Determination of Aldehydic Lipid Peroxidation Products

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[42] D e t e r m i n a t i o n o f A l d e h y d i c L i p i d P e r o x i d a t i o n Products: Malonaldehyde and 4-Hydroxynonenal By HERMANN ESTERaAUERand KEVlN H. CHEESEMAN Introduction

Aldehydes are always produced when lipid hydroperoxides break down in biological systems,X-3 and it is of interest to identify and measure these compounds both as an index of the extent of lipid peroxidation and as an aid to elucidate the role of aldehydes as causative agents in certain pathological conditions. 2-4 We deal here with current analytical methods used for the qualitative and quantitative determination of aldehydes in biological systems, and we pay particular attention to 4-hydroxynonenal (HNE) and malondialdehyde (MDA). 4-Hydroxynonenal is produced as a major product of the peroxidative decomposition of to6 polyunsaturated fatty acids (PUFA) and possesses cytotoxic, hepatotoxic, mutagenic, and genoroxic properties. 2,4,5 Increased levels of HNE were found in plasma and various organs under conditions of oxidative stress (for review, see Refs. 6 and 7). In addition to HNE, lipid peroxidation generates many other aldehydes (alkanals, 2alkenals, 2,4-alkadienals, protein- and phospholipid-bound aldehydes) which may also be of toxicological significance.2,4.6,8 Malondialdehyde is in many instances the most abundant individual aldehyde resulting from lipid peroxidation, and its determination by thiobarbituric acid (TBA) is one of the most common assays in lipid peroxidation studies. In vitro MDA can alter proteins, DNA, RNA, and many other biomolecules.8 Recently, it has been demonstrated with t H. Esterbauer, in "Free Radicals, Lipid Peroxidation and Cancer" (D. C. H. McBrien and T. F. Slater, eds.), p. 101. Academic Press, London, 1982. 2 H. Esterbauer, in "Free Radicals in Liver Injury" (G. Poli, K. H. Cheeseman, M. U. Dianzani, and T. F. Slater, eds.), p. 29. IRL Press, Oxford, 1985. 3 W. Grosch, in "Autoxidation of Unsaturated Lipids" (H. W. S. Chan, ed.), p. 95. Academic Press, London, New York, 1987. 4 M. Comporti, Lab. Invest. 53, 599 (1985). 5 H. Esterbauer, H. Zollner, and R. J. Schaur, IS1Atlas Sci. Biochem. 1, 311 (1988). 6 H. Esterbauer, H. Zollner, and R. J. Schaur, in "Membrane Lipid Oxidation" (C. VigoPelfrey, ed.), Voi. 1, p. 239. CRC Press, Boca Raton, Florida, 1990. 7 H. Esterbauer and H. Zollner, Free Radical Biol. Med. 7, 197 (1989). 8 E. Schauenstein, H. Esterbauer, and H. Zollner, "Aldehydes in Biological Systems: Their Natural Occurrence and Biological Activities." Pion Press, London, 1977.

METHODS IN ENZYMOLOGY, VOL. 186

Copyright © 1990by Academic Press, Inc. All rights of reproduction in any form reserved.

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ASSAY AND REPAIR OF BIOLOGICAL DAMAGE

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monoclonal antibodies that malonaldehyde-altered protein occurs in atheroma of hyperlipidemic rabbits. 9 Standard Determination of Malonaldehyde with Thiobarbituric Acid In the TBA test reaction one molecule of MDA reacts with two molecules of TBA with the production of a pink pigment having an absorption maximum at 532-535 nm. The reaction should be performed at pH 2-3 at 90-100 ° for 10-15 min. Typically, the tissue sample (e.g., a liver microsomal suspension) is mixed with 2 volumes of cold 10% (w/v) trichloroacetic acid (TCA) to precipitate protein. The precipitate is pelleted by centfifugation, and an aliquot of the supernatant is reacted with an equal volume of 0.67% (w/v) TBA in a boiling water bath for 10 min. After cooling the absorbance is read at 532 nm and the concentration of MDA calculated based on an e value of 153,000. This value is the average of several slightly differing figures reported in the literature. 1° The crystalline M D A - T B A adduct in water shows an absorption maximum at 532 nm (e 159,200)) 1 The TBA reagent should be prepared as an aqueous solution and requires heating to disgolve the TBA solid. A standard curve can be prepared using malonaldehyde bisdimethyl- or bisdiethylacetal as the source of MDA. A 10 mM stock solution is prepared by adding 1 mmol of the acetal to 100 ml of 1% (v/v) sulfuric acid and leaving the mixture at room temperature for 2 hr to achieve complete hydrolysis. For the preparation of the standard curve the MDA stock is further diluted to about 110/~M and then reacted with TBA as above. The concentration of the MDA solution can be checked by measuring the UV spectrum. In 1% H 2 S O 4 the absorption maximum is at 245 nm (e 13,700). 12 In alkaline solution (10 mM NaaPO4) the maximum is at 267 nm (e 3 1 , 5 0 0 ) . 13 Many factors influence the results obtained with the TBA test, as discussed previously in this series. 12,14Briefly, conditions and procedures to be avoided if free MDA is to be measured are as follows: preparation of the TBA reagent in strong acid solutions, high concentrations of metals, such as iron, high concentrations of sugars, such as sucrose, and use of the whole tissue sample in the assay. To ensure that no lipid oxidation 9 M. E. Haberland, D. Fong, and L. Cheng, Science 241, 215 (1988). 10 H. Esterbauer, K. H. Cheeseman, M. U. Dianzani, G. Poli, and T. F. Slater, Biochem. J. 2118, 129 (1982). 11 V. Nair and G. A. Turner, Lipids 19, 804 (1984). t2 H. Esterbauer, J. Lang, S. Zadravec, and T. F. Slater, this series, Vol. 105, p. 319. ~s T. W. Kwon and B. M. Watts, J. Food Sci. 28, 627 (1963). 14 R. P. Bird and H. H. Draper, this series, Vol. 105, p. 299.

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TABLE I TBA REACTION WITH DIFFERENT COMPOUNDSa Compound Malonaldehyde Alkanals 2-Alkenals

2,4-Alkadienals

4-Hydroxyalkenals

Amino acids preincubated with 0.9 m M Fe Sugars preincubated with 0.9 m M Fe Monohydroperoxides from arachidonic acid

Conditions b A A,B,C A without A with Fe B C A without A with Fe B C A without A with Fe C D D E without E with Fe

Fe

Fe

Fe

Fe

e value 153,000 0 14-66 30-90 100-200 130-160 48-160 184-280 1100-2600 4500 12-119 38-124 320 50-620 90-2700 3200-8100 12400-34100

a Absorbance at 530-535 nm is expressed per mole of compound. b A, 5% TCA, 10 rain, 100°, in the presence or absence of 3 /zM FeSO4 (Ref. 10); B, water, 60 rain, 95° (Ref. 15); C, 1 N glacial acetic acid, 120 min, 100° (Ref. 16); D, glacial acetic acid, 30 min, 100° (Ref. 17); and E, 10% TCA, 30 min, 100°, in the presence or absence of 2.5 m M FeSO4 (Ref. 18).

occurs during the assay, butylated hydroxytoluene (0.01 vol % of a 2% BHT solution in ethanol) and EDTA (1 mM final concentration) can be added to the sample prior to TCA precipitation. Determinations from TBA Test Measurements It is well documented that the TBA test is not specific for MDA. A great variety of substances other than MDA under appropriate conditions also form pink TBA complexes; moreover, MDA or MDA-like substances can arise during the assay from acid-catalyzed or thermal decomposition of precursors (other aldehydes, MDA bound to proteins, oxidized lipids, amino acids, sialic acid) (Table I). 1s-18It would seem, however, that using 15 R. Marcuse and L. Johansson, J. Am. Oil Chem. Soc. 50, 387 (1973). ~6G. Witz, N. J. Lawrie, A. Zaccaria, H. E. Ferran, Jr., and B. D. Goldstein, J. Free Radicals Biol. Med. 2, 33 (1986). 17 j. M. C. Gutteridge, FEBS Leu. 128, 343 (1981). 18j. Terao and S. Matsushita, Lipids 16, 98 (1981).

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the protocol described above for peroxidized tissue samples, e.g., microsomes, there is little artifactual production of MDA or interference with other TBA-positive substances. This is not merely conjecture but has been demonstrated in practice. 10,12,19,20In liver microsomal suspensions in which lipid peroxidation has been stimulated by ADP-iron, CC14, o r ascorbate-iron, the direct determination of free MDA by the HPLC method described below gave precisely the same value as did the TBA test, indicating that in those systems the standard TBA test measures only free MDA and not MDA-like substances. Also, in oxidized low density lipoprotein 80% of the TBA-reactive substances (TBARS) were free MDA. 21 This does not contradict the low specificity of the TBA test but can be explained as follows. First, in the standard procedure most of the potential MDA precursors, e.g., protein-MDA complexes or oxidized lipids, are removed by TCA precipitation in the cold prior to the actual assay. Second, other TBA-positive compounds that could be present in the deproteinized supernatant, such as aldehydes, amino acids, sugars, and fatty acid hydroperoxides, give only a very weak color in the standard TBA assay. On a molar basis, the absorption at 530-535 nm produced by such compounds is several orders of magnitude lower than the absorption produced by MDA (Table I). The TBA-positive compounds would therefore have to be present in the sample in extremely high concentrations to interfere significantly with the standard determination of MDA. Suspensions of peroxidized rat liver microsomes (ADP-iron, 30 min) contain, e.g., 58 nmol free MDA and 95 nmol of the other aldehydes listed in Table I. 1°A rough estimate shows that 99.7% of the absorbance at 535 nm which would be found in the standard TBA assay results from MDA (e 153,000), and only 0.3% or less is due to all other aldehydes (assumed average e 300). The situation may, however, be completely different if the standard reaction conditions are significantly altered, e.g., heating in the presence of the complete tissue fraction, prolonged reaction times, the use of other acids, and supplementation of the reaction mixture with iron. There can be no doubt that such modified TBA tests are much less specific, and it seems appropriate to refer in such cases to the measurement of TBApositive substances, TBARS, or simply the TBA value rather than specifying MDA. Frequently used modifications of the TBA test employ the whole acidt9 H. Esterbauer and T. F. Slater, 1RCS Med. Sci. 9, 749 (1981) 2o j. Lang, P. Heckenast, H. Esterbauer, and T. F. Slater, in "Oxygen Radicals in Chemistry and Biology" (W. Bors, M. Saran, and D. Tait, eds.), p. 351. de Gruyter, Berlin, New York, 1984. 21 H. Esterbauer, G. Jiirgens, O. Quehenberger, and E. KoUer, J. Lipid Res. 28, 495 (1987).

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ified sample. Typically22.23 1 volume of tissue sample, e.g., 10% (w/v) homogenate, is mixed with 6 volumes of 1% phosphoric acid and 2 volumes 0.6% aqueous TBA solution and heated for 45-60 min in a boiling water bath. After cooling, 8 volumes of n-butanol are added and mixed vigorously. The butanol phase which contains the colored TBA reaction products is separated by centrifugation and its absorbance measured at 532 nm. The method with phosphoric acid is basically developed to measure MDA and/or TBARS bound to proteins; it seems that under the assay conditions the binding is, at least in part, reversible. With this method significantly elevated levels of TBARS were found in various fresh tissues of rats fed a vitamin E-deficient diet, e.g., 570 nmol/g liver of vitamin E-deficient rats compared to 76 nmol in controls. 23 In another procedure, 0.2 ml of 35% TCA and 1 ml of 0.5% TBA are added to 1 ml of sample, and the mixture is heated at 60° for 90 min, after which dispersed lipids are extracted with 3 ml CH2C12 and the clear aqueous phase measured at 532 nm. 24--26With this method 0.22-0.40 nmol TBARS/ml were found in control plasma. A u s t 27 reported a method where 1 ml of sample is mixed with 2 ml of a TCA-TBA-HCI reagent [15% (w/v) TCA, 0.375% (w/v) TBA, 0.25 N HCI], the complete mixture is heated on a boiling water bath for 15 min, and after centrifugation, the absorbance is measured at 535 nm. Numerous other variations have been introduced, but heating the complete assay sample with TBA in acidic solution is common to all. These methods can be subject to various sources of errors if only the absorption at 530-535 nm is measured. For example, yellow compounds with maxima at 450-490 nm and significant absorption at 530-535 nm are often formed and would lead to an overestimation of TBARS. It is therefore strongly recommended that the spectrum in the 430-600 nm range be recorded to prove the existence of the 532 nm maximum typical for the MDA-TBA complex and to correct for possible background absorption by interpolation. The exact amount of the MDA-TBA complex in the reaction mixture can also be determined by HPLC. 14,28 The neutralized reaction mixture or a butanol extract is separated on an ODS column with methanol-water (85 : 15) and detected at 530-535 nm. In a TCA extract of fresh pork liver 30 nmol TBARS was found by the spectrometric method, 36% of which was MDA as determined by HPLC. 28 22 M. Uchiyama and M. Mihara, Anal. Biochem. 86, 271 (1978). 23 M. Mihara, M. Uchiyama, and K. Fukuzawa, Biochem. Med. 23, 302 (1980). 24 K.-L. Fong, P. B. McCay, and J. L. Poyer, J. Biol. Chem. 248, 7792 (1973). 25 D. M. Lee, Biochem. Biophys. Res. Commun. 95, 1663 (1980). 26 F. Bernheim, M. L. Bernheim, and K. M. Wilber, J. Biol. Chem. 174, 257 (1948). 27 S. D. Aust, this series, Vol. 52, p. 302. 28 R. P. Bird, S. S. O. Hung, M. Hadley, and H. H. Draper, Anal. Biochem. 128, 240 (1983).

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Other modifications of the TBA assay were designed to analyze lipid hydroperoxides. As can be seen from Table I, including iron in the TBA reaction mixture significantly increases the yield of pink products from various substances. A particularly high absorption is obtained with monohydroperoxides from arachidonic acid, and this is the basis of a TBA test for microdetermination of lipid peroxides. The recommended procedure 18,29is briefly as follows. One milliliter of methanol solution containing the oxidized lipid is mixed with 2 ml of 20% TCA containing 20/~mol ferrous sulfate and 1 ml 0.67% TBA. The mixture is heated in a boiling water bath for 30 min, and after cooling 2 ml CHCI3 is added (to extract turbid material). The mixture is centrifuged, and the absorbance of the clear supernatant is measured. In the case of methyl arachidonate monohydroperoxide isomers (MeHPETE), a linear relationship is found between the absorption at 532 nm and the concentration of MeHPETE. Although each isomer gave a different response, the lowest amount of MDA was found from the 8-OOH isomer (0.081 mol MDA/mol MeHPETE); the highest amount gave the 5-OOH and 15-OOH isomers (0.22 mol MDA/mol MeHPETE). To prevent additional lipid oxidation during the color development, 0.01 vol% of 2% BHT in ethanol can be added to the TBA reagent just prior to use. Another TBA test to measure preferentially lipid peroxides was developed by Yagi for the analysis of serum or blood. 3°,31 Here, the proteins and lipids are first precipitated with phosphotungstic acid. The sediment (equivalent to 20/.d serum) is then suspended in 4 ml water, 0.5 ml glacial acetic acid, and 0.5 ml 0.33% aqueous TBA solution. The mixture is heated for 60 min at 95 ° and after cooling extracted with 5 ml n-butanol. The concentration of the T B A - M D A complex in the butanol extract is determined fluorimetrically at 553 nm with excitation at 515 nm. The amount of TBARS (assumed to be lipid peroxides) found by this test in normal subjects was in the range of 1.8-3.9 nmol/ml serum. This is about 10 times the amount found with another modified TBA test 25 and clearly shows that different procedures yield very different results. Direct comparison of data reported by different investigators is often not possible. In conclusion, what is measured by the TBA assay is strongly influenced by the reaction conditions. In assays where the whole sample is heated in an acidic TBA solution, the resulting absorption at 530-535 nm (or fluorescence at 553 nm) can come from all preexisting MDA, proteinbound MDA, and lipid peroxides, as well as any other substances that give rise to MDA or TBARS in the hot acid. Tests using only the TCA29 T. Asakawa and S. Matsushita, Lipids 15, 137 (1980). 3o K. Yagi, this series, Vol. 105, p. 328. 31 K. Yagi, in "Lipid Peroxides in Biology and Medicine" (K. Yagi, ed.), p. 223. Academic Press, London, New York, 1982.

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soluble fraction of the sample are more specific for free MDA. Here again, however, interference can be caused by other TCA-soluble compounds, in particular, if free MDA is low, such as in fresh tissue samples. In any case, additional analyses should be performed to elucidate the nature and the source of the pink color. Such analyses include the demonstration of the TBA-MDA complex by HPLC 14,28 and the direct detection of free MDA as described below. Direct Determination of Malondialdehyde by HPLC An HPLC method for the determination of free MDA in biological samples has been described at length previously in this series, 12 and we only outline it here. The principle of the method is that an aqueous sample containing MDA at pH 6.5-8.0 is separated by HPLC using an aminophase column with acetonitrile-30 mM Tris buffer, pH 7.4 (1 : 9, v/v), as the mobile phase. The effluent is monitored at 267 nm, the absorption maximum of the enolate anion form of free MDA (e 31,500 at this pH and wavelength). The system is calibrated and the sample MDA peak identified by comparison with a solution of MDA. Using an injection volume of 20/zl, the smallest concentration of MDA in the original solution that can be quantified this way is 0.25/zM. To protect the column it is best to deproteinize the sample, and we find that addition of an equal volume of acetonitrile followed by centrifugation is satisfactory. A typical estimation would be as follows. An aliquot of sample (e.g., liver microsomal suspension) is mixed with an equal volume of acetonitrile and the protein precipitate pelleted at 3000 g for 10 min. A sample (20/zl) of the supernatant is injected into the HPLC and separated on an S-5 Spherisorb-NH2 column (Phase Separations Ltd.) at I ml/min. With this method it w a s s h o w n 1°A2,19,2° that what is measured with the standard TBA test (use of TCA supernatant) in peroxidized microsomes or mitochondria is exclusively free MDA. In addition, this method was u s e d 6 to compare the amount of free MDA and TBARS formed during oxidation of various PUFA (Fig. 1). At the stage when the PUFA were completely oxidized, the yields of free MDA on a molar basis were 0.5% for linoleic acid, 4.5% for linolenic acid, 4.9% for y-linolenic acid, 4.7% for arachidonic acid, and 7.6% for docosahexaenoic acid. The corresponding yields of TBARS were slightly higher, namely, 0.55, 4.9, 5.1, 6. l, and 8.6%. The very low yield of MDA from linoleic acid agrees with the proposal 32 that MDA is only formed from PUFAs with three or more double bonds. We have also used this method in a system completely unrelated to 32 W. A. Pryor, J. P. Stanley, and E. Blair,

Lipids 11, 370 (1976).

414


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incubation time, hours FiG. 1. Free malonaldehyde (MDA) and MDA-like substances (TBARS) formed during autoxidation of arachidonic acid (20 : 4) and docosahexaenoic acid (22 : 6). The fatty acids (0.1 mg/ml) were incubated in Tris buffer, pH 7.4, with ascorbate-iron (10 raM-0.4 raM) at 37°. Consumption of the fatty acids was measured by GC, free MDA by HPLC, and TBARS by the standard TBA assay as described in the text.

lipid peroxidation: the production of MDA from cleoxyribose degradation by OH. attack. 33 Further, a modification of this method has been reported in which the proportion of acetonitrile is increased to 80% in order to achieve better separation from interfering substances when measuring plasma. 34 Two other HPLC methods for measuring free MDA directly have been published. One method 35 uses an ODS column with acetonitrile-water (14 : 86), 50 mM myristyltrimethylammonium bromide, 1 mM phosphate buffer, pH 6.8, as mobile phase at 1 ml/min, with detection at 267 nm. The basis of the separation is ion-pairing chromatography. A reasonably good equivalence between this direct determination and the TBA test was found when measuring peroxidized microsomes. The other method 36 uses a size-exclusion column (Spherisorb TSK G 1000 PW, Phase Separation 33 K. H. Cheeseman, A. Beavis, and H. Esterbauer, Biochem. J. 252, 649 (1988). 34 C. Largilliere and S. B. Melancon, Anal. Biochem. 170, 123 (1988). 35 A. W. Bull and J. Marnett, Anal. Biochem. 149, 284 (1985). 36 A. S. Csallany, M. D. Guan, J. D. Manwaring, and P. B. Addis, Anal. Biochem. 142, 277 (1984).

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Ltd.) with 0.1 M phosphate buffer, pH 8.0, and detection at 267 nm. A poor equivalence was found by this method when measuring MDA in beef or pork muscle or rat liver, e.g., 43 nmol (TBA) versus 11 nmol (HPLC) per 1 g of rat liver. Several other chromatographic methods for the detection of MDA were reported. In one procedure, 37 developed for vegetable oil, the sampie-(0.1 g) is reacted with dansylhydrazine in hydrochloric acid containing FeC13. The formed dansylpyrazole is separated by HPLC with fluorimetric detection. In another method, 38 developed for investigation of the formation of MDA from lipid peroxidation products, the oxidized lipid (20-25 mg) is treated for 18 hr at ambient temperature with 1 ml of 5% anhydrous HC1 in methanol and I ml trimethyl orthoformate. The amount of MDA-tetramethylacetal formed is determined by gas chromatography. Both methods certainly do not measure free MDA but rather the amount of MDA that can be formed from precursors by acid-catalyzed decomposition. Although in the systems we have studied the TBA test is demonstrated as measuring free M D A , 12'19'20'33 this will not be true in all systems. If the investigator is concerned in knowing whether MDA is the only TBAreactive product in the test system, then the measurement should be validated with a direct measurement of free MDA by HPLC. If the two determinations are equivalent, the investigator can use the more convenient TBA test. Determination of Aldehydes via Dinitrophenylhydrazone Formation

The methods most frequently used for determination of aldehydes in biological tissues are based on treatment of the sample with 2,4-dinitrophenylhydrazine. Aldehydes react with dinitrophenylhydrazine to form the corresponding dinitrophenylhydrazone (DNPH) derivatives. In contrast to most free aldehydes the hydrazone derivatives are stable and not volatile, greatly facilitating the subsequent workup procedure. Moreover the D N P H derivatives have a strong yellow color (hmax 360-380 nm, e 25000-28000 M -1 c m - l ) which is of great help in detecting the compounds on TLC plates or by HPLC. An outline of the procedure we routinely use is as follows. The sample is mixed with dinitrophenylhydrazine reagent and allowed to react. The DNPH derivatives are extracted into an organic solvent, concentrated, and preseparated by TLC to yield DNPH classes of different polarity (here termed zones I, II, and III from their positions on the TLC plate). 37T. Hirayama, N. Yamada, M. Nohara, and S. Fukui, J. Sci. FoodAgric. 35, 338 (1984). E. N. Frankei and W. E. Neff, Biochim. Biophys. Acta 754, 264 (1983).

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The individual classes are recovered and separated by HPLC for identification of their constituent individual aldehydes. The importance of the preliminary separation by TLC should be stressed as it performs several important functions. First, it enables the removal of excess dinitrophenylhydrazine reagent. Second, it enables certain contaminating carbonyls to be eliminated; the DNPH forms of formaldehyde, acetone, and acetaldehyde are always found at this stage even in the reagent blank. Apparently these carbonyls are always present in laboratory air and standard solutions. Finally, analysis of the hydrazones in each zone (I, II, and III) greatly facilitates clear separation of the individual compounds and provides more confident identification of the peaks in the HPLC chromatogram. For example, zone III can only contain alkanals, 2-alkenals, and 2,4-alkadienals and cannot contain the more polar 4-hydroxyalkenals that are restricted to zone I. It is possible to apply all of the DNPH derivatives directly in HPLC without preliminary TLC, e.g., by using gradient programs; however, the resulting chromatograms are complicated, and it is extremely difficult to make definite peak identifications. A typical determination of aldehydes produced during lipid peroxidation in liver microsomes, 1°,39hepatocytes, 39 or low density lipoproteins, 21 as examples for other biological samples, is as follows. To 1 ml of the sample, e.g., microsomes at I mg protein/ml, add 0.1 ml of 1% EDTA, 10/zl of 2% BHT, and 0.5 ml freshly prepared DNPH reagent (2,4-dinitrophenylhydrazine recrystallized from butanol dissolved in 1 N HCI at a concentration of 0.35 mg/ml). Mix vigorously and keep in the dark for 2 hr at ambient temperature and then for 1 hr at 4°. The reaction mixture is extracted with CH2C12 (2 times 5 ml each); phase separation can be achieved by centrifugation. The pooled extract is left in a freezer for at least 2 hr and then rapidly filtered through a folded filter to remove ice crystals. The extract is brought to dryness on a rotary evaporator (-