to inhibit MAO in vitro and in vivo l (BARSKY et al. 1959; ZELLER and BARSKY 1952; ZELLER et al. 1952a, 1952c). The following MAO inhibitors have mainly been used in human therapy (CROWTHER et al. 1962; JEXNI and PLETSCHER 1962):
CO-NH-:NH-CH
CH a phenylethylhydrazine phenylisopropylhydrazine -CH2-
r
H -NH-NH2
OHa
IX-methylbenzylhydrazine
Mo 1255
COOC 2H 5
d) 2-Methyl-3- pi peridinopyrazi ne """N,,/CH3 I
~N/"N/-
:
I
-------
In vivo, this inhibitor has a very potent, long-lasting action on brain MAO probably due to an oxidized metabolite (DUBNICK et a1. 19620, 1963; GYLYS et a1. 1963). l\IAO inhibition in vivo by methylpiperidinopyrazines requires a single alkyl group (methyl or ethyl) on the pyrazine residue adjacent to the piperidine. Halogenation of the pyrazine nucleus, formation of pyrazine-N-oxides, and replacement d the latter by quinoxaline abolishes MAO inhibition, whereas the piperidine residue may be transformed into a N-oxide or replaced without marked changes of potency by other cyclic amino groups or even by an acyclic tertiary amino group. 2. Distribution and metabolism in tissues a) H ydrazines a) Distribution and excretion. In animals the concentration of iproniazid in blood and tissues is maximal one hour after injection. The inhibitor penetrates into the brain in measurable amounts within 1-5 minutes following i.v. administration. Excretion is almost complete within 24 hours (HESS and WEISSBACH 1958; HESS et a1. 1958; KOECRLIN and ILIEV 1959; NAIR 1959; NAIR et a1.1960, 1962). lsocarboxazid and nialamide behave similarly to iproniazid (SCHNEIDER et a1. 1959; M. A. SCHWARTZ 1960). In humans the turnover of orally administered iproniazid and isocarboxazide is somewhat slower than in animals; within 24 hours only about 50-60% are recovered in the urine (KOECHLIN et a1. 1962). Phenelzine has been reported to accumulate more in the mesencephalon than in the cortical and bulbar regions of the guinea pig brain (ZARA and BELSANTI 1961); it seems to enter the brain more rapidly than iproniazid (HOROWITZ 1960). ~) Metabolism. In the brain in vivo as well as in brain slices, but not in mitochondria, the uptake of iproniazid is faster than the development of MAO inhibition (GLUCKMAN and HOROWITZ 1959). Acylhydrazines need an aerobic preincubation for 15-20 minutes for maximal MAO inhibition even with mitochondria, whereas aralkylhydrazines require oxygen, but not as much as their acyl derivatives (DAVISON 1957; A. L. GREEN 1962, 1964; HARDEGG and HEILBRONN 1961; NICKERSON and PARMAR 1960; M. A. SCHWARTZ 1962; SEIDEN and WESTLEY 1963; SMITH et a1. 1964; TAYLOR et a1. 1960a; ZELLER 1961; ZELLER et a1. 1955a). MAO inhibition in vivo persists for a longer time than unchanged iproniazid or isonicotinic acid can be detected in the tissues (HESS et a1. 1958; NAIR
608
Monoamine oxidase inhibitors
1959; NAIR et al. 1960). This suggests that iproniazid has to be transformed into a compound representing the actual inhibitor (HElM and DmMER 1960; PLETSCHER 1958b). Indeed the isopropylhydrazine moiety is the essential part of the iproniazid molecule (BARSKY et al. 1959; ZELLER et al. 1955 b) and represents the active metabolite in vivo (KOECHLIN 1959; ROTH and RIEDER 1964). Recent in vitro-studies demonstrate that the actual inhibitor is a volatile compound, i.e. isopropylhydrazine or an oxidized product derived from isopropylhydrazine respectively. The formation of the actual inhibitor requires an electron acceptor, e.g. oxygen or under anaerobic conditions tetrazolium salts (DAVISON 1957; A. L. GREEN 1964; KORY and MmGIOLI 1964; M. A. SCHWARTZ 1962; SEIDEN and WESTLEY 1962, 1963; SMITH et al. 1964; ZELLER 1955a; ZELLER and SARKAR 1962). Cyanide or thiourea increase the formation of the actual inhibitor and possibly its attachment to the active center of MAO (BARBATO and ABOOD 1963; DAVISON 1957; A. L. GREEN 1964; SMITH et al. 1964). Cupric ions also catalyze this reaction (EBERSON and PERSSON 1962; A. L. GREEN 1964). The formation of the actual inhibitor may occur nonenzymatically, but is possibly enhanced by tissue constituents, e.g. porphyrine derivatives or chymotrypsin (ALBERT and REES 1955; BEAVAN and WHITE 1954; KORY and MINGIOLI 1964; KRUGERTHIEMER 1955; LONG, unpubl.; LUTWACK et al. 1957; SEIDEN and WESTLEY 1963; SMITH et al. 1964). Based on studies on the cupric ion-catalyzed autoxidation of MAO inhibitors with hydrazine moiety, it has also been proposed that one electron of the hydrazine could be transferred to MAO followed by oxidation of the hydrazine radical by molecular oxygen (EBERSON and PERSSON 1962). It is very likely that other MAO inhibitors with acylhydrazine structure undergo corresponding transformations. Thus, the actual inhibitor of isocarboxazid is probably benzylhydrazine or a more highly oxidized metabolite (KORY and MINGIOLI 1964; ROTH and RIEDER 1964; M. A. SCHWARTZ 1962; SMITH et al. 1964). Besides hydrolysis of the acylhydrazine bond, the metabolism of hydrazides also includes cleavage of the alkyl or aralkyl substituent, although the first reaction is probably of major quantitative importance. Iproniazid is metabolized to isonicotinic acid, isopropylhydrazine, and to a small extend to isoniazid. Isonicotinic acid appears mostly in the urine and has also been detected in brain. Isopropylhydrazine is mainly oxidized to CO 2 , but it also forms hydrazones with physiological keto compounds (HESS et al. 1958; HORITA 1962a; KOECHLIN and ILmv 1959; KOECHLIN et al. 1962; NAIR 1959, 1962; RITTER et al. 1952; ROTH and RIEDER 1964; RUBIN et al. 1952; SEYDEN and WESTLEY 1963; SMITH et al. 1964). The further metabolism of isoniazid includes N-acetylation (main pathway), formation of hydrazones with ex-keto acids, or hydrolysis to isonicotinic acid (CHALMERS 1962; PETERS 1960; ZAMBONI and DEFRANCESCHI 1954). Isocarboxazid yields benzylhydrazine as the major metabolite in blood and tissues. Benzylhydrazine is probably oxidized to benzoate which appears in the urine as hippurate. In vitro isocarboxazid undergoes rapid hydrolysis into benzylhydrazine which actually seems to be the inhibitor (KOECHLIN et al. 1962; ROTH and RmDER 1964; M. A. SCHWARTZ 1960, 1961a, 1961 b, 1962). Nialamide probably follows the two pathways indicated for iproniazid, but in addition the aInide bond in the alkyl side chain is split (PINSON et al. 1962). Sulfonic acid aralkylhydrazides have been reported to yield metabolites siInilar to the corresponding acyl-aralkylhydrazines (ROONEY et al. 1962). The metabolism of unacylated alkyl- and aralkylhydrazineJl has not been investigated thus far. In analogy to the fate of phenylhydrazine (McIsAAC et al.
Effect on monoamine oxidase
609
1958) it may be concluded that aralkylhydrazines are hydroxylated in paraposition and also form hydrazones with physiological IX-keto acids (HORITA and MATSUMOTO 1962). In any case, the metabolism of aralkylhydrazines, e.g. pheniprazine, proceeds in vitro and in vivo more rapidly than that of acylated hydrazines, e.g. iproniazid (HORITA 1963b). b) Other compounds Tranylcypromine does not seem to be altered by MAO in vitro (BELLEAU and MORAN 1962). In the rat in vivo, however, the main part of the compound is metabolized within 24 hours. i.e. by cleavage of the cyclopropane ring resulting in urinary excretion of hippurate and of 4 metabolites of minor quantitative importance. A considerable portion of tranylcypromine is excreted unchanged in the urine (ALLEVA 1963). Pargyline, after oral administration, reaches a maximal concentration in blood and tissues within 1-2 hours followed by a slow decline over 48 hours. The inhibitor is excreted partly unchanged and partly metabolized (TAYLOR and KRAUSE 1962). In tissue homogenates. pargyline does not seem to be rapidl~ metabolized (HORITA 1963b). 2-Jlethyl-3-piperidinopyrazille has to be transformed into an oxidized deri,-ative in order to inhibit :\IAO. In vitro. this transformation may be catalyzed by the unspecific oxidase of liver microsomes. In vivo. the actual inhibitor, which docs not seem to be an X-oxide or a product with an opened piperidine ring. probably undergoes further metabolism. After ix. injection. 80 0 0 of the drug is excreted in the urine within 24 hours (BLUM et al. 1964; DFBNICK et al. 1962 c, 1963).
3. Effect on monoamine oxidase a) H ydrazine derivatives
In t'itro hydrazine derivatives inhibit MAO of numerous tissues to a compara ble degree (BARSKY 1958; CANAL et al. 1958: CORNE and GRAHAM 1957: DAVISON 1956, 1958a, 1958b; FnwER 1960; FUNDERBURK et al. 1962; GRIESEMER and 'YELLS 1956; HORITA 1958b, 1962a: IISALO 1957, 1962; KRISHNA et al. 1961; }IAXWELL et al. 1961; NAKAI 1958; PEPEU et al. 1961; SCHIATTI 1960; SHORE et al. 1957; SJOERDSMA et al. 1955; ZELLER et al. 1952c, 1955a). The extent of inhibition in tissue slices and cell fractions, e.g. mitochondria and microsomes, is similar, whereas total homogenates are less susceptible to some hydrazine derivatives, e.g. iproniazid. This might be due either to unidentified factors protecting MAO, to side reactions between the inhibitor and some tissue components, or to metabolic inactivation of the inhibitor in total homogenates (BARSKY 1958; GLUCKMAN et al. 1958; A. L. GREEN 1962; HORITA 1963a, 1963b; HORITA and MATSUMOTO 1962; ZELLER et al. 1955a). The inhibition of mitochondrial MAO is only slightly influenced by the functional state of the mitochondria (AEBI 1962; AEBI et al. 1962b). MAO inhibition by hydrazides, e.g. iproniazid, isocarboxazid, and nial· amide, becomes maximal only after aerobic preincubation in the absence of substrates for about 15-20 minutes (see above). This progression must be attributed to the formation of free isopropyl- or benzylhydrazines and oxidized metabolites respectively, which in the case of iproniazid and isocarboxazid seem to be the actual }IAO inhibitors. Although the inhibition of }IAO by iproniazid is at least a bimolecular reaction, its kinetics are of first order (DAVISON 1957; A. L. GREEN 1962; HARD EGG and HEILBRONN 1961), probably because iproniazid is much in excess of the enzyme (WEBB 1963). With iproniazid and isopropylhydrazine respectively an inhibition rate of 1,7 X 103 and 1,6 X 104 liters mole-1 minutes-1 as well Handb. d. expo Pharmakol.. Erg.-Werk Ed. XIX 39
610
Monoamine oxidase inhibitors
as an overall enthalpy of activation of about 30 and 26 kcaljmole was determined (DAVISON 1957). This corresponds to a very high increase of entropy (WEBB 1963) which could explain the irreversibility of MAO inhibition (see below). Since, however, MAO inhibition by iproniazid and isopropylhydrazine is certainly not a monomolecular reaction (see above), it remains uncertain whether formation of the actual inhibitor or its attachment to the enzyme is mainly responsible for the change of entropy. MAO inhibition by hydrazine derivatives with or without acyl residue has been found by most investigators to be competitive with the substrate as long as the inhibition is not fully established (ARAI 1960; DAVISON 1957; GLUCKMAN and MARAZZI 1958; A. L. GREEN 1962; NICKERSON and PARMAR 1960; SEIDEN and WESTLEY 1962; SMITH et aL 1964; TUNG et aL 1960; ZELLER et aL 1955a). Correspondingly, the rate of formation of the enzyme-inhibitor-complex depends on the affinity of the substrate used (HARDEGG and HEILBRONN 1961). The dissociation constant for the complex MAO-iproniazid is about 2 . 10-4 • Therefore, the affinity of iproniazid for the enzyme is about 15 times stronger than that of a good substrate such as tyramine (DAVISON 1957). Once the inhibition of MAO has been established, it seems to be uncompetitive with the substrate (SEIDEN and WESTLEY 1962, 1963) and is almost .irreversible by dialysis, by washing of the mitochondria, or by urea treatment (ARAI 1960; BARSKY 1958; DA"~SON 1957; GLUCKMAN and MARAZZI 1958; A. L. GREEN 1962; SMITH et al. 1964; VIOLLIER et aL 1953; ZELLER et aL 1955a). At :noc the oxidative deamination of substrates with different affinity, e.g. 5-HT, tyramine, norepinephrine (NE), epinephrine, normetanephrine, isoamylamine, is inhibited to a similar degree (BARSKY 1958; GEY and PLETSCHER, unpubL; HARD EGG 1961; HOPE and SMITH 1960; KoBAYASHI and SCHAYER 1955; SJOERDSMA et aL 1955; SEPCTOR et aL 1960d; TABACHNICK 1959; UDENFRIEND and WEISSBACH 1958). The question how MAO inhibitors interfere with the enzyme has not been fully elucidated. The actual inhibitor with hydrazine structure presumably combines with the active center of the enzyme because of its structural resemblance to monoamines. Hydrazine derivatives may be considered to be pseudoamines, since the isosteric residues -NH- and -CH2- behave very similarly in various chemical and physical respects. The hydrazines, in contrast to the substrates, might be irreversibly bound to the electrophilic site of MAO (presumably covalent bond). Hydrazine derivatives do not require a free IX-hydrogen for interaction with MAO (BARSKY et aL 1959; BELLEAU and MORAN 1963; DAVISON 1957; DAVISON and SANDLER 1958; A. L. GREEN 1964; ZELLER 1960, 1963d; ZELLER and FOUTS 1963; ZELLER et al. 1952b, 1955a). No oxygen is required for the attachment of the actual inhibitor to MAO (KORY and MINGIOLI 1964; A. L. GREEN 1964; SMITH et aL 1964). In vivo, MAO inhibition by hydrazine derivatives has been demonstrated in numerous tissues, e.g. brain, sympathetic ganglia, liver, heart, spleen (APRISON and FERSTER 1961; BARSKY 1958; CORNE and GRAHAM 1957; DAVISON 1958a, 1958b; DAVISON et aL 1957; FINGER 1960; GRIESEMER et aL 1953; HESS et aL 1958; HElM and DIEMER 1960; IrSALO 1962; KAMIJO et aL 1956; LEVINE 1962; LOTLIKAR and MCCUTCHEON 1961; MCGRATH and HORITA 1962; NARANJO and PAI,ACIOS 1961; OZAKI et aL 1960; RANDALL and BAGDON 1959; RANDALL et aI., unpubL; SCHNEIDER et aL 1959; SJOERDSMA et aL 1955; VIOLLIER et aL 1953; WEISSBACH et aL 1957a; WITT et aL 1961; ZELLER et aL 1952a, 1955a). After large single doses of the inhibitors the reduction of enzyme activity persists for several days or weeks (BURKARD et al. 1960; DAVISON et aL 1957; HESS et al. 1958; HORITA 1958a; RANDALL and BAGDON 1959; WITT et aL 1961; ZELLER
Effect on monoamine oxidase
611
et a1. 1955a) and outlasts by far the presence of the unchanged inhibitor in blood and tissues (HESS et a1. 1958; HORITA and CHIXN 1\l64; NAIR 1959; ~AIR et aI. 1960). MAO inhibition in vivo may vary according to the type of tissue and the route of administration of the inhibitor. Hydrazides, e.g. iproniazid and nialamide, given parenterally or orally inhibit }IAO in liver and kidncy faster and more markedly, but for a shortC'r period than in brain. In consequC'nce, 1-2weeks aftC'r a single administration of the inhibitors, MAO is practically no longer affeeted in linr, but still markC'rlly inhibited in brain (BARSKY 1958; CAXAL d aI. 1\l58: DAVISOX !f)58b; DAVISOX et a1. 1957; FewER 1960; FOLKERTH and ApRIsox 1962; GOC:EI{Ty and HORITA H)59; HEDf and DIE-'IER 1960; HESS et a1. 1958: HOPE and S-'IlTH H)60: HORITA 1959, lfHil: RHDALL C't al., unpubl.: SPECTOR ct a1. 1960d: T_\YLOR and KRArsE HHi2: TAYLOR et a1. H)60a; "-EIKEL and SALrtWX 1962). TherC'by, thC' inhibition in midbrain, medulla, pons, and cC'rebral hemispheres outla8ts that in cC'rebellum (FOLKERTJI and APRISOX 1962). By the parcnteral route, single dose,; of aralkylhydrazines, e.g. pheniprazinc and phenelzine. inhibit }IAO in brain more markedly than in liver. but on oral administration the,\- "ho,,· the opposite pattern, i.e. tlll',\- beha,-e in general similarly to h,\-drazides like iproniazid, nialamide, isocarboxazicl (GOGERTY and HCJRIT.\ 1960; HORITA 1959, 1960, H)61; HClRIT.\ and }IcGRATH 1960b; HORITA and PARKER 1958; K.UIIXSKI et a1. 1964-; }ICGRATH and HORITA 1962). Since till' restitution of }[AO acth-it,\- is probably duC' to ele novo formation of the enzyme, the a bm-e differcnces in the time coun,e might indicate a lower turnov('f of }IAO in brain than in other organs. Beha,-ioural change,; caused b,\- }IAO inhibitors of the hydrazine t,\·pe rather parallel the reduction of }IAO aetivity in brain than that in liver (APRISOX and FERSTER H)61: HEISE and BOFF 1960; HESS and DOPFXER H)(31). Some hydrazine deri,-atives have been describcd which on oral administration inhibit }IAO in brain at least as much as in liver, e.g. an aeylderi,-atiYe of X1,X~-diisopropylhydrazine (GEY and PLETSCHER, unpub1.), X-aralkylhydrazines (.JORI et 011. 196:3), and X1-benzyl-~1-(1,4-benzodioxan-2-ylmethyl hydrazine (BOVET-XITTI et a1. 1961). Repeated administration of ~IAO inhibitors of the h,\-drazine type in therapeutic doses, e.g. iproniazid and Ro 4-2637 1, reduces the enzymc activity more markedly in brain than in liver and kidney of animals (HOPE and S-'UTH 1960; S-'WSZKOVICZ and GI'EIG 1961) as well as of humans (BERNHEI~IER et a1. 1962; GANROT et a1. 1962b). Therapeutic doses of hydrazine derivatives inhibit MAO in seyeral regions of the human brain up to 100~o (BERXHEDIER et a!. 1962: GAXROT et a1. 1962b). Preex position oj JIAO in vitro and in vivo to reversiblc inhibiton; like amphetamine (PAR-'IAR and XICKERSOX 1961) or harmane derivatiyes (HORITA and CHIXX 1964-; HORITA and }IcGRATH H)60a; LONG, unpubI.) prevents the action of irreversible inhibitors with hydra7,ine structure. This indicates that the re,-ersible inhibitors and the h,\-dra7,ines compete for the samC' active site of }IAO. Once inhibited by a hydrazine compound, e. g. pheniprazil1e, }IAO can no longer bind othC'r hydrazine dcriva bves, e. g. iproniazid or its metabolites (S:mTH ei a!. 1H64). b) Phellylcyclopropylaillille
In vitro, this compound reduces :VIAO acti,-ity more markedly than inhibitors of the other classes. The inhibition is either competitiye or uncompetitiye aC'cording to the substrate, almost irreversible, maximal ,vithout aerobic preincubation, 1
Ro 4-2():n
=
para· toluene sulfonic acid :\l-benzylhydrazide.
39*
612
Monoamine oxidase inhibitors
and independent from cyanide. MAO of different tissues is inhibited to a varying degree (BARRATO and ABOOD 1961, 1963; BELLEAU and MORAN 1962, 1963; GEY et aI., unpubI.; H. GREEN and ERICKSON 1960; MAASS and NIMMO 1959; D. R. MAXWELL et aI. 1961; SARKAR et aI. 1960a; D. H. TEDESCHI et aI. 1960a: R. E. TEDESCHI et aI. 1959b; ZELLER 1961, 1963a; ZELLER and SARKAR 1962; ZELLER et aI. 1962). It can be assumed that tranylcypromine is bound to the active site of MAO, because its steric and electronic properties are very similar to those of the intermediary forms of the monoamines during oxidation (BELLEAU and MORAN 1962, 1963; BELLEAU et aI. 1961; SARKAR et aI. 1960a; ZELLER and SARKAR 1962). In vivo, a single oral or parenteral dose of tranylcypromine strongly inhibits MAO in brain already after 1 hour. This inhibition lasts for about 15 hours (H. GREEN and ERICKSON 1960; SARKAR et aI. 1960a). In liver the effect is less pronounced and of shorter duration (HORITA 1960; SARKAR et aI. 1960). Preexposition of MAO with tranylcypromine only slightly reduces MAO inhi· bition by hydrazine compounds (SMITH et aI. 1964). This and the competition between tranylcypromine and some substrates (see above) suggest that the complex between MAO and tranylcypromine may be reversed by compounds with a higher affinity for the active site of the enzyme. Preexposition of MAO in vivo to reversible inhibitors like harmaline and harmine does not prevent the action of tranylcypromine (HORITA and MCGRATH 1960a), since the latter presumably persists in the tissue longer than the harmane derivatives (HORITA and CHINN 1964).
c) N-Benzyl-N-methyl-propargylamine and derivatives In vitro, this drug reduces MAO activity slightly more than iproniazid and pheniprazine. The inhibition, similarly to iproniazid, is not reversed by washing and maximal only after preincubation (SWETT et aI. 1963; TAYLOR et aI. 1960a; ZELLER 1963a). It is conceivable that the acetylene linkage of pargyline reacts with sulfhydryl groups essential for MAO (BELLEAU and MORAN 1963). In vivo, single oral doses inhibit MAO in liver slightly more than in brain within the first hours; 1-5 days after oral or intraperitoneal administration, the degree of inhibition in brain, heart, and liver is similar but higher than in the stomach and jejunum (LEVINE and SJOERDSMA 1963a). The subsequent restoration of MAO activity in brain and heart proceeds at a slower rate than in the other tissues (TAYLOR et aI. 1960a, 1960b); LEVINE and SJOERDSMA 1963). The enzyme activity of other tissues is also reduced, e.g. in sympathetic ganglia (LEVINE 1962). Far lower doses are required to produce significant reduction of MAO activity in man than in animals (LEVINE and SJOERDSMA 1962a, 1963a). One oral or parenteral application of 50--100 mg to hypertensive patients results in marked inhibition of MAO activity (87-95%) in jejunal mucosa with complete recovery within 7-9 days (LEVINE and SJOERDSMA 1962a, 1963). Preexposition of MAO with pargyline does not prevent the attachment of iproniazid, i.e. of its inhibitory metabolite, to the enzyme (SMITH et aI. 1964). d) 2-Methyl-3-piperidinopyrazine In vitro, 2-methyl-3-piperidinopyrazine has a rather weak action on MAO if it is not oxidized to the actual inhibitor. The latter competes with the substrates for MAO. In vivo, the compound has a rapid onset and a long duration of action on MAO resembling the more effective aralkylhydrazines. Thereby, MAO of brain is
Effects on other enzyme systems
613
inhibited more markedly and for a longer period of time than MAO of liver. On oral administration the pyrazine derivative is more potent than after intravenous injection. Preexposition of intact mice to harmaline prevents the MAO inhibition due to 2-methyl-3-piperidinopyrazine (DUBNICK et al. 1962c, 1963). 4. Effects on other enzyme systems Several enzymes are influenced by MAO inhibitors of the hydrazine type. Interference seems, however, to be dependent on the structure of the hydrazine moiety. Thus, hydrazines not inhibiting }IAO (e.g. isoniazid, semicarbazide) are also effective, and MAO inhibitors without the hydrazine residue are inactin. Up to now, no enzyme system besides MAO has been found to be related to the therapeutic effects of MAO inhibitors.
aj Other amine oxidases a) Amphetamiul' oxidasl'. Amphetamine oxidase (oxidase of non-terminal amines: triphosphopyridine nucleotide-dependent unspecific oxidase of microsomes) deaminates ~-alkylated aralkylamines which cause competitive and reversible inhibition of MAO in yitro; furthermore, the enzyme acts on several classes of drugs, e.g. barbiturates, aminopyrine, acetanilide (AXELROD 1955, 1959c; BRODIE et al. 1958a; GAUDETTE and BRODIE 1959; ZELLER et al. 1958a). MAO inhibitors with hydrazine structure reduce the activity of amphetamine oxidase slightly in vitro (EBERHOLST et al. 1958; FOUTS and BRODIE 1955, 1956; SERROXE and FUJIMOTO 1960); in vivo short and weak inhibition is followed by an activation (SCHOTT and CLARK 1952; STOCK and 'WESTERMANN 1962a; 'VESTERMANN and STOCK 1962). The alteration of this oxidative system might at least in part be responsible for the enhancement of drug effects (e.g. amphetamine excitation) by MAO inhibitors of the hydrazine type (ARRIGONI-MARTELLI et al. 1958; BIANCHI and DAVID 1960; DANDIYA et al. 1959; GAUDETTE and COATNEY 1961; JACOB and ECHINARD-GARIN 1961; SHEE 1960). Furthermore, the biphasic effect on the enzyme possibly explains why barbiturate narcosis is initially enhanced and subsequently diminished (see below). Some }IAO inhibitors of the non-hydrazine type have no effect on amphetamine oxidase in vivo (STOCK and 'VESTERMANN 1962a; WESTERMANN and STOCK 1962). Other typical inhibitors of amphetamine oxidase, e.g. SKF-525A, lack the pharmacological effects of MAO inhibitors (AXELROD et al. 1954; BucHEL and STURTZ-MoURY 1957; COOPER et al. 1954; FOUTS and BRODIE 1955; LA Du et al. 1955).
(3) Diamine oxidases (EC1.4.3.6) (BAKERandCHAYKIN 1960; BLASCHKO 1952, 1962; BUFFONI and BLASCHKO 1963; BURKARD et al. 1962a, 1963; FOUTS et al. 1957; KAPELLER-ADLER and KRAEL 1931; KAPELLER-ADLER and }!ACFARLAXE 1963; R. KOHN 1931; }IcEwEN and COHEN 1963; PLETSCHER et al. 1960 c ; POHL 1893; PUGH and QUASTEL 1937a, 1937b; SCHIEVELBEIX and WERLE 1957: TABOR et al. 1964; WERXER and SEILER 1963; YAMADA and YASUNOBU 1962a. 1962b, 1962c, 1963; YAMADA et al. 1963, 1964; ZELLER 1940a, 1942, 1951, 1956b; ZELLER et al. 1956a, 1958b, 1963c). In vitro, hydrazine derivatives with and without MAO inhibition reduce the activity of the classical diamine oxidase (DAO) (ARAI 1960; BARSKY et al. 1959; BLASCHKO et al. 1959; BURKARD et al. 1960; DALY et al. 1962; GEY et al. 1960; HEILBRONX 1960; HOLMSTEDT and THAM 1959: KOBAYASHI and OKUYAMA 1962:
614
Monoamine oxidase inhibitors
LINDAHL et al. 1957; SCHULER and MEYER 1955; SHORE and COHN 1960; TABACHNIK 1959; UDENFRIEND et al. 1958b; WATON 1956; ZELLER et al. 1952c, 1955b, 1957a), of spermine oxidase (EC 1.5.3.3) (BARSKY 1958; BLASCHKO 1962; BLASCHKO et al. 1959; TABOR et al. 1954), of mezcaline oxidase (ARAI 1960; BARSKY 1958; DALY et al. 1962; TABACHNIK 1960; ZELLER et al. 1958b) and of methylamine oxidase (WERNER and SEILER 1963). In vivo iproniazid and similar hydrazides, regardless of their effect on MAO, inhibit mezcaline oxidase, methylamine oxidase, and the classical DAO. The activity of the latter may even be Table 3. Comparison of diamine oxidase and monoamine oxidase inhibition in cat kidney by various compounds Diamine oxidase
Inhibitor
lIonoamine oxidase -~--
number of cats
ED.. in I'ffioles!kg
-C~CH-NH2
ED.. in I'ffioles/kg
1
>168
3,6 (1.8-7.0)
nlllllbrr I
of cats
11
CH 2 cis-trans-phenylcyclopropylamine -
--
-CH2-I-CH2-C~CH
-------- - - -
16
450 (360-1450)
CH3 N-benzyl-N-methyl-propargylamine
1ocarboxazid (BECKENBRIDGE and NORMAN 1962; BELFORD and J;'EINLIEB 1nti 1; LACROIX and LEUSEN 1961; LACROIX et al. 1962a). The tyramine-induced activation of myocardial phosphorylase, however, is enhanced by iproniazid (LACROIX et a1. l!)62b). Iproniazid pretreatment of mice docs not markedly affe;ing 11 liO-IOO % ilwrca:;e of JllonO; depend:; considembly on the chemieal :;trncture.
*
Table 4. j)ose oj the 'IIIonoallline oxirlascinhibitors which muse a 5U% increase of brain Ij-liydrox!lif!I1)/allline JIi hours after i . ]). adlllinistmiion of the drugs (R /)',0) in various 'III(w/'lilalian s1lecics
M()lI~(l ,
I PI'()lIillZill
I ~ oc arl)oxa;"icl
il40 (2HO-!l:lO) 17(i
44 (:\7- 4!l) 2H (17-:\7) 14 (H- 20) 74 (:\0- 107)
(Im~H)(i)
(:uilloa pig
Rabbit Fi g ur(l~ III
* lng/ kg.
(i4(:Ifi -\)H)
2HO (lilO-40()) parnlltileH(lK:
,
fidll(:iallimit~ for
l'i\':tlo.\'lhell;"~·(-
Trall~'I-
h.\' dra;"irw
e~ ' pJ'()lllillt'-
7(i (li7 - H4) l:l!l (ti7- 17H)
2:1 (20- 21i) (i (2- \J) 2H (14-4H)
iH
(10-20) 42" (2:"iH- il!iii)
Oil % prohnbility
(PLIp,g
10 pg
37 + 7* .J1 ~1: .J- *
78
~1:
22
I
Isoniazid
93 ± 13 I:J5 :1:: 25 90 ± 19
Significant (Iecroaso of tho amplitude of contmetion as compared to non-treated animals (Ji r, also been sho,,,n to actiya te DCA. pro ba bly beca use the i80nicotinoylhydrazone of pyridoxal5' -phospha te may function as a coenzyme nnder certain experimental conditions (GONNARD 1958, 1962; GONNARD and Borm;;:€; 1961: GONNARD and FENARD 1962; GOXXARD and Xcn':YEX CHI 1958a. 1959a, 1959b: GOXXARD et a!. 1964a, 1964b; PAL:\I 1958). Some hydrazine dcrivatiues inhibiting JIAO arc also weak inhibitors of DCA, e.g. iproniazid (DAVISON 1956; GONNARD and Xcn;YEX CHI 1958b; GREIG et aI. 1959; HAGEN and WELCH 1956; J. LEVY and MICHEL-BER 1960b; SCHULER and WySS 1960); pheniprazine (BRODIE et aI. 1958a; SCHFLER and "TySS 1960); phenelzine (DUBNICK et aI. 1959a, 1962a; TABACHNICK 1959). In vh'o, i.e. after application to intact animals, the following hydrazines cause some DCA inhibition: H ydrazine (HANSSON and CLARK 1962 b) : isoniazid (BURKARD et aI. 1962 c; CAXAL 1961): phenelzine (DrBxlcK et al. 1959a, 1962a): semicarbazide (H.\xssoX and CLARK 1962b; WEISSBACH et aI. 1957b); thioseml:carbazide (WISS and 'VEBER 1958, 1963). The compounds mentioned above are of limited interest as inhibitors of DCA, because their potency in vivo is rather low. Yew and potent DCA inhibdors. Some hydro:cyphenylalkylhydrazines and oxygen isosters have been found to be potent inhibitors of DCA in vitro and in viyo. They are listed in Table 7 and will be the main topic of chapter 2. These substances strongly inhibit DCA in vitro. They are at least 100 times more effective than a-methyldopa. NSD 1034 seems to inhibit the enzyme competitively (HIRSCH et al. 1962 a). With Ro 4-4602 it has been shown that the inhibition is independent from coenzyme addition and irreversible by dialysis (BCRK.\RD et al. 1962b). The inhibition by Ro 4-5127 (trihydroxybenzylhydrazine) seems to be pseudoirreversible and competitive (BURKARD et aI. 1964c). DCA inhibition by Ro 4-4602 increases if the compound is preincubated with the enzyme before addition of the substrate. This is probably due to hydrolysis into the free hydrazine (Ro 4-5127) which seems to be the effective part of
Inhibitors of decarboxylase of aromatic I-amino acids
662
Table 7_ Inhibition of DCA by hydrazine derivatives and isosteTs In vitro 50 % inhibition 1, Z,', 6, 8, g --
Structure
No.
L
3-hydroxybenzyloxyamine
CR
OH
1 X 10-6 2 X 10-8
1
-------
----
1-
~-CH 2-O-NH2
4· bromo-3-hydroxybenzyloxyamine
-----
~
--
-
--
--
81).6
--
20.1
--
---
-
-
---
CH 3 2 X 10- 7
2 X lOt
93.7
7.6
1.8
1
-------- - - - - - - - - - - - - - - - - -
-1----
--
OHOH
LI
4-4602
12.5
---
4xl0 6
NH-NH2 DL-oc-hydrazino-oc-methyl3,4-dihydroxyphenylpropionic acid
HO-#'
2.2
-------
------
\-CH 2-6-cOOH
"'=/
Ro
-
2 X 103
4x 10-9
,,~/
HOJ
--
---------
-----
---
OH Br-#'
34_3
7 X 104 i
4 X 10-6
Nl-(3-hydroxybenzyl)N1-methylhydrazine
NSD 485
"it~ 'mg!kgl~m!k~
mg/kg
3
~-CH2-~-NH2 "'=-_/
#'--
NSD 1055
ED"
bition :
I
O"'-CH2-O-NH 2
NSD 1034
ViV0 3,CI,7.10
-- % inhi- 1
relative molar concentration i molarity'
OH NSD 1024
In
~-CH2-NH-NH-OC-CH-CH OH 4
~~/
1
X 10-6
2
**,
i
----
4 X 104 **
!H.5
-
2.1
NH2 NI-DL-seryl-N2-(2,3,4-trihydroxybenzyl)hydrazine
10.0 I I
---
-
---
---
----
OHOH
LI
Ro 4-5127
HO-
