Enzymology and subcellular localization of aldehyde oxidation in rat liver. Oxidation of 3,4-dihydroxyphenylacetaldehyde derived from dopamine to 3,4-dihydroxyphenylacetic acid

Presence of cytosolic aldehyde dehydrogenase isozymes in adult and fetal rat liver

A colony of Wistar-strain rats bred at Purdue University was composed of animals with two different isozyme patterns of liver cytosolic aldehyde dehydrogenase (EC 1.2.1.3, ALDH) as determined by isoelectric focusing. One cytosolic isozyme pattern had a major activity band with a pI = 5.8 and a minor activity band at pI = 6.2. The other pattern contained three major isozymes with pI values of 5.3, 5.4 and 5.6 along with the pI 6.2 isozyme and a trace of the 5.8 one. The 5.8 and 6.2 isozymes were recognized by antibodies produced against horse and beef liver cytosolic ALDH, whereas the set of three (5.3-5.6) were not. The cytosolic isozymes were inhibited by low levels of disulfiram and had Km values for acetaldehyde in the 100 microM range, properties typical for cytosolic ALDHs. All animals contained the same isozymes of liver mitochondrial ALDH. These were a major activity with a pI = 5.2 and minor activities associated with isozymes of pI = 6.4 and 6.6. These isozymes were recognized by antibodies produced against pure horse and beef liver mitochondrial ALDHs. Both cytosolic and mitochondrial ALDHs were found in fetal liver as early as day 15 of gestation. The total activity for mitochondrial ALDH increased between day 15 and day 21 whereas that for cytosolic ALDHs remained relatively constant during development. It appeared that both cytosolic and mitochondrial ALDH were present by at least the third trimester and could afford the fetus some protection against the toxic action of endogenous or exogenous aldehydes.

Purification and characterization of beef and pig liver aldehyde dehydrogenases

Beef liver cytosolic, mitochondrial, and pig liver mitochondrial aldehyde dehydrogenases (ALDH) had been purified to homogeneity. The two mitochondrial enzymes as with other mammalian mitochondrial enzymes had properties very similar to that of the corresponding human enzyme. These include immunological as well as basic kinetic properties such as low Km for aldehyde, activation by Mg2+ ions, and lack of inhibition by disulfiram. A major difference between these two enzymes and the human mitochondrial enzyme was that they contained an N-terminal-blocked amino acid. Cytosolic ALDHs from human and horse liver have been shown to possess an N-acetyl serine as the N-terminal residue; beef cytosolic ALDH was also found to be blocked. Tissue preparations and subcellular fractions from beef or pig liver could be used to study acetaldehyde oxidation. This is the subject of the accompanying paper (Cao Q-N, Tu G-C, Weiner H, Alcohol Clin Exp Res 12:xxx-xxx, 1988).

Mitochondria as the primary site of acetaldehyde metabolism in beef and pig liver slices

Aldehyde dehydrogenase (ALDH) is the major enzyme involved in the oxidation of acetaldehyde. It has been shown that the liver enzyme is located in both cytosol and mitochondria. It has not been established where the subcellular oxidation of acetaldehyde occurs in species other than rat. Using slices isolated from beef and pig livers and selectively inhibiting the mitochondria enzyme with cyanamide or the cytosolic enzyme with disulfiram, it was possible to address this question. It was found that with both beef and pig liver slices 60% of the oxidation was catalyzed by the mitochondrial ALDH and 20% by the higher Km cytosolic enzyme. The remainder of the metabolism was the result of non-ALDH involvement. Furthermore, any decrease in the level of the low Km mitochondrial aldehyde dehydrogenase activity resulted in a decreased rate of acetaldehyde oxidation showing that its activity governed the rate of acetaldehyde oxidation. These were the same conclusions previously reached using rat liver tissue slices. Thus, it appears that for all mammalian tissue, mitochondria is the primary location of acetaldehyde oxidation.

Human liver aldehyde dehydrogenase: subcellular distribution in alcoholics and nonalcoholics

Activity assay and isoelectric focusing analysis of human biopsy and autopsy liver specimens showed the existence of two major aldehyde dehydrogenases (ALDH I, ALDH II). Subcellular distribution of these isozymes was determined in autopsy livers from alcoholics and nonalcoholics. Nearly 70% of the total ALDH activity was recovered in the cytosol which contained both the major isozymes. Densitometric evaluation of isozyme bands showed that about 65% of the cytosolic enzyme activity was due to ALDH II and the rest due to ALDH I isozyme. Only about 5% of the total ALDH activity was found in the mitochondrial fraction (70% ALDH I and 30% ALDH II). Significantly reduced total and specific ALDH activities were noted in all the subcellular fractions from cirrhotic liver specimens. Apparently, ALDH I isozyme from cytosol and mitochondria is primarily responsible for the oxidation of small amounts of acetaldehyde normally found in the blood of nonalcoholics after drinking moderate amounts of alcohol. However, in alcoholics who exhibit higher blood acetaldehyde concentrations after drinking alcohol, ALDH II isozyme may be of greater physiological significance.

Effects of induction of rat liver cytosolic aldehyde dehydrogenase on the oxidation of biogenic aldehydes

Phenobarbital and tetrachlorodibenzo-p-dioxin (TCDD) induce two different forms of aldehyde dehydrogenase (EC 1.2.1.3, ALDH), designated phi and tau respectively, in the rat liver cytosol. The physiological substrates for these enzymes are as yet unknown. In this study we investigated whether the induction of these enzymes forms affected the metabolism of dopamine and norepinephrine in rat liver slices. A 10-fold increase in phi-ALDH produced by phenobarbital treatment resulted in small increases in the formation of 3,4-dihydroxyphenylacetic acid and 3,4-dihydroxymandelic acid from the biogenic amines. The 50- to 100-fold elevation of the tau-isozyme did not alter the rate of formation of the acids. When liver slices were incubated with 40 mM ethanol, the formation of the reduced products of dopamine and norepinephrine, 3,4-dihydroxyphenylethanol and 3,4-dihydroxyphenylglycol, respectively, was favored. Under these conditions, the induction of the phi-isoenzyme again produced only a small increase in the formation of the acid products, whereas the induction of the tau-isoenzyme had no effect on acid production from biogenic amine metabolism. The results suggest that neither the phi- nor the tau-forms of ALDH are involved in the hepatic metabolism of dopamine or norepinephrine and support the conclusion that the oxidation of the aldehyde derived from dopamine occurs in mitochondria [A. W. Tank, H. Weiner and J. Thurman, Biochem. Pharmac. 30, 3265 (1981)].

Human aldehyde dehydrogenase: kinetic identification of the isozyme for which biogenic aldehydes and acetaldehyde compete

Michaelis constants and maximal velocities for phenylacetaldehyde (a metabolite of phenylethylamine), 3,4-dihydroxyphenylacetaldehyde (a metabolite of dopamine), 5-hydroxyindole acetaldehyde (a metabolite of serotonin), and 3,4-dihydroxyphenylglycolaldehyde (a metabolite of epinephrine and norepinephrine) have been determined for both cytoplasmic (E1) and mitochondrial (E2) isozymes of human liver aldehyde dehydrogenase (EC 1.2.1.3). Kinetic constants with biogenic aldehydes have never been previously determined for individual homogeneous isozymes of aldehyde dehydrogenase from any species. Mathematical treatment of these constants suggests that competition with acetaldehyde during alcohol metabolism would severely inhibit dehydrogenation of biogenic aldehydes with the mitochondrial and not the cytoplasmic isozyme of human liver aldehyde dehydrogenase.

Halogenated hydrocarbon and hydroperoxide-induced peroxidation in rat tissue slices

Tissue slices were used to compare relative peroxidation capacity of bromotrichloromethane (BrCCl3) and t-butyl hydroperoxide (BHP) by measurement of both peroxidation products and biochemical indices of damage. In liver and testes slices, BHP increased thiobarbituric acid reactive-substances (TBARS) and total aldehydes, measured as cyclohexanedione-reactive substances (CHDRS), to a greater extent than did an equimolar amount of BrCCl3. GSH was decreased more by BHP than by BrCCl3. Neither compound released lactate dehydrogenase or glutamic-pyruvic transaminase from liver slices. Treatment of rats with cyanamide, an aldehyde dehydrogenase inhibitor, increased the total CHDRS in liver slices and medium after incubation with BHP or BrCCl3. HPLC of the CHDRS showed hexanal and propanal increased to the greatest extent. The hydroperoxide, BHP, which does not require metabolism to an active species, was a better initiator of peroxidation than the halogenated hydrocarbon, BrCCl3, which must be metabolized to a radical species before it can initiate peroxidation.

Effect of tolbutamide and chlorpropamide on acetaldehyde metabolism in two inbred strains of mice

The mechanisms by which chlorpropamide and tolbutamide disrupt acetaldehyde metabolism were studied in C57BL and DBA mice. Acute po administration of varying doses of tolbutamide or chlorpropamide 2.5 hr before a 3.0 g/kg ip dose of ethanol to C57BL and DBA mice resulted in significant elevations of blood acetaldehyde when measured 2.5 hr after ethanol dosing. Dose-response analysis revealed a significant (p less than .05) difference in ED50 values for the elevated blood acetaldehyde response to tolbutamide in DBA (60 mg/kg) and C57BL (100 mg/kg) mice. The ED50 value for potentiation by chlorpropamide of blood acetaldehyde concentration was similar (23 to 32 mg/kg) in both inbred strains. At higher doses of chlorpropamide, DBA mice displayed elevations of blood acetaldehyde nearly threefold greater than those measured in C57BL mice treated identically. Measurements of aldehyde dehydrogenase (ALDH) in hepatic subcellular fractions, obtained from both inbred strains treated with 100 mg/kg tolbutamide or chlorpropamide prior to a 3.0 g/kg dose of ethanol, revealed a 50 to 80% inhibition of the low-Km ALDH present in mitochondria. Chlorpropamide and tolbutamide did not inhibit ALDH in vitro, suggesting that metabolites of these hypoglycemic agents may be responsible for the genotypic-dependent alterations in in vivo acetaldehyde oxidation.

Effect of triiodothyronine on alcohol dehydrogenase and aldehyde dehydrogenase activities in rat liver. Implications for the control of ethanol metabolism

Treatment of rats with 20 micrograms of 3,3',5-triiodo-L-thyronine (T3) per 100 g body wt for a period of 6 days led to a 45% decrease in total liver alcohol dehydrogenase and a 36% decrease in total liver aldehyde dehydrogenase. Most of the latter decrease was directly attributable to a 57% fall in the level of the physiologically-important low Km mitochondrial isoenzyme. The high Km isoenzyme of the postmitochondrial and soluble fractions was much less affected by T3-treatment. T3, at concentrations up to 0.1 mM, did not inhibit the activity of aldehyde dehydrogenase in vitro. Despite these large losses of the two enzymes most intimately involved in ethanol metabolism, the rate of ethanol elimination in vivo was the same in T3-treated and control animals. Moreover, there was no difference between the two groups in the susceptibility of ethanol elimination to inhibition by 4-methylpyrazole, making it unlikely that an alternative route of ethanol metabolism had been significantly induced by treatment with T3. As it had been suggested that T3 might create a "hypermetabolic state" in which constraints normally imposed on alcohol dehydrogenase and aldehyde dehydrogenase are removed thereby compensating for any loss in total enzymic activity, 2,4-dinitrophenol (0.1 mmoles/kg body wt) was administered to rats in order to raise the general metabolic rate. However, the uncoupler proved to be lethal to T3-treated animals and did not stimulate ethanol elimination in controls. The results do not support the notion that ethanol elimination in vivo is normally governed either by the level of alcohol dehydrogenase or by that of hepatic aldehyde dehydrogenase. However, the mode of control remains unclear.

Effects of disulfiram on the oxidation of benzaldehyde and acetaldehyde in rat liver

The in vitro oxidation of benzaldehyde and acetaldehyde was studied in liver samples from disulfiram-treated and control rats. With 25 microM substrate, both cytosol and mitochondria appeared to make a nearly equal contribution to the oxidation of benzaldehyde, whereas ca. 90% of acetaldehyde oxidation occurred in mitochondria. When the Km values for benzaldehyde with aldehyde dehydrogenase (ALDH) were determined, two Km values (3 and 120 microM) were obtained with mitochondria, but only a single Km value (25 microM) was obtained with the cytosolic fraction. The relatively high Km (2.9 mM) found with microsomes makes it unlikely that microsomes are important in the oxidation of benzaldehyde. In intact mitochondria, with 200 microM acetaldehyde or benzaldehyde the matrix space enzyme accounted for 77 and 62%, respectively, of the total ALDH activity. When the activity was determined in a mixture containing both substrates, the activity was found not to be additive, indicating that both substrates are oxidized by the same matrix space enzyme. With subcellular fractions, from livers of disulfiram-treated and control rats, a greater degree of inhibition of ALDH was obtained when acetaldehyde was a substrate compared to that with benzaldehyde in cytosol and mitochondria. Microsomal ALDH was not inhibited by disulfiram. In liver slices from rats given disulfiram, a statistically significant inhibition was found when either 25 or 250 microM acetaldehyde was used (46 and 33%). With benzaldehyde, a significant inhibition (24%) was observed only with the lower substrate concentration. Finding that both mitochondrial fractions and slices were less inhibited at the higher substrate concentration implies that the high Km enzyme is not inhibited. It can be concluded that, in rat, disulfiram inhibiting liver ALDH not only affects oxidation of acetaldehyde, but also that of benzaldehyde.

Analysis of human alcohol- and aldehyde-metabolizing isozymes by electrophoresis and isoelectric focusing

Isoelectric focusing and electrophoresis were used to identify the various isozymes of alcohol dehydrogenase (ADH), aldehyde dehydrogenase (ALDH), aldehyde oxidase (AOX), and xanthine oxidase (XOX). ADH types I, II, and III were located primarily in the cytosol fraction of liver, but some activity was found also in the small granule fraction. The ALDH-I and -IV isozymes were found in the large granule fraction, while ALDH-II and -III were present in the cytosol and ALDH-V in the small granule fraction. AOX and XOX each appeared as a single cytosolic form with some small granule activity. The tissue distribution of these isozymes is presented and the physiological role of each enzyme is discussed.

Aldehyde dehydrogenase activity as the rate-limiting factor for acetaldehyde metabolism in rat liver

The velocity of acetaldehyde metabolism in rat liver may be governed either by the rate of regeneration of NAD from NADH through the electron transport system or by the activity of aldehyde dehydrogenase (ALDH). Measurements of oxygen consumption revealed that the electron transport system was capable of reoxidizing ALDH-generated NADH much faster than it was produced and hence was not rate-limiting for aldehyde metabolism. To confirm that ALDH activity was the rate-limiting factor, low-Km ALDH in slices or intact mitochondria was partially inhibited by treatment with cyanamide and the rate of acetaldehyde metabolism measured. Any inhibition of low-Km ALDH resulted in a decreased rate of acetaldehyde metabolism, indicating that no excess of low-Km ALDH existed. Approximately 40% of the metabolism of 200 microM acetaldehyde in slices was not catalyzed by low-Km ALDH. Fifteen of this 40% was catalyzed by high-Km ALDH. A possible contribution by aldehyde oxidase was ruled out through the use of a competitive inhibitor, quinacrine. Acetaldehyde binding to cytosolic proteins may account for the remainder. By measuring acetaldehyde accumulation during ethanol metabolism, it was also established that low-Km ALDH activity was rate-limiting for acetaldehyde oxidation during concomitant ethanol oxidation.

Application of the reverse dept polarization-transfer pulse sequence to monitor in vitro and in vivo metabolism of 13C-ethanol by 1H-NMR spectroscopy

Using the reverse 13C----1H DEPT polarization-transfer pulse sequence the metabolism of 13C ethanol in vitro and in vivo has been monitored by 1H-NMR spectroscopy. Using yeast alcohol dehydrogenase, acetaldehyde, the hydrated form of acetaldehyde and acetate were identified as metabolites of [2-13C]-ethanol. The ratio of hydrated to free acetaldehyde was dependent upon the protein concentration of the reaction mixture. Binding of acetaldehyde in an irreversible Schiffs base resulted in optimal enzyme activity. Hepatocytes from rats fasted for 20 h, metabolised [1-13C] and [2-13C]ethanol in a linear fashion, but no [13C]acetaldehyde was detected. Metabolic integrity of the hepatocytes was confirmed with [2-13C]acetate. The addition of disulfiram (50 micron) to hepatocyte suspensions which had been incubated with [1-13C]ethanol, resulted in the resynthesis of [13C]ethanol. The amount of [13C]ethanol resynthesized under these conditions represents intracellular acetaldehyde whose concentration was in the range of 400-800 mumol/g wet weight of hepatocytes when 50 mM ethanol had been originally incubated with the hepatocyte suspension. These studies show how NMR-polarization transfer pulse sequences can be used to monitor the metabolism of 13C-ethanol in vivo, and provide a unique tool to measure in vivo concentrations of acetaldehyde. The studies also suggest that cytoplasmic aldehyde dehydrogenase may play a major role in hepatic ethanol metabolism.

Subcellular distribution of aldehyde dehydrogenase activities in human liver

The subcellular distributions of aldehyde dehydrogenase activities towards acetaldehyde have been determined in wedge-biopsy samples of human liver. A form with Km values of less than 1 microM and 285 microM towards acetaldehyde and NAD+ respectively was present in the mitochondrial fraction. This enzyme had no detectable activity towards N-tele-methylimidazole acetaldehyde, the aldehyde derived from the oxidation of N-tele-methylhistamine. The activity in the cytosol was more sensitive to inhibition by disulfiram and had Km values of 270 microM and 25 microM for acetaldehyde and NAD+, respectively. It was active towards N-tele-methylimidazole acetaldehyde with a Km value of 2.5 microM and a maximum velocity that was 40% of that determined with acetaldehyde. Both these cytosolic activities had alkaline pH optima.

Use of cyanamide to determine localization of acetaldehyde metabolism in rat liver

Various techniques have been employed previously to show that acetaldehyde is primarily oxidized in the mitochondrial matrix of rat liver. In this study, a new approach was tested. Mitochondrial low-Km aldehyde dehydrogenase (ALDH) was partially inactivated and the effect on acetaldehyde oxidation measured. Cyanamide was chosen as the ALDH inhibitor. An enzymatic activation of cyanamide, probably by catalase, was necessary for the drug to inhibit ALDH activity. The level of remaining ALDH activity after cyanamide treatment was correlated with the ability of either rat liver mitochondria or liver slices to oxidize acetaldehyde. Any inhibition of ALDH resulted in a decreased rate of acetaldehyde oxidation, indicating that there is no excess of ALDH in the cell above what is needed to oxidize acetaldehyde. Approximately 15% of the acetaldehyde disappearance at 200 microM was catalyzed by high-Km ALDH, and nearly 30% of the acetaldehyde was lost through binding to cytosolic proteins.

Comparative subcellular distribution of aldehyde dehydrogenase in rat, mouse and rabbit liver

The subcellular distribution of hepatic aldehyde dehydrogenase (ALDH) activity was determined in Buffalo, Fischer 344, Long-Evans, Sprague-Dawley, Wistar and Purdue/Wistar rats. These subcellular distributions were compared to the distribution of mouse and rabbit liver ALDH. For the six rat strains, at millimolar propionaldehyde concentrations, NAD-dependent ALDH activity was associated primarily with mitochondria (51%) and microsomes (30%). At millimolar acetaldehyde concentrations, NAD-dependent ALDH was primarily mitochondrial (up to 80%). Less than 1% of total NAD-dependent aldehyde dehydrogenase was found in the cytosol. The highly inbred Purdue/Wistar line possessed significantly less acetaldehyde-NAD ALDH activity as well as less total NADP-dependent ALDH activity than the other strains. In CD-1 mouse liver, millimolar Km, NAD-dependent ALDH activity was found in mitochondria (60%), microsomes (23%) and cytosol (5%). In rabbit liver, millimolar Km, NAD-dependent ALDH was also distributed among mitochondria (36%), microsomes (19%) and cytosol (28%). At micromolar substrate concentrations, mitochondria possessed the majority of rat, mouse and rabbit liver ALDH activity. In all three species, NADP-dependent ALDH activity was found predominantly in the microsomal fraction (up to 65%). The cytosol possessed little NADP-dependent ALDH in any species. We conclude that there are significant species differences in the subcellular distribution of aldehyde dehydrogenase between rat, mouse and rabbit liver. In all three species, mitochondria and microsomes possessed the majority of hepatic aldehyde dehydrogenase activity. However, the cytosol of mouse and rabbit liver also made a significant contribution to total ALDH activity. For the six rat strains examined, liver cytosol possessed little or no ALDH activity.

Metabolism of malondialdehyde by rat liver aldehyde dehydrogenase

Mammalian liver contains a group of pyridine nucleotide linked aldehyde dehydrogenases [E.C. 1.2.1.3] which are present in high specific activity and possess wide substrate specificities. Malondialdehyde (MDA), a difunctional three-carbon aldehyde thought to be toxic, is generated during membrane lipid peroxidation in hepatocytes. The role of aldehyde dehydrogenase (ALDH) in the metabolism of MDA was tested in vitro with subcellular fractions and semipurified cytosolic preparations from rat livers. The cytosolic fraction accounted for virtually all of the MDA (50 microM) metabolizing activity observed in the postnuclear supernatant fraction. The rate of MDA disappearance was relatively low in the mitochondrial fraction and was not detectable in reaction mixtures which contained microsomes. Rat liver cytosol contained two ALDHs with MDA metabolizing activity. These enzymes were separated by DEAE-cellulose ion exchange chromatography and had apparent Km values of 16 microM and 128 microM for malondialdehyde. Mitochondria contained an ALDH enzyme with lower affinity (Km of 7.3 mM with NAD+) for malondialdehyde. These data show that rat liver contains at least three ALDH enzymes which oxidize malondialdehyde.

Ethanol oxidation in systems containing soluble and mitochondrial fractions of rat liver. Regulation by acetaldehyde

Systems containing soluble fraction of rat liver, with or without mitochondrial fraction, oxidised [1-14C] ethanol to acetaldehyde, 14CO2 and non-volatile 14C-products of which acetate was the principal, and possibly the only, component. Ethanol oxidation was stimulated by pyruvate which served as an electron sink thereby allowing rapid regeneration of NAD. When no mitochondria were present acetaldehyde accumulated, rapidly at first but eventually reaching a plateau. The rate of ethanol oxidation in these systems was much lower than the measured maximum activity of alcohol dehydrogenase (ADH) and it was concluded that ADH was inhibited by the accumulated acetaldehyde. Mitochondria, because of their relatively high aldehyde dehydrogenase (ALDH) activity, prevented the accumulation of acetaldehyde, or quickly removed acetaldehyde already accumulated. This action was accompanied by a sharp increase in the rate of ethanol oxidation, presumably due to the deinhibition of ADH. Cyanamide, an inhibitor of mitochondrial ALDH, blocked the stimulatory effect of mitochondria on ethanol oxidation. It was concluded that, in the reconstituted systems, acetaldehyde played a dominant role in controlling the rate of ethanol oxidation. The possible importance of acetaldehyde in governing ethanol oxidation in vivo is discussed.

Effects of magnesium and calcium on mitochondrial and cytosolic liver aldehyde dehydrogenases

The effects of Mg2+ and Ca2+ ions on the activities of mitochondrial and cytosolic aldehyde dehydrogenases isolated from horse, rat, and beef livers were investigated in 0.1 M sodium phosphate, pH 7.5, at 25 degrees C. As with the Mg2+-enhancement of the horse liver mitochondrial enzyme [17], Mg2+-activation was observed for the mitochondrial enzymes from rat and beef. The cytosolic enzymes from horse, rat, and beef livers were inhibited in the presence of Mg2+ ions. The effects of Ca2+ ions on the activity were essentially the same as those observed in the presence of Mg2+ ions; the mitochondrial isozymes were activated while the cytosolic isozymes were inhibited. The fact that only the activity of mitochondrial forms of mammalian liver aldehyde dehydrogenase was enhanced by Ca2+ or Mg2+ ions may be related to the in vivo regulation of aldehyde metabolism, a presumed mitochondrial event.

The role of cytoplasmic aldehyde dehydrogenase in the metabolism of N-tele-methylhistamine

The subcellular distributions of aldehyde dehydrogenase activities towards acetaldehyde have been compared with those toward N-tele-methylimidazole acetaldehyde, the aldehyde derived from the oxidation of N-tele-methylhistamine. At high concentrations of acetaldehyde (3.0 mM), significant aldehyde dehydrogenase activity can be found in the mitochondrial, light mitochondrial, microsomal and cytoplasmic fractions whereas, when the activity is determined with 15 microM acetaldehyde, the enzyme activity is enriched only in the mitochondrial fraction suggesting that this organelle will be the dominant site for the metabolism of acetaldehyde derived from ingested ethanol. The activity towards N-tele-methylimidazole acetaldehyde was determined by generating this compound in the assay by the oxidation of N-tele-methylhistamine in the presence of beef plasma amine oxidase. At the low steady-state aldehyde concentrations that will be present in such an assay, only the cytoplasmic form of aldehyde dehydrogenase showed activity towards this substrate.

Subcellular distribution and kinetic parameters of HS mouse liver aldehyde dehydrogenase

1. Aldehyde dehydrogenase subcellular distribution studies were performed in a heterogeneous stock (HS) of male and female mice (Mus musculus) with propionaldehyde (5 mM and 50 microM) and formaldehyde (1 mM) and NAD+ or NADP+. 2. The relative percents of distribution were: cytosolic 55-68%, mitochondrial 12-20%, microsomal 9-18% and lysosomal 3-15% for both propionaldehyde concentrations and NAD+. 3. Kinetic experiments using propionaldehyde and acetaldehyde with NAD+ revealed two separate enzymes, Enzyme I (low Km) and Enzyme II (high Km) in the cytosolic and mitochondrial fractions. 4. The kinetic data also indicated a spectrum of cytosolic low Km values that exhibited a bimodal distribution with one congruent to 40 microM and one congruent to 5 microM. 5. It was concluded that there was no significant difference in aldehyde-metabolizing capability between male and female HS mice, compared on a per gram of liver basis. The cytosolic low Km enzyme plays a major role in aldehyde oxidation at moderate to low aldehyde concentrations.