step during biosynthesis of the 3-hydroxymandelonitrile glycosides, at least ... lyase that is usually present in cyanogenic plants effectively enhances, and ...
15. The biology of the cyanogenic glycosides: new developments PROCEEDINGS PHYTOCHEMICAL
OF THE
SOCIETY
OF EUROPE
Nitrogen Metabolism of Plants Edited by
K. MENGEL Institut für Pjlanzenernährung Justus-Liebig-Universität, Giessen, Germany and
D.J. PILBEAM Department 0/ Pure and Applied Biology The University 0/ Leeds, UK
ADOLF NAHRSTEDT Institut für Pharmazeutische Biologie und Phytochemie der Westf. Wilhelms-Universität, Hittorfstr. 56, D-4400 Münster, FRG
Introduction Several reviews of the cyanogenic compounds have been presented during the past years, particularly of the cyanogenic glycosides. Among them are reviews on their occurrence (Hegnauer 1986; Nahrstedt 1987), their function (Nahrstedt 1985; Hegnauer 1986), toxicity (Poulton 1983), biosynthesis (Conn 1988), and their occurrence and physiology in insects (Nahrstedt 1988). A comprehensive volume, Cyanide compounds in biology, was published in 1988 by the Ciba Foundation. All these activities indicate that the cyanogenic compounds are an important subject for biological research. This chapter aims to summarize recent progress in this area.
Metabolism The term cyanogenesis is used for the production of hydrogen cyanide (H C N) under physiological conditions, excluding the so-called pseudocyanogenic glycosides (Seigier 1975). Figure 15.1 shows cyanogenic pathways that have been more or less established for higher plants. In small amounts (about I ",M and less) cyanide is produced: (I) during the formation of ethylene from l-amino-cyclopropane-lcarboxylic acid (ACC) (Yang and Hoffmann 1984); (2) by the action of horseradish (Porter and Bright 1987); and
peroxidase (H R P) on t-arnino
acids
(3) from glyoxylate and hydroxylamine via an enzymatically catalysed reaction (Hucklesby et al. 1982).
CLARENDONPRESS·OXFORD 1992
The last pathway mayaiso be used by some micro-algae which also produce cyanide from n-amino acids (Vennesland et al. 1981) (not shown in Fig.15.1). Another pathway is able to produce large amounts of cyanide, up to approximately 1 per cent in plants (Poulton 1983) and 0.2 per cent in insects (Davis and Nahrstedt 1985). It too originates from amino acids such as valine, isoleucine, leucine, phenylalanine, and tyrosine as weil as the
250 Adolj Nahrstedt
Biology
NH2 R1 -
I
CH-CH-COOH
I
ACC
R2 N-OH-
ACC-oxidase
!
amino acids
! ! !
HCN
Aldoximes
~El ADP ~
Nitriles
r O-R I 3
OH
I R -C-CN 1 I
R2 Hydroxynitriles (cyanohydrins)
L-amino acid viaenamine
~
---+ R
Glyoxylate + hydroxylamine
Cyanogenic glycosides
-C-CN 1
I
R2
Esters 01 cyanohydrins Cyanogenic lipids
Fig.15.1. Hydrogen cyanide (HCN) in Iiving organisms generally arises from the hydrolysis of cyanogenic glycosides biogenetically derived from amino acids (left column); in minor quantities, HCN is formed du ring the biosynthesis of ethylene from I-amino-cyclopropane-I-carboxylic acid (ACC), by the action of horseradish peroxidase (H R P) on amino acids, and by enzymatic oxidation of glyoxylate and hydroxyl amine in the presence of manganese ions.
non-proteinogenic amino acids cyclopentenylglycine and, probably, nicotinic acid (Nahrstedt et al. I982a) as precursors. Products accumulated are cyanohydrins, which occur freely in so me plants and arthropods, or which (usually) are stabilized either by glycosylation to form the cyanogenic glycosides, or (more seldom) by esterification as in mandelonitrile benzoate found in some miIIipedes (Duffey 1981); esterification with fatty acids leads to the special group of the cyanolipids that occur exclusively in the seeds of many sapindaceous plants (Mikolajczak 1977). This pathway has been thoroughly investigated by Professor E. E. Conn using mainly his laboratory plant Sorghum bicolor Moench (Poaceae), and was found to be bound to the microsomal fraction as a highly channeled system (Conn 1983). Halkier and Moller [1989] have recently purified the dhurrin-synthesizing system from etiolated seedlings of S. bicolor on a Sephacryl S- I000 column and succeeded in a tenfold increase of specific activity. Further purification using different methods was not successful as the activity was reduced drastically, probably by dissociation of the essential cornponents from the enzyme system. (See note added in proof, p. 269.)
0/ cyanogenic g/ycosides
251
An open quest ion was the identity of the precursor of mandelonitrile glycosides bearing am-hydroxyl on their aromatic ring such as holocalin (Fig, 15.10) or xeranthin (Fig. 15.11). Schütte (1973) has argued that m-tyrosine might be the precursor, while others suggest tyrosine (van Valen 1978) or phenylalanine (Nahrstedt 1976). The fruits of Xeranthemum cylindraceum contain both m-hydroxylated and non-hydroxylated mandelonitrile glycosides (Schwind et al. 1990) (see below). Feeding the ripening inflorescences with 0.5 mM solutions of L-tyr, D,L-meta-tyr, and t-phe affected the concentration of the cyanogenic glucoside zierin (2-ß-Dglucopyranosyloxy-2S-2-(3-hydroxy) phenyl acetonitriIe) at a rate of zero, 46 per cent decrease and 28 per cent increase respectively. L-[U-'4C]phe was incorporated into zierin at a rate of approximately 0.5 per cent, whereas L-[U-'4C]tyr was not. When glyphosate, an inhibitor of enolpyruvylshikimate phosphate synthase (Amrhein 1986) was fed (0.6 mM), a decrease of approximately 50 per cent zierin was observed that was clearly antagonized by 2 mM t-phe but not by t-tyr or D,L-m-tyr (Fig. 15.2). These results show that phenylalanine is the precursor and that hydroxylation occurs at a later step during biosynthesis of the 3-hydroxymandelonitrile glycosides, at least in X. cylindraceum (Schwind 1990). The cyanogenic glycosides are readily hydrolysed by more or less specific
0.7 0.6
E
0.5
'0
~
0.4
'0
C ~ 0.3 o Ci
~ 0.2 0.1
glyphosate 0.6mM
2mM phe
2mM tyr
2mM m-tyr
Fig. 15.2. The inflorescences of the asteracean plant Xeranthemum xylindraceum show a decreased accumulation of the meta-hydroxylated aromatic cyanogenic glucoside zierin at an amount of c.0.2 j.tmol g -'d.m. when incubated in 0.6 mM glyphosate". This effect is antagonized by phenylalanine (phe) but not by tyrosine (tyr) and meta-tyrosine (m-tyr) indicating that the non-hydroxylated amino acid phenylalanine is the precursor.
252 Adolf Nahrstedt
I
&"
NC-CH2-CH-COOH /
ß-cyanoalanine
ß-CAS
/
