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

ALDH2 Inhibition by Lead and Ethanol Elicits Redox Imbalance and Mitochondrial Dysfunction in SH-SY5Y Human Neuroblastoma Cell Line: Reversion by Alda-1

Lead (Pb), a common environmental contaminant, and ethanol (EtOH), a widely available drug of abuse, are well-known neurotoxicants. In vivo, experimental evidence indicates that Pb exposure affects oxidative EtOH metabolism with a high impact on living organisms. On these bases, we evaluated the consequences of combined Pb and EtOH exposure on aldehyde dehydrogenase 2 (ALDH2) functionality. In vitro exposure to 10µM Pb, 200mM EtOH, or their combination for 24h reduced ALDH2 activity and content in SH-SY5Y human neuroblastoma cells. In this scenario, we observed mitochondrial dysfunction characterized by reduced mass and membrane potential, decreased maximal respiration, and spare capacity. We also evaluated the oxidative balance in these cells finding a significant increase in reactive oxygen species (ROS) production and lipid peroxidation products under all treatments accompanied by an increase in catalase (CAT) activity and content. These data suggest that ALDH2 inhibition induces the activation of converging cytotoxic mechanisms resulting in an interplay between mitochondrial dysfunction and oxidative stress. Notably, NAD⁺ (1mM for 24h) restored ALDH2 activity in all groups, while an ALDH2 enhancer (Alda-1, 20µM for 24h) also reversed some of the deleterious effects resulting from impaired ALDH2 function. Overall, these results reveal the crucial role of this enzyme on the Pb and EtOH interaction and the potential of activators such as Alda-1 as therapeutic approaches against several conditions involving aldehydes accumulation.

Quantitative analysis of the interaction of ethanol metabolism with gluconeogenesis and fatty acid oxidation in the perfused liver of fasted rats

Ethanol is known to significantly affect gluconeogenesis and lipid metabolism in the liver, primarily by altering the redox ratio in both cytosol and mitochondria. The effect of ethanol was analyzed using a comprehensive, dynamic model of liver metabolism that takes into account sub-cellular compartmentation, detailed kinetics for the citric acid cycle, ethanol and acetaldehyde oxidation, and gluconeogenesis, and inter-compartmental transport of metabolites, including the malate-aspartate shuttle. The kinetic expression for alcohol dehydrogenase takes into account inhibition by ethanol and NADH. Simulations of perfusions of the rat liver were performed with various combinations of substrates (lactate, pyruvate, and fatty acids), with subsequent addition of ethanol to the perfusate. The model successfully predicts NADH/NAD⁺, in both cytosol and mitochondria, the expected directional flux of reducing equivalents between the two compartments during perfusion with different gluconeogenic precursors, and the effect of ethanol on glucose and ketone body production. This model can serve as a platform for in silico experiments investigating the effects of ethanol on the many dehydrogenases, and thus the major carbohydrate and lipid metabolic pathways in the liver, as well as potential effects of various drugs that may interact with ethanol.

The fructose-dependent acceleration of ethanol metabolism

The aim of the present study was to elucidate how fructose is able to increase the rate of ethanol metabolism in the liver, an observation previously termed the fructose effect. Previous studies suggest that an increase in ATP consumption driven by glucose synthesis from fructose stimulates the oxidation of NADH in the mitochondrial respiratory chain, allowing faster oxidation of ethanol by alcohol dehydrogenase; however, this idea has been frequently challenged. We tested the effects of fructose, sorbose and tagatose both in vitro and in vivo. Both ethanol and each sugar were either added to isolated hepatocytes or injected intraperitoneally in the rat. In the in vitro experiments, samples were taken from the hepatocyte suspension in a time-dependent manner and deproteinized with perchloric acid. In the in vivo experiments, blood samples were taken every 15 min and the metabolites were determined in the plasma. These metabolites include ethanol, glucose, glycerol, sorbitol, lactate, fructose and sorbose. Ethanol oxidation by rat hepatocytes was increased by more than 50% with the addition of fructose. The stimulation was accompanied by increased glucose, glycerol, lactate and sorbitol production. A similar effect was observed with sorbose, while tagatose had no effect. The same pattern was observed in the in vivo experiments. This effect was abolished by inhibiting alcohol dehydrogenase with 4-methylpyrazole, whereas inhibition of the respiratory chain with cyanide did not affect the fructose effect. In conclusion, present results provide evidence that, by reducing glyceraldehyde and glycerol and fructose to sorbitol, respectively, NADH is consumed, allowing an increase in the elimination of ethanol. Hence, this effect is not linked to a stimulation of mitochondrial re-oxidation of NADH driven by ATP consumption.

Mitochondrial free fatty acid β-oxidation supports oxidative phosphorylation and proliferation in cancer cells

Oxidative phosphorylation (OxPhos) is functional and sustains tumor proliferation in several cancer cell types. To establish whether mitochondrial β-oxidation of free fatty acids (FFAs) contributes to cancer OxPhos functioning, its protein contents and enzyme activities, as well as respiratory rates and electrical membrane potential (ΔΨm) driven by FFA oxidation were assessed in rat AS-30D hepatoma and liver (RLM) mitochondria. Higher protein contents (1.4-3 times) of β-oxidation (CPT1, SCAD) as well as proteins and enzyme activities (1.7-13-times) of Krebs cycle (KC: ICD, 2OGDH, PDH, ME, GA), and respiratory chain (RC: COX) were determined in hepatoma mitochondria vs. RLM. Although increased cholesterol content (9-times vs. RLM) was determined in the hepatoma mitochondrial membranes, FFAs and other NAD-linked substrates were oxidized faster (1.6-6.6 times) by hepatoma mitochondria than RLM, maintaining similar ΔΨm values. The contents of β-oxidation, KC and RC enzymes were also assessed in cells. The mitochondrial enzyme levels in human cervix cancer HeLa and AS-30D cells were higher than those observed in rat hepatocytes whereas in human breast cancer biopsies, CPT1 and SCAD contents were lower than in human breast normal tissue. The presence of CPT1 and SCAD in AS-30D mitochondria and HeLa cells correlated with an active FFA utilization in HeLa cells. Furthermore, the β-oxidation inhibitor perhexiline blocked FFA utilization, OxPhos and proliferation in HeLa and other cancer cells. In conclusion, functional mitochondria supported by FFA β-oxidation are essential for the accelerated cancer cell proliferation and hence anti-β-oxidation therapeutics appears as an alternative promising approach to deter malignant tumor growth.

(13) C magnetic resonance spectroscopic imaging of hyperpolarized [1-(13) C, U-(2) H5 ] ethanol oxidation can be used to assess aldehyde dehydrogenase activity in vivo

PURPOSE

Aldehyde dehydrogenase (ALDH2) is an emerging drug target for the treatment of heart disease, cocaine and alcohol dependence, and conditions caused by genetic polymorphisms in ALDH2. Noninvasive measurement of ALDH2 activity in vivo could inform the development of these drugs and accelerate their translation to the clinic.

METHODS

[1-(13) C, U-(2) H5 ] ethanol was hyperpolarized using dynamic nuclear polarization, injected into mice and its oxidation in the liver monitored using (13) C MR spectroscopy and spectroscopic imaging.

RESULTS

Oxidation of [1-(13) C, U-(2) H5 ] ethanol to [1-(13) C] acetate was observed. Saturation of the acetaldehyde resonance, which was below the level of detection in vivo, demonstrated that acetate was produced via acetaldehyde. Irreversible inhibition of ALDH2 activity with disulfiram resulted in a proportional decrease in the amplitude of the acetate resonance.

CONCLUSION

(13) C magnetic resonance spectroscopy measurements of hyperpolarized [1-(13) C, U-(2) H5 ] ethanol oxidation allow real-time assessment of ALDH2 activity in liver in vivo.

Contribution of liver alcohol dehydrogenase to metabolism of alcohols in rats

The kinetics of oxidation of various alcohols by purified rat liver alcohol dehydrogenase (ADH) were compared with the kinetics of elimination of the alcohols in rats in order to investigate the roles of ADH and other factors that contribute to the rates of metabolism of alcohols. Primary alcohols (ethanol, 1-propanol, 1-butanol, 2-methyl-1-propanol, 3-methyl-1-butanol) and diols (1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol) were eliminated in rats with zero-order kinetics at doses of 5-20 mmol/kg. Ethanol was eliminated most rapidly, at 7.9 mmol/kgh. Secondary alcohols (2-propanol-d7, 2-propanol, 2-butanol, 3-pentanol, cyclopentanol, cyclohexanol) were eliminated with first order kinetics at doses of 5-10 mmol/kg, and the corresponding ketones were formed and slowly eliminated with zero or first order kinetics. The rates of elimination of various alcohols were inhibited on average 73% (55% for 2-propanol to 90% for ethanol) by 1 mmol/kg of 4-methylpyrazole, a good inhibitor of ADH, indicating a major role for ADH in the metabolism of the alcohols. The Michaelis kinetic constants from in vitro studies (pH 7.3, 37 °C) with isolated rat liver enzyme were used to calculate the expected relative rates of metabolism in rats. The rates of elimination generally increased with increased activity of ADH, but a maximum rate of 6±1 mmol/kg h was observed for the best substrates, suggesting that ADH activity is not solely rate-limiting. Because secondary alcohols only require one NAD(+) for the conversion to ketones whereas primary alcohols require two equivalents of NAD(+) for oxidation to the carboxylic acids, it appears that the rate of oxidation of NADH to NAD(+) is not a major limiting factor for metabolism of these alcohols, but the rate-limiting factors are yet to be identified.

Deficiency of retinaldehyde dehydrogenase 1 induces BMP2 and increases bone mass in vivo

The effects of retinoids, the structural derivatives of vitamin A (retinol), on post-natal peak bone density acquisition and skeletal remodeling are complex and compartment specific. Emerging data indicates that retinoids, such as all trans retinoic acid (ATRA) and its precursor all trans retinaldehyde (Rald), exhibit distinct and divergent transcriptional effects in metabolism. Despite these observations, the role of enzymes that control retinoid metabolism in bone remains undefined. In this study, we examined the skeletal phenotype of mice deficient in retinaldehyde dehydrogenase 1 (Aldh1a1), the enzyme responsible for converting Rald to ATRA in adult animals. Bone densitometry and micro-computed tomography (µCT) demonstrated that Aldh1a1-deficient (Aldh1a1^−/−) female mice had higher trabecular and cortical bone mass compared to age and sex-matched control C57Bl/6 wild type (WT) mice at multiple time points. Histomorphometry confirmed increased cortical bone thickness and demonstrated significantly higher bone marrow adiposity in Aldh1a1^−/− mice. In serum assays, Aldh1a1^−/− mice also had higher serum IGF-1 levels. In vitro, primary Aldh1a1^−/− mesenchymal stem cells (MSCs) expressed significantly higher levels of bone morphogenetic protein 2 (BMP2) and demonstrated enhanced osteoblastogenesis and adipogenesis versus WT MSCs. BMP2 was also expressed at higher levels in the femurs and tibias of Aldh1a1^−/− mice with accompanying induction of BMP2-regulated responses, including expression of Runx2 and alkaline phosphatase, and Smad phosphorylation. In vitro, Rald, which accumulates in Aldh1a1^−/− mice, potently induced BMP2 in WT MSCs in a retinoic acid receptor (RAR)-dependent manner, suggesting that Rald is involved in the BMP2 increases seen in Aldh1a1 deficiency in vivo. Collectively, these data implicate Aldh1a1 as a novel determinant of cortical bone density and marrow adiposity in the skeleton in vivo through modulation of BMP signaling.

Potent inhibition of aldehyde dehydrogenase-2 by diphenyleneiodonium: focus on nitroglycerin bioactivation

Aldehyde dehydrogenase-2 (ALDH2) catalyzes vascular bioactivation of the antianginal drug nitroglycerin (GTN) to yield nitric oxide (NO) or a related species that activates soluble guanylate cyclase (sGC), resulting in cGMP-mediated vasodilation. Accordingly, established ALDH2 inhibitors attenuate GTN-induced vasorelaxation in vitro and in vivo. However, the ALDH2 hypothesis has not been reconciled with early studies demonstrating potent inhibition of the GTN response by diphenyleneiodonium (DPI), a widely used inhibitor of flavoproteins, in particular NADPH oxidases. We addressed this issue and investigated the effects of DPI on GTN-induced relaxation of rat aortic rings and the function of purified ALDH2. DPI (0.3 µM) inhibited the high affinity component of aortic relaxation to GTN without affecting the response to NO, indicating that the drug interfered with GTN bioactivation. Denitration and bioactivation of 1-2 µM GTN, assayed as 1,2-glycerol dinitrate formation and activation of purified sGC, respectively, were inhibited by DPI with a half-maximally active concentration of about 0.2 µM in a GTN-competitive manner. Molecular modeling indicated that DPI binds to the catalytic site of ALDH2, and this was confirmed by experiments showing substrate-competitive inhibition of the dehydrogenase and esterase activities of the enzyme. Our data identify ALDH2 as highly sensitive target of DPI and explain inhibition of GTN-induced relaxation by this drug observed previously. In addition, the data provide new evidence for the essential role of ALDH2 in GTN bioactivation and may have implications to other fields of ALDH2 research, such as hepatic ethanol metabolism and cardiac ischemia/reperfusion injury.

c-Fos immunoreactivity in prefrontal, basal ganglia and limbic areas of the rat brain after central and peripheral administration of ethanol and its metabolite acetaldehyde

Considerable evidence indicates that the metabolite of ethanol (EtOH), acetaldehyde, is biologically active. Acetaldehyde can be formed from EtOH peripherally mainly by alcohol dehydrogenase (ADH), and also centrally by catalase. EtOH and acetaldehyde show differences in their behavioral effects depending upon the route of administration. In terms of their effects on motor activity and motivated behaviors, when administered peripherally acetaldehyde tends to be more potent than EtOH but shows very similar potency administered centrally. Since dopamine (DA) rich areas have an important role in regulating both motor activity and motivation, the present studies were undertaken to compare the effects of central (intraventricular, ICV) and peripheral (intraperitoneal, IP) administration of EtOH and acetaldehyde on a cellular marker of brain activity, c-Fos immunoreactivity, in DA innervated areas. Male Sprague-Dawley rats received an IP injection of vehicle, EtOH (0.5 or 2.5 g/kg) or acetaldehyde (0.1 or 0.5 g/kg) or an ICV injection of vehicle, EtOH or acetaldehyde (2.8 or 14.0 μmoles). IP administration of EtOH minimally induced c-Fos in some regions of the prefrontal cortex and basal ganglia, mainly at the low dose (0.5 g/kg), while IP acetaldehyde induced c-Fos in virtually all the structures studied at both doses. Acetaldehyde administered centrally increased c-Fos in all areas studied, a pattern that was very similar to EtOH. Thus, IP administered acetaldehyde was more efficacious than EtOH at inducing c-Fos expression. However, the general pattern of c-Fos induction promoted by ICV EtOH and acetaldehyde was similar. These results are consistent with the pattern observed in behavioral studies in which both substances produced the same magnitude of effect when injected centrally, and produced differences in potency after peripheral administration.

Mitochondrial NAD dependent aldehyde dehydrogenase either from yeast or human replaces yeast cytoplasmic NADP dependent aldehyde dehydrogenase for the aerobic growth of yeast on ethanol

BACKGROUND

In a previous study, we deleted three aldehyde dehydrogenase (ALDH) genes, involved in ethanol metabolism, from yeast Saccharomyces cerevisiae and found that the triple deleted yeast strain did not grow on ethanol as sole carbon source. The ALDHs were NADP dependent cytosolic ALDH1, NAD dependent mitochondrial ALDH2 and NAD/NADP dependent mitochondrial ALDH5. Double deleted strain ΔALDH2+ΔALDH5 or ΔALDH1+ΔALDH5 could grow on ethanol. However, the double deleted strain ΔALDH1+ΔALDH2 did not grow in ethanol.

METHODS

Triple deleted yeast strain was used. Mitochondrial NAD dependent ALDH from yeast or human was placed in yeast cytosol.

RESULTS

In the present study we found that a mutant form of cytoplasmic ALDH1 with very low activity barely supported the growth of the triple deleted strain (ΔALDH1+ΔALDH2+ΔALDH5) on ethanol. Finding the importance of NADP dependent ALDH1 on the growth of the strain on ethanol we examined if NAD dependent mitochondrial ALDH2 either from yeast or human would be able to support the growth of the triple deleted strain on ethanol if the mitochondrial form was placed in cytosol. We found that the NAD dependent mitochondrial ALDH2 from yeast or human was active in cytosol and supported the growth of the triple deleted strain on ethanol.

CONCLUSION

This study showed that coenzyme preference of ALDH is not critical in cytosol of yeast for the growth on ethanol.

GENERAL SIGNIFICANCE

The present study provides a basis to understand the coenzyme preference of ALDH in ethanol metabolism in yeast.

Post-translational modifications of mitochondrial aldehyde dehydrogenase and biomedical implications

Aldehyde dehydrogenases (ALDHs) represent large family members of NAD(P)+-dependent dehydrogenases responsible for the irreversible metabolism of many endogenous and exogenous aldehydes to the corresponding acids. Among 19 ALDH isozymes, mitochondrial ALDH2 is a low Km enzyme responsible for the metabolism of acetaldehyde and lipid peroxides such as malondialdehyde and 4-hydroxynonenal, both of which are highly reactive and toxic. Consequently, inhibition of ALDH2 would lead to elevated levels of acetaldehyde and other reactive lipid peroxides following ethanol intake and/or exposure to toxic chemicals. In addition, many East Asian people with a dominant negative mutation in ALDH2 gene possess a decreased ALDH2 activity with increased risks for various types of cancer, myocardial infarct, alcoholic liver disease, and other pathological conditions. The aim of this review is to briefly describe the multiple post-translational modifications of mitochondrial ALDH2, as an example, after exposure to toxic chemicals or under different disease states and their pathophysiological roles in promoting alcohol/drug-mediated tissue damage. We also briefly mention exciting preclinical translational research opportunities to identify small molecule activators of ALDH2 and its isozymes as potentially therapeutic/preventive agents against various disease states where the expression or activity of ALDH enzymes is altered or inactivated.

Identification of S-nitrosated mitochondrial proteins by S-nitrosothiol difference in gel electrophoresis (SNO-DIGE): implications for the regulation of mitochondrial function by reversible S-nitrosation

The S-nitrosation of mitochondrial proteins as a consequence of NO metabolism is of physiological and pathological significance. We previously developed a MitoSNO (mitochondria-targeted S-nitrosothiol) that selectively S-nitrosates mitochondrial proteins. To identify these S-nitrosated proteins, here we have developed a selective proteomic methodology, SNO-DIGE (S-nitrosothiol difference in gel electrophoresis). Protein thiols in control and MitoSNO-treated samples were blocked, then incubated with copper(II) and ascorbate to selectively reduce S-nitrosothiols. The samples were then treated with thiol-reactive Cy3 (indocarbocyanine) or Cy5 (indodicarbocyanine) fluorescent tags, mixed together and individual protein spots were resolved by 2D (two-dimensional) gel electrophoresis. Fluorescent scanning of these gels revealed S-nitrosated proteins by an increase in Cy5 red fluorescence, allowing for their identification by MS. Parallel analysis by Redox-DIGE enabled us to distinguish S-nitrosated thiol proteins from those which became oxidized due to NO metabolism. We identified 13 S-nitrosated mitochondrial proteins, and a further four that were oxidized, probably due to evanescent S-nitrosation relaxing to a reversible thiol modification. We investigated the consequences of S-nitrosation for three of the enzymes identified using SNO-DIGE (aconitase, mitochondrial aldehyde dehydrogenase and α-ketoglutarate dehydrogenase) and found that their activity was selectively and reversibly inhibited by S-nitrosation. We conclude that the reversible regulation of enzyme activity by S-nitrosation modifies enzymes central to mitochondrial metabolism, whereas identification and functional characterization of these novel targets provides mechanistic insight into the potential physiological and pathological roles played by this modification. More generally, the development of SNO-DIGE facilitates robust investigation of protein S-nitrosation across the proteome.

Inhibition of hepatic mitochondrial aldehyde dehydrogenase by carbon tetrachloride through JNK-mediated phosphorylation

The aim of this study was to investigate the mechanism of inhibition of mitochondrial aldehyde dehydrogenase (ALDH2) by carbon tetrachloride (CCl(4)). CCl(4) administration caused marked hepatocyte ballooning and necrosis in the pericentral region. CCl(4) also inhibited hepatic ALDH2 activity in a time-dependent manner without altering the protein level, suggesting ALDH2 inhibition through covalent modifications such as phosphorylation by JNK. To demonstrate phosphorylation, the isoelectric point (pI) of ALDH2 in CCl(4)-exposed rats was compared to that of untreated controls. Immunoblot analysis revealed that immunoreactive ALDH2 bands in CCl(4)-exposed rats were shifted to acidic pI ranges on two-dimensional electrophoresis (2-DE) gels. Incubation with alkaline phosphatase significantly restored the suppressed ALDH2 activity with a concurrent alkaline pI shift of the ALDH2 spots. Both JNK and activated JNK were translocated to mitochondria after CCl(4) exposure. In addition, incubation with catalytically active JNK led to significant inhibition of ALDH2 activity, with an acidic pI shift on 2-DE gels. Furthermore, immunoprecipitation followed by immunoblot analysis with anti-phospho-Ser-Pro antibody revealed phosphorylation of a Ser residue(s) of ALDH2. These results collectively indicate a novel underlying mechanism by which CCl(4) exposure activates JNK, which translocates to mitochondria and phosphorylates ALDH2, contributing to inhibition of ALDH2 activity accompanied by decreased cellular defense capacity and increased lipid peroxidation.

Polymorphisms in mitochondrial genes encoding complex I subunits are maternal factors of voluntary alcohol consumption in the rat

OBJECTIVE

Alcohol is detoxified in the liver by oxidizing enzymes that require nicotinamide adenine dinucleotide (NAD+) such that, in the rat, the availability of NAD+ contributes to control voluntary ethanol intake. The UChA and UChB lines of Wistar rats drink low and high amounts of ethanol respectively and differ in the capacity of their mitochondria to oxidize NADH into NAD+. This function resides in complex I of the respiratory chain and its variation is linked to genes transmitted through the maternal line. The aim of this study was to identify the genetic basis for the difference in the reoxidation of NADH in these nondrinker (UChA) and drinker (UChB) rats.

METHODS

Seven mitochondrial genes and two chromosome X genes encoding complex I subunits from rats of both lineages were amplified from liver DNA and sequenced.

RESULTS

The UChA and UChB rat lines differ in their Nd2, Nd4, Nd5 and Nd6 mitochondrial genes and in the encoded proteins. Most noteworthy are ND2 and ND4 whose amino acid variations lead to changes in three-dimensional structure models. The ND2 proteins also differ in the number of predicted transmembrane domains. The Nd1 and Nd3 genes have silent substitutions, whereas Nd4L and the exonic sequences of the nuclear genes Ndufa1 and Ndufb11 show no differences between the UChA and UChB lines.

CONCLUSION

Amino acid variations in four complex I subunits encoded in the mitochondrial genome may contribute to explain the differences between UChA and UChB rats in their capacity to reoxidize NADH and in their alcohol intake, suggesting that mitochondrial genes may constitute maternal factors of alcoholism.

Drug interaction between ethanol and 3,4-methylenedioxymethamphetamine ("ecstasy")

Alcohol (ethanol) and 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) are frequently co-abused, but recent findings indicate a harmful drug interaction between these two agents. In our previous study, we showed that MDMA exposure inhibits the activity of the acetaldehyde (ACH) metabolizing enzyme, aldehyde dehydrogenase2 (ALDH2). Based on this finding, we hypothesized that the co-administration of MDMA and ethanol would reduce the metabolism of ACH and result in increased accumulation of ACH. Rats were treated with MDMA or vehicle and then administered a single dose of ethanol. Liver ALDH2 activity decreased by 35% in the MDMA-treated rats compared to control rats. The peak concentration and the area under the concentration versus time curve of plasma ACH were 31% and 59% higher, respectively, in the MDMA-ethanol group compared to the ethanol-only group. In addition, the MDMA-ethanol group had 80% higher plasma transaminase levels than the ethanol-only group, indicating greater hepatocellular damage. Our results not only support a drug interaction between MDMA and ethanol but a novel underlying mechanism for the interaction.

Inactivation of cytosolic aldehyde dehydrogenase via S-nitrosylation in ethanol-exposed rat liver

Aldehyde dehydrogenase (ALDH) isozymes are critically important in the metabolism of acetaldehyde, thus preventing its accumulation after ethanol-exposure. We previously reported that mitochondrial ALDH2 could be inactivated via S-nitrosylation in ethanol-exposed rats. This study was aimed at investigating whether cytosolic ALDH1, with a relatively-low-Km value (11-18 microM) for acetaldehyde, could be also inhibited in ethanol-exposed rats. Chronic or binge ethanol-exposure significantly decreased ALDH1 activity, which was restored by addition of dithiothreitol. Immunoblot analysis with the anti-S-nitroso-Cys antibody showed one immunoreactive band in the immunoprecipitated ALDH1 only from ethanol-exposed rats, but not from pair-fed controls, suggesting S-nitrosylation of ALDH1. Therefore inactivation of ALDH1 via S-nitrosylation can result in accumulation of acetaldehyde upon ethanol-exposure.

Characterization of an Aldehyde Dehydrogenase from Euglena gracilis

The free-living protist Euglena gracilis showed an enhanced growth when cultured in the dark with high concentrations of ethanol as carbon source. In a medium containing glutamate/malate plus 1% ethanol, E. gracilis reached a density of 3 x 10(7) cells/ml after 100 h of culture, which was 5 times higher than that attained with glutamate/malate or ethanol separately. This observation suggested the involvement of a highly active aldehyde dehydrogenase in the metabolism of ethanol. Purification of the E. gracilis aldehyde dehydrogenase from the mitochondrial fraction by affinity chromatography yielded an enrichment of 34 times and recovery of 33% of the total mitochondrial activity. SDS-PAGE and molecular exclusion chromatography revealed a native tetrameric protein of 160 kDa. Kinetic analysis showed Km values of 5 and 50 microM for propionaldehyde and NAD(+), respectively, and a Vm value of 1,300 nmol (min x mg protein)(-1). NAD(+) and NADH stimulated the esterase activity of the purified aldehyde dehydrogenase. The present data indicated that the E. gracilis aldehyde dehydrogenase has kinetic and structural properties similar to those of human aldehyde dehydrogenases class 1 and 2.

Inhibition of mitochondrial aldehyde dehydrogenase by nitric oxide-mediated S-nitrosylation

Mitochondrial aldehyde dehydrogenase (ALDH2) is responsible for the metabolism of acetaldehyde and other toxic lipid aldehydes. Despite many reports about the inhibition of ALDH2 by toxic chemicals, it is unknown whether nitric oxide (NO) can alter the ALDH2 activity in intact cells or in vivo animals. The aim of this study was to investigate the effects of NO on ALDH2 activity in H4IIE-C3 rat hepatoma cells. NO donors such as S-nitrosoglutathione (GSNO), S-nitroso-N-acetylpenicillamine, and 3-morpholinosydnonimine significantly increased the nitrite concentration while they inhibited the ALDH2 activity. Addition of GSH-ethylester (GSH-EE) completely blocked the GSNO-mediated ALDH2 inhibition and increased nitrite concentration. To directly demonstrate the NO-mediated S-nitrosylation and inactivation, ALDH2 was immunopurified from control or GSNO-treated cells and subjected to immunoblot analysis. The anti-nitrosocysteine antibody recognized the immunopurified ALDH2 only from the GSNO-treated samples. All these results indicate that S-nitrosylation of ALDH2 in intact cells leads to reversible inhibition of ALDH2 activity.

Isolation and characterization of an aldehyde dehydrogenase encoded by the aldB gene of Escherichia coli

An aldehyde dehydrogenase was detected in crude cell extracts of Escherichia coli DH5alpha. Growth studies indicated that the aldehyde dehydrogenase activity was growth phase dependent and increased in cells grown with ethanol. The N-terminal amino acid sequence of the purified enzyme identified the latter as an aldehyde dehydrogenase encoded by aldB, which was thought to play a role in the removal of aldehydes and alcohols in cells that were under stress. The purified enzyme showed an estimated molecular mass of 220 +/- 8 kDa, consisting of four identical subunits, and preferred to use NADP and acetaldehyde. MgCl2 increased the activity of the NADP-dependent enzyme with various substrates. A comparison of the effect of Mg2+ ions on the bacterial enzyme with the effect of Mg2+ ions on human liver mitochondrial aldehyde dehydrogenase revealed that the bacterial enzyme shared kinetic properties with the mammalian enzyme. An R197E mutant of the bacterial enzyme appeared to retain very little NADP-dependent activity on acetaldehyde.

Mutations in mitochondrial aldehyde dehydrogenase (ALDH2) change cofactor affinity and segregate with voluntary alcohol consumption in rats

Genetic factors influence alcohol consumption and alcoholism. A number of groups have bred alcohol drinker and non drinker rat strains, but genetic determinants remain unknown. The University of Chile rat lines UChA (low drinkers) and UChB (high drinkers) display differences in the relative K(m) for NAD+ of mitochondrial aldehyde dehydrogenase (ALDH2) but no V(max) differences. The relative K(m) differences may be due to mitochondrial changes or to genetic differences coding for ALDH2. We investigated whether there are differences in the coding regions of ALDH2 cDNA in these lines and whether the Aldh2 genotype predicts the phenotype of alcohol consumption and the K(m) of ALDH2 for NAD+. Liver cDNA was prepared, and the Aldh2 transcript was amplified, cloned and sequenced. Genotyping was conducted by DNA amplification and restriction enzyme digestion. When compared to Aldh21 of Sprague-Dawley, 94% of the UChA (low drinker) rats (n = 61), presented a mutation that changes Gln67 to Arg in the mature enzyme (allele referred to as Aldh22). In UChB (high drinker) rats (n = 69), 58% presented the Aldh21 allele, while 42% presented the Gln67Arg change plus a second mutation that changed Glu479 to Lys (allele Aldh23). The Aldh22 allele was absent in high drinker rats. Rats of different Aldh2 genotypes displayed marked phenotypic differences in both ethanol consumption (g/kg/day; means +/- SE): (Aldh21/Aldh21) = 5.7 +/- 0.2, (Aldh22/Aldh22) = 0.9 +/- 0.2 and (Aldh23/Aldh23) = 4.6 +/- 0.2; and K(m)s for NAD+ of 43 +/- 3 microm, 132 +/- 13 microm and 41 +/- 2 microm, respectively (Aldh22 versus Aldh21 or Aldh23; P < 0.0001 for both phenotypes). Overall, the data show that alleles of Aldh2 strongly segregate with the phenotype of ethanol consumption and the relative K(m) for NAD+ of ALDH2. Bases mutated suggest that non drinker Aldh22 is ancestral with regard to the coding changes in either Aldh21 or Aldh23, variants which would allow ethanol consumption and may provide an evolutionary advantage by promoting calorie intake from fermented products along with carbohydrates.

Use of an "acetaldehyde clamp" in the determination of low-KM aldehyde dehydrogenase activity in H4-II-E-C3 rat hepatoma cells

The high-affinity (K(M)<1 microM) mitochondrial class 2 aldehyde dehydrogenase (ALDH2) metabolizes most of the acetaldehyde generated in the hepatic oxidation of ethanol. H4-II-E-C3 rat hepatoma cells have been found to express ALDH2. We report a method to assess ALDH2 activity in intact hepatoma cells that does not require mitochondrial isolation. To determine only the high-affinity ALDH2 activity it is necessary to keep constant low concentrations of acetaldehyde in the cells to minimize its metabolism by high-K(M) aldehyde dehydrogenases. To maintain both low and constant concentrations of acetaldehyde we used an "acetaldehyde clamp," which keeps acetaldehyde at a concentration of 4.2+/-0.4 microM. The clamp is attained by addition of excess yeast alcohol dehydrogenase, 14C-ethanol, and oxidized form of nicotinamide adenine dinucleotide (NAD(+)) to the hepatoma cell culture medium. The concentration of 14C-acetaldehyde attained follows the equilibrium constant of the alcohol dehydrogenase reaction. Thus, 14C-acetate is generated virtually by the low-K(M) aldehyde dehydrogenase activity. 14C-acetate is separated from the culture medium by an anionic resin and its radioactivity is determined. We showed that (1) acetate production is linear for 120 min, (2) addition of 160 microM cyanamide to the culture medium leads to a 75%-80% reduction of acetate generated, and (3) ALDH2 activity is dependent on cell-to-cell contact and increases after cells reach confluence. The clamp system allows the determination of ALDH2 activity in less than one million H4-II-E-C3 rat hepatoma cells. The specificity and sensitivity of the "acetaldehyde clamp" assay should be of value in evaluation of the effects of new agents that modify Aldh2 gene expression, as well as in the study of ALDH2 regulation in intact cells.

Synthesis of potential antidipsotropic isoflavones: inhibitors of the mitochondrial monoamine oxidase-aldehyde dehydrogenase pathway

Recently we have shown that daidzin, the major active principle of an ancient herbal treatment for "alcohol addiction", suppresses ethanol intake in alcohol-preferring laboratory animals. Further, we have identified the monoamine oxidase (MAO)-aldehyde dehydrogenase (ALDH-2) pathway of the mitochondria as the potential site of action of daidzin. Daidzin analogues that potently inhibit ALDH-2 but have no or little effect on MAO are most antidipsotropic, whereas those that also inhibit MAO exhibit little, if any, antidipsotropic activity. Therefore, in the design and synthesis of more potent antidipsotropic analogues, structural features important for the inhibition of both ALDH-2 and MAO must be taken into consideration. To gain further information on the structure-activity relationships at the inhibitor binding sites of ALDH-2 and MAO, we prepared 44 analogues of daidzin and determined their potencies for ALDH-2 and MAO inhibition. Results indicate that a sufficient set of criteria for a potent antidipsotropic analogue is an isoflavone with a free 4'-OH function and a straight-chain alkyl substituent at the 7 position that has a terminal polar function such as -OH, -COOH, or -NH(2). The preferable chain lengths for the 7-O-omega-hydroxy, 7-O-omega-carboxy, and 7-O-omega-amino subsitutents are 2 < or = n < or = 6, 5 < or = n < or = 10, and n > or = 4, respectively. Analogues that meet these criteria have increased potency for ALDH-2 inhibition and/or decreased potency for MAO inhibition and therefore are likely to be potent antidipsotropic agents.

Making an Oriental equivalent of the yeast cytosolic aldehyde dehydrogenase as well as making one with positive cooperativity in coenzyme binding by mutations of glutamate 492 and arginine 480

Yeast has at least three partially characterized aldehyde dehydrogenases. Previous studies by gene disrupted in our laboratory revealed that the Saccharomyces cerevisiae cytosol ALDH1 played an important role in ethanol metabolism as did the class 2 mitochondrial enzyme. To date, few mutagenesis studies have been performed with the yeast enzymes. An important human variant of ALDH is one found in Asian People. In it, the glutamate at position 487 is replaced by a lysine. This glutamate interacts with an arginine (475) that is located in the subunit that makes up the dimer pair in the tetrameric enzyme. Sequence alignment shows that these two residues are located at positions 492 and 480, respectively, in the yeast class 1 enzyme which shares just 45% sequence identity with the human enzymes. Mutating glutamate 492 to lysine produced an enzyme with altered kinetic properties when compared to the wild-type glutamate-enzyme. The K(m) for NADP of E492K increased to nearly 3600 microM compare to 40 microM for wild-type enzyme. The specific activity decreased more than 10-fold with respect to the recombinant wild-type yeast enzyme. Moreover, substituting a glutamine for a glutamate was not detrimental in that the E492Q had wild-type-like K(m) for NADP and V(max). These properties were similar to the changes found with the human class 2 E487K mutant form. Further, mutating arginine 480 to glutamine produced an enzyme that exhibited positive cooperativity in NADP binding. The K(m) for NADP increased 11-fold with a Hill coefficient of 1.6. The NADP-dependent activity of R480Q mutant was 60% of wild-type enzyme. Again, these results are very similar to what we recently showed to occur with the human enzyme [Biochemistry 39 (2000) 5295-5302]. These findings show that the even though the glutamate and arginine residues are not conserved, similar changes occur in both the human and the yeast enzyme when either is mutated.

Differences in the roles of conserved glutamic acid residues in the active site of human class 3 and class 2 aldehyde dehydrogenases

Although the three-dimensional structure of the dimeric class 3 rat aldehyde dehydrogenase has recently been published (Liu ZJ et al., 1997, Nature Struct Biol 4:317-326), few mechanistic studies have been conducted on this isoenzyme. We have characterized the enzymatic properties of recombinant class 3 human stomach aldehyde dehydrogenase, which is very similar in amino acid sequence to the class 3 rat aldehyde dehydrogenase. We have determined that the rate-limiting step for the human class 3 isozyme is hydride transfer rather than deacylation as observed for the human liver class 2 mitochondrial enzyme. No enhancement of NADH fluorescence was observed upon binding to the class 3 enzyme, while fluorescence enhancement of NADH has been previously observed upon binding to the class 2 isoenzyme. It was also observed that binding of the NAD cofactor inhibited the esterase activity of the class 3 enzyme while activating the esterase activity of the class 2 enzyme. Site-directed mutagenesis of two conserved glutamic acid residues (209 and 333) to glutamine residues indicated that, unlike in the class 2 enzyme, Glu333 served as the general base in the catalytic reaction and E209Q had only marginal effects on enzyme activity, thus confirming the proposed mechanism (Hempel J et al., 1999, Adv Exp Med Biol 436:53-59). Together, these data suggest that even though the subunit structures and active site residues of the isozymes are similar, the enzymes have very distinct properties besides their oligomeric state (dimer vs. tetramer) and substrate specificity.

Generation of acetate and production of ethyl-lysine in the reaction of acetaldehyde plus serum albumin

We report that incubation of acetaldehyde with bovine serum albumin results in the generation of acetate in a reaction that is directly proportional to the levels of albumin and exponentially dependent on the concentration of acetaldehyde. Both reactants need to be present for acetate to be formed. The oxidation of acetaldehyde into acetate requires that a reduced product also be generated in the reaction. It was hypothesized that, at high concentrations, acetaldehyde itself may reduce the Schiff bases formed in the reaction of a second molecule of acetaldehyde with amino groups in the protein, resulting in the generation of ethyl-lysine moieties. Incubation of acetaldehyde (240 mM) with bovine serum albumin was found to generate ethyl-lysine moieties as determined by a specific monoclonal antibody. Immunization of rabbits with products of the reaction of bovine serum albumin with acetaldehyde led to the generation of antibodies that reacted to reduced adducts formed in the reaction of acetaldehyde and proteins in the presence of sodium cyanoborohydride. However, the generation of acetate from acetaldehyde plus albumin was 60-fold greater than could be explained by the reduction of Schiff bases, as determined by the maximal incorporation of [14C]-acetaldehyde into an acid-precipitable protein fraction. Thus, other mechanisms to generate acetate also occur. The present findings provide an explanation for earlier reports that acetaldehyde adducts formed under "nonreducing" conditions generate antibodies that recognize reduced acetaldehyde protein adducts. However, the mechanism by which the bulk of acetate is generated in the reaction of acetaldehyde and bovine serum albumin remains to be elucidated.

Molecular cloning, characterization, and potential roles of cytosolic and mitochondrial aldehyde dehydrogenases in ethanol metabolism in Saccharomyces cerevisiae

The full-length DNAs for two Saccharomyces cerevisiae aldehyde dehydrogenase (ALDH) genes were cloned and expressed in Escherichia coli. A 2,744-bp DNA fragment contained an open reading frame encoding cytosolic ALDH1, with 500 amino acids, which was located on chromosome XVI. A 2,661-bp DNA fragment contained an open reading frame encoding mitochondrial ALDH5, with 519 amino acids, of which the N-terminal 23 amino acids were identified as the putative leader sequence. The ALDH5 gene was located on chromosome V. The commercial ALDH (designated ALDH2) was partially sequenced and appears to be a mitochondrial enzyme encoded by a gene located on chromosome XV. The recombinant ALDH1 enzyme was found to be essentially NADP dependent, while the ALDH5 enzyme could utilize either NADP or NAD as a cofactor. The activity of ALDH1 was stimulated two- to fourfold by divalent cations but was unaffected by K+ ions. In contrast, the activity of ALDH5 increased in the presence of K+ ions: 15-fold with NADP and 40-fold with NAD, respectively. Activity staining of isoelectric focusing gels showed that cytosolic ALDH1 contributed 30 to 70% of the overall activity, depending on the cofactor used, while mitochondrial ALDH2 contributed the rest. Neither ALDH5 nor the other ALDH-like proteins identified from the genomic sequence contributed to the in vitro oxidation of acetaldehyde. To evaluate the physiological roles of these three ALDH isoenzymes, the genes encoding cytosolic ALDH1 and mitochondrial ALDH2 and ALDH5 were disrupted in the genome of strain TWY397 separately or simultaneously. The growth of single-disruption delta ald1 and delta ald2 strains on ethanol was marginally slower than that of the parent strain. The delta ald1 delta ald2 double-disruption strain failed to grow on glucose alone, but growth was restored by the addition of acetate, indicating that both ALDHs might catalyze the oxidation of acetaldehyde produced during fermentation. The double-disruption strain grew very slowly on ethanol. The role of mitochondrial ALDH5 in acetaldehyde metabolism has not been defined but appears to be unimportant.

Structure of mitochondrial aldehyde dehydrogenase: the genetic component of ethanol aversion

BACKGROUND

The single genetic factor most strongly correlated with reduced alcohol consumption and incidence of alcoholism is a naturally occurring variant of mitochondrial aldehyde dehydrogenase (ALDH2). This variant contains a glutamate to lysine substitution at position 487 (E487K). The E487K variant of ALDH2 is found in approximately 50% of the Asian population, and is associated with a phenotypic loss of ALDH2 activity in both heterozygotes and homozygotes. ALDH2-deficient individuals exhibit an averse response to ethanol consumption, which is probably caused by elevated levels of blood acetaldehyde. The structure of ALDH2 is important for the elucidation of its catalytic mechanism, to gain a clear understanding of the contribution of ALDH2 to the genetic component of alcoholism and for the development of specific ALDH2 inhibitors as potential drugs for use in the treatment of alcoholism.

RESULTS

The X-ray structure of bovine ALDH2 has been solved to 2.65 A in its free form and to 2.75 A in a complex with NAD+. The enzyme structure contains three domains; two dinucleotide-binding domains and a small three-stranded beta-sheet domain, which is involved in subunit interactions in this tetrameric enzyme. The E487K mutation occurs in this small oligomerization domain and is located at a key interface between subunits immediately below the active site of another monomer. The active site of ALDH2 is divided into two halves by the nicotinamide ring of NAD+. Adjacent to the A-side (Pro-R) of the nicotinamide ring is a cluster of three cysteines (Cys301, Cys302 and Cys303) and adjacent to the B-side (Pro-S) are Thr244, Glu268, Glu476 and an ordered water molecule bound to Thr244 and Glu476.

CONCLUSIONS

Although there is a recognizable Rossmann-type fold, the coenzyme-binding region of ALDH2 binds NAD+ in a manner not seen in other NAD+-binding enzymes. The positions of the residues near the nicotinamide ring of NAD+ suggest a chemical mechanism whereby Glu268 functions as a general base through a bound water molecule. The sidechain amide nitrogen of Asn169 and the peptide nitrogen of Cys302 are in position to stabilize the oxyanion present in the tetrahedral transition state prior to hydride transfer. The functional importance of residue Glu487 now appears to be due to indirect interactions of this residue with the substrate-binding site via Arg264 and Arg475.

Allosteric inhibition of human liver aldehyde dehydrogenase by the isoflavone prunetin

Isoflavonoid derivatives including prunetin (4',5-dihydroxy-7-methoxyisoflavone) were shown to be potent inhibitors of human aldehyde dehydrogenases (Keung W-M and Vallee BL, Proc Natl Acad Sci USA 90: 1247-1251, 1993). The inhibition reaction was reinvestigated using recombinantly expressed human aldehyde dehydrogenases. The kinetic analyses showed that prunetin inhibits competitively against both NAD and propionaldehyde with the mitochondrial and cytoplasmic enzymes. The Ki value for the mitochondrial enzyme was much lower than for the cytoplasmicenzyme. A mixed pattern of inhibition was obtaiend with the mitochondrial enzyme in the presence of Mg2+. Only one mole of prunetin binds per mole of tetrameric mitochondrial enzyme, which remains unaltered in the presence of Mg2+. Prunetin did not displace NADH from the enzyme-NADH complex. Propionaldehyde did not reverse the loss of fluorescence obtained due to enzyme-prunetin complex formation, indicating that prunetin may not be interacting at the substrate site. The esterase activity of the mitochondrial enzyme was also inhibited by prunetin in a competitive manner. The replacement of lysine 192 by glutamine resulted in a mutant with a 20% kcat and a 100-fold increase in the Km for NAI) compared with the native enzyme. However, the Ki value of prunetin against NAD was similar to that observed with the native enzyme. Prunetin, even at a very high concentration, was not an inhibitor of alcohol and malate dehydrogenase. It was concluded that prunetin may act as an allosteric inhibitor of aldehyde dehydrogenase.

Heterotetramers of human liver mitochondrial (class 2) aldehyde dehydrogenase expressed in Escherichia coli. A model to study the heterotetramers expected to be found in Oriental people

About 50% of the Oriental population have less liver mitochondrial aldehyde dehydrogenase (ALDH2) activity than do other people. It was found that they possessed an enzyme with a lysine at position 487 (E487K) instead of glutamate (Glu487). We previously found that the Km for NAD of recombinant human and rat E487K enzymes increased more than 150-fold (Farrés, J., Wang X., Takahashi, K., Cunningham, S. J. , Wang, T.T., and Weiner, H (1994) J. Biol. Chem. 269, 13854-13860). Many aldehyde dehydrogenase-deficient people were found to be heterozygous when genotyped for ALDH2. In this study liver tissue from heterozygous people was analyzed and found to possess mRNAs for both the glutamate and the lysine subunits. Western blot analysis showed that the glutamate subunit was present. The cDNAs for Glu487 and E487K were coexpressed on one plasmid in Escherichia coli, and the enzyme forms were separated from each other by isoelectric focusing to show that heterotetramers were formed. Only one Km value for NAD could be measured with the purified heterotetrameric enzyme that possessed just 16-18% activity of the glutamate homotetrameric enzyme. The E487K homotetramers had 8% specific activity of the Glu487 enzyme. There was no pre-steady state burst of NADH formation with the heterotetramer, a property found with the glutamate enzyme. Similar results were found for the coexpressed rat liver enzyme, except that a higher specific activity, 48%, was obtained. Thus, we conclude that presence of the lysine subunit altered the activity of the glutamate subunit in the heterotetramer to make it function more like an E487K enzyme.

Effect of aniline on ethanol oxidation and carbohydrate metabolism in isolated hepatocytes

The addition of aniline to isolated hepatocytes derived from fasted rats and incubated with ethanol, caused a 30-60% decrease in the rate of ethanol oxidation. The degree of inhibition was dependent on aniline concentration, 5 mM causing near-maximal inhibition. Aniline reduced the activity of alcohol dehydrogenase in a noncompetitive manner, but had no effect on aldehyde dehydrogenase activity nor on reducing-equivalent transfer between the cytoplasm and mitochondria. The inhibition of alcohol dehydrogenase by aniline was associated with a decrease in the inhibitory effects of ethanol on glycolysis. Aniline, added to hepatocytes in the presence or absence of ethanol, inhibited gluconeogenesis from lactate and pyruvate, but not from sorbitol or fructose.

Possible role of liver cytosolic and mitochondrial aldehyde dehydrogenases in acetaldehyde metabolism

To provide a molecular basis for understanding the possible mechanism of action of antidipsotropic agents in laboratory animals, aldehyde dehydrogenase (ALDH) isozymes were purified and characterized from the livers of hamsters and rats and compared with those from humans. The mitochondrial ALDHs from these species exhibit virtually identical kinetic properties in the oxidation and hydrolysis reactions. However, the cytosolic ALDH of human origin differs significantly from those of the rodents. Thus, for human ALDH-1, the Km value for acetaldehyde is 180 +/- 10 micromolar, whereas those for hamster ALDH-1 and rat ALDH-1 are 12 +/- 3 and 15 +/- 3 micromolar, respectively. Km values determined at pH 9.5 are virtually identical to those measured at pH 7.5. In vitro human ALDH-1 is 10 times less sensitive to disulfiram inhibition than are the hamster and rat cytosolic ALDHs. Competition between acetaldehyde and aromatic aldehydes or naphthaldehydes for the binding and catalytic sites of ALDHs shows their topography to be complex with more than one binding site. This also follows from data on substrate inhibition and activation, effects of NAD+ on ALDH-catalyzed hydrolysis of p-nitrophenyl esters, substrate specificity toward aldehydes and p-nitrophenyl esters, and inhibition by disulfiram in relation to oxidation and hydrolysis catalyzed by the ALDHs. The data further suggest that acetaldehyde cannot be considered as a "standard" ALDH substrate for studies aimed at aromatic ALDH substrates, e.g. biogenic aldehydes. Apparently, in human liver, only mitochondrial ALDH oxidizes acetaldehyde at physiological concentrations, whereas in hamster or rat liver, both the mitochondrial and cytosolic isozymes will do so.

Hormonal and chemical influences on the expression of class 2 aldehyde dehydrogenases in rat H4IIEC3 and human HuH7 hepatoma cells

We studied the effect a variety of hormones and chemical stimuli on the activity of low Km aldehyde dehydrogenase (ALDH) in rat H4IIEC3 hepatoma cells and ALDH activity in human HuH7 hepatoma cells. The low Km enzyme in H4IIEC3 cells reflects ALDH2 activity, and the ALDH activity in HuH7 likely represents ALDH5. Of the steroid hormone family, thyroid hormone, progesterone, and dihydrotestosterone increased low Km ALDH activity approximately 50%, whereas dexamethasone and estradiol had little effect. Insulin decreased the activity of low Km ALDH. None of these hormones affected the activity of ALDH in HuH7 cells. Among second messengers, 8-bromo-cAMP and A23187 increased low Km ALDH activity; HuH7 ALDH activity again was unchanged. Exposure of the cells to 22 mM ethanol reduced low Km activity by approximately 20%, whereas hydrogen peroxide, tumor necrosis factor-alpha, and interleukin-1 beta had little effect. Ultraviolet light increased the HuH7 ALDH activity. Retinaldehyde or retinolc acid reduced the HuH7 ALDH activity, but had no effect on low Km ALDH activity. These data suggest that low Km ALDH2 can be regulated by hormones and may not be constitutive as previously thought, and that the HuH7 ALDH is regulated differently.

Inhibition of gene expression by triple helix formation in hepatoma cells

The aim of this study was to selectively inhibit human mitochondrial aldehyde dehydrogenase (ALDH2) gene expression by triple helix assembly. Eight 21-mer oligodeoxyribonucleotides were designed to bind to two purine-rich sequences in the 5'-flanking region of the human ALDH2 gene. Gel mobility shift assays showed that triplex formation is sequence-specific for the target duplex and the third strand oligonucleotide. In the presence of Mg2+, but absence of K+, triplex-forming oligonucleotides bind to their target sites with apparent dissociation constants (Kd) in the 10(-7) to 10(-9) M range. Potassium cation virtually suppressed the triplex formation of G-C-rich duplex DNA with natural oligonucleotides, but did not prevent triplex formation with phosphorothioate-modified oligonucleotides. Phosphorothioate-modified oligonucleotides were delivered into human hepatoma Hep G2 cells by cationic liposomes. The reduction in ALDH2 mRNA levels in the cells was determined by the competitive reverse transcription-polymerase chain reaction. One of the phosphorothioate-modified oligonucleotides designed to forma an antiparallel triplex with a target in the 5'-flanking region of human ALDH2 gene (-105 to -125 from the translation initiation codon ATG) reduced by 80-90% the ALDH2 mRNA levels without affecting albumin mRNA levels. Data suggest that triple-helix formation may provide a means to selectively inhibit hepatic ALDH2 gene expression for therapeutic use.

The aldehyde dehydrogenase ALDH2*2 allele exhibits dominance over ALDH2*1 in transduced HeLa cells

Individuals heterozygous or homozygous for the variant aldehyde dehydrogenase (ALDH2) allele (ALDH2*2), which encodes a protein differing only at residue 487 from the normal protein, have decreased ALDH2 activity in liver extracts and experience cutaneous flushing when they drink alcohol. The mechanisms by which this allele exerts its dominant effect is unknown. To study this effect, the human ALDH2*1 cDNA was cloned and the ALDH2*2 allele was generated by site-directed mutagenesis. These cDNAs were transduced using retroviral vectors into HeLa and CV1 cells, which do not express ALDH2. The normal allele directed synthesis of immunoreactive ALDH2 protein (ALDH2E) with the expected isoelectric point. Extracts of these cells contained increased aldehyde dehydrogenase activity with low Km for the aldehyde substrate. The ALDH2*2 allele directed synthesis of mRNA and immunoreactive protein (ALDH2K), but the protein lacked enzymatic activity. When ALDH2*1-expressing cells were transduced with ALDH2*2 vectors, both mRNAs were expressed and immunoreactive proteins with isoelectric points ranging between those of ALDH2E and ALDH2K were present, indicating that the subunits formed heteromers. ALDH2 activity in these cells was reduced below that of the parental ALDH2*1-expressing cells. Thus, the ALDH2*2 allele is sufficient to cause ALDH2 deficiency in vitro.

Daidzin suppresses ethanol consumption by Syrian golden hamsters without blocking acetaldehyde metabolism

Daidzin is a potent, selective, and reversible inhibitor of human mitochondrial aldehyde dehydrogenase (ALDH) that suppresses free-choice ethanol intake by Syrian golden hamsters. Other ALDH inhibitors, such as disulfiram (Antabuse) and calcium citrate carbimide (Temposil), have also been shown to suppress ethanol intake of laboratory animals and are thought to act by inhibiting the metabolism of acetaldehyde produced from ingested ethanol. To determine whether or not daidzin inhibits acetaldehyde metabolism in vivo, plasma acetaldehyde in daidzin-treated hamsters was measured after the administration of a test dose of ethanol. Daidzin treatment (150 mg/kg per day i.p. for 6 days) significantly suppresses (> 70%) hamster ethanol intake but does not affect overall acetaldehyde metabolism. In contrast, after administration of the same ethanol dose, plasma acetaldehyde concentration in disulfiram-treated hamsters reaches 0.9 mM, 70 times higher than that of the control. In vitro, daidzin suppresses hamster liver mitochondria-catalyzed acetaldehyde oxidation very potently with an IC50 value of 0.4 microM, which is substantially lower than the daidzin concentration (70 microM) found in the liver mitochondria of daidzin-treated hamsters. These results indicate that (i) the action of daidzin differs from that proposed for the classic, broad-acting ALDH inhibitors (e.g., disulfiram), and (ii) the daidzin-sensitive mitochondrial ALDH is not the one and only enzyme that is essential for acetaldehyde metabolism in golden hamsters.

Cloning and expression of the full-length cDNAS encoding human liver class 1 and class 2 aldehyde dehydrogenase

The amino acid sequences of both human class 1 and 2 aldehyde dehydrogenase (ALDH) and the sequences of the genes coding for them are known. Based on this sequence data, we designed primers and isolated the full-length cDNAs encoding both isozymes from a human liver mRNA pool. cDNAs were subcloned in the plasmid pT7-7 and expressed in Escherichia coli with a yield of approximately 3 mg ALDH protein/liter of cell culture, although only one-third of the enzyme was soluble. The soluble recombinantly expressed ALDHs were purified to homogeneity using a hydroxyacetophenone-Sepharose affinity column. The mitochondrial isozyme had a subunit molecular weight of 55 kDa, an isoelectric point of 4.9, and a specific activity of 1.10 units/mg, which were in good agreement with that from the native enzyme. The expressed cytosolic isozyme had the same subunit molecular weight (55 kDa) and pI (5.4) as that reported for the native enzyme and had a specific activity of 0.26 units/mg. The expressed mitochondrial isozyme could be recognized by antibodies raised against rat mitochondrial ALDH, whereas the cytosolic isozyme could be recognized by antibody raised against horse cytosolic ALDH.

Crystallization and preliminary X-ray investigation of bovine liver mitochondrial aldehyde dehydrogenase

Aldehyde dehydrogenase from bovine liver mitochondria has been crystallized using the sitting drop method of vapor diffusion at 22 degrees C. The crystals formed from solutions containing, 40 mM-sodium citrate, 1 mM-NAD+ and 21% to 24% polyethylene glycol 3400 (pH 5.3 to 5.5). X-ray diffraction data collected from these crystals indicate that the crystals belong to the orthorhombic space group P2(1)2(1)2(1) with cell dimensions of a = 153.7 A, b = 159.37 A and c = 101.45 A. The crystals diffract to at least 2.9 A and a tetramer may comprise the asymmetric unit.

Polymorphism of the rat liver mitochondrial aldehyde dehydrogenase cDNA

In humans, a deficiency in mitochondrial aldehyde dehydrogenase (Class 2 ALDH) activity due to a single base-pair exchange in its structural gene serves as a deterrent to excessive alcohol consumption. Differences in Class 2 ALDH isozyme patterns on isoelectric focusing gels have been observed in the selectively bred, alcohol-preferring (P) and alcohol-nonpreferring (NP) lines of rats. To determine whether the differences are the result of sequence variation in the structural gene, we sequenced the cDNAs for Class 2 ALDH from P and NP rats. A synonymous exchange was seen in the codon for amino acid 473 in both lines, when compared with published sequences. Additionally, when the cDNA from P rats was used as reference, a substitution (G for A) was identified in the cDNA of NP rats which changes amino acid 67 from Gln (CAG codon; ALDH2Q allele) to Arg (CGG codon; ALDH2R allele). The Arg for Gln substitution makes the enzyme more basic and could account for the different electrophoretic mobilities. To determine whether the polymorphism was associated with drinking behavior, we genotyped the ALDH2 locus by amplifying rat genomic DNA encompassing the nucleotide exchange followed by probing with allele-specific oligonucleotides. There are highly significant differences in the frequencies of the two alleles in the P and NP rat lines. The frequency of the ALDH2R allele is 63% in the NP line and only 18% in the P line, whereas the frequency of the ALDH2Q allele is 82% in the P line and 37% in the NP line.

Purification and characterization of catalytically active precursor of rat liver mitochondrial aldehyde dehydrogenase expressed in Escherichia coli

The cDNA coding for the precursor (p-ALDH) or mature (m-ALDH) rat liver mitochondrial aldehyde dehydrogenase was cloned in an expression vector pT7-7 and expressed in Escherichia coli strain BL21 (DE3)/plysS. The p-ALDH expressed in E. coli was a soluble tetrameric protein. It exhibited virtually the same specific activity and KmS for substrates as m-ALDH. N-terminal sequencing of isolated p-ALDH provided the evidence that the catalytic activity was not derived from a partially processed mature-like enzyme. The assembly states of both p-ALDH and m-ALDH synthesized in a rabbit reticulocyte lysate were also determined. Both of them were monomers and could not bind to a 5'-AMP-Sepharose column, showing that the monomeric form of the enzyme is inactive. The stabilities in vivo and in vitro were compared between p-ALDH and m-ALDH expressed in E. coli. p-ALDH was less stable than was m-ALDH both in vivo and in vitro. Thus, although the conformations of p-ALDH and m-ALDH are similar, the presence of signal peptide is a destabilizing factor to the p-ALDH. p-ALDH expressed in E. coli could bind to and be translocated into rat liver mitochondria, however, with lower efficiency when compared to the import of p-ALDH synthesized in reticulocyte lysate.

Molecular cloning of the mitochondrial aldehyde dehydrogenase gene of Saccharomyces cerevisiae by genetic complementation

Mutants of Saccharomyces cerevisiae deficient in mitochondrial aldehyde dehydrogenase (ALDH) activity were isolated by chemical mutagenesis with ethyl methanesulfonate. The mutants were selected by their inability to grow on ethanol as the sole carbon source. The ALDH mutants were distinguished from alcohol dehydrogenase mutants by an aldehyde indicator plate test and by immunoscreening. The ALDH gene was isolated from a yeast genomic DNA library on a 5.7-kb insert of a recombinant DNA plasmid by functional complementation of the aldh mutation in S. cerevisiae. An open reading frame which specifies 533 codons was found within the 2.0-kb BamHI-BstEII fragment in the 5.7-kb genomic insert which can encode a protein with a molecular weight of 58,630. The N-terminal portion of the protein contains many positively charged residues which may serve as a signal sequence that targets the protein to the mitochondria. The amino acid sequence of the proposed mature yeast enzyme shows 30% identity to each of the known ALDH sequences from eukaryotes or prokaryotes. The amino acid residues corresponding to mammalian cysteine 302 and glutamates 268 and 487, implicated to be involved at the active site, were conserved. S. cerevisiae ALDH was found to be localized in the mitochondria as a tetrameric enzyme. Thus, that organelle is responsible for acetaldehyde oxidation, as was found in mammalian liver.

Sequence of the precursor of bovine liver mitochondrial aldehyde dehydrogenase as determined from its cDNA, its gene, and its functionality

The partial sequence coding for bovine liver mitochondrial aldehyde dehydrogenase was previously reported (Farres, J., Guan, K.L., and Weiner, H., 1989, Eur. J. Biochem. 180, 67-74); cDNA coding for the N-terminal region has now been obtained by primer extension. The deduced 520 amino acids contained 499 residues corresponding to the mature mitochondrial aldehyde dehydrogenase which was one residue shorter than other mammalian mitochondrial aldehyde dehydrogenases. The N-terminal signal peptide had only 21 amino acids but showed high sequence identity to the signal sequences of human and rat enzymes, which have 17 and 19 amino acids, respectively, and had general characteristic features of typical mitochondrial signal peptides. Although the direct protein sequence of the signal peptide was not available, a mRNA coding for a 57-kDa precursor was proven to exist. The bovine signal peptide was able to direct the import of bovine aldehyde dehydrogenase precursor into isolated liver mitochondria. The complete nuclear gene coding for bovine mitochondrial aldehyde dehydrogenase was found to span at least 25 kb. A restriction map of this gene was constructed. Two potential transcription start sites were identified by primer extension and S1 nuclease protection. Several SpI transcription factor recognition sequences were found in the 5'-region of the gene. TATA box and CAAT box like sequences were found to be exist 18 and 64 bp upstream from the major transcription start site, respectively.

Liver mitochondrial aldehyde dehydrogenase: in vitro expression, in vitro import, and effect of alcohols on import

An in vitro expression plasmid (pGRAP) that contained the cDNA coding for the rat mitochondrial aldehyde dehydrogenase precursor was constructed, mRNA was synthesized then translated, and the in vitro synthesized precursor of aldehyde dehydrogenase was used in an in vitro import assay. As expected the 19 amino acid signal peptide of the precursor allowed import of the precursor into rat liver mitochondria. This in vitro system was used to examine the effect of alcohols on import. It was found that the alcohols (ethyl, butyl, hexyl, and octyl) tested inhibited the import of the aldehyde dehydrogenase precursor. Pretreatment of the mitochondria with alcohol was responsible for the inhibition. The inhibition appeared to be relatively specific for pre-aldehyde dehydrogenase as the precursor of ornithine transcarbamylase was still imported in the presence of alcohols. Of potential physiological significance was finding that ethanol inhibited import in a dose-response fashion; 50% inhibition occurred at 75 mM, a concentration achievable during the ingestion of alcohol. In addition, the concentrations of alcohols required to produce an inhibitory effect on import decreased as the hydrocarbon chain length of alcohols increased. The inhibitory effect of alcohols appeared to be specific as other solvents examined did not inhibit import. We postulate that alcohols may perturb the mitochondrial membrane and affect the receptor-translocator necessary for the import of the aldehyde dehydrogenase precursor.

The metabolism of ethanol-derived acetaldehyde by alcohol dehydrogenase (EC 1.1.1.1) and aldehyde dehydrogenase (EC 1.2.1.3) in Drosophila melanogaster larvae

Both aldehyde dehydrogenase (ALDH, EC 1.2.1.3) and the aldehyde dehydrogenase activity of alcohol dehydrogenase (ADH, EC 1.1.1.1) were found to coexist in Drosophila melanogaster larvae. The enzymes, however, showed different inhibition patterns with respect to pyrazole, cyanamide and disulphiram. ALDH-1 and ALDH-2 isoenzymes were detected in larvae by electrophoretic methods. Nonetheless, in tracer studies in vivo, more than 75% of the acetaldehyde converted to acetate by the ADH ethanol-degrading pathway appeared to be also catalysed by the ADH enzyme. The larval fat body probably was the major site of this pathway.

Primary structures of rat and bovine liver mitochondrial aldehyde dehydrogenases deduced from cDNA sequences

The cDNA coding for rat liver mitochondrial aldehyde dehydrogenase was cloned and sequenced. It contained an open reading frame of 1557 bp. Of the deduced 519 amino acid residues, 19 were proposed to correspond to the signal peptide necessary to allow the protein to enter the mitochondria [Farrés, J., Guan, K.-L. and Weiner, H. (1987) Biochem. Biophys. Res. Commun. 150, 1083 - 1087]. The sequence of the 500 amino acid residues comprising the mature subunit was 96% identical to that of the corresponding human liver mitochondrial enzyme. The longest cDNA isolated coding for the bovine liver enzyme contained sequence information corresponding to residues 72 - 500 of the rat or human enzyme. The deduced protein sequence of these residues was 94% identical to those of the human enzyme. Over 60% of the bases in the coding region of the rat cDNA were G and C. These residues were clustered non-randomly and two potentially stable stem-loop structures could be calculated as existing in the mRNA. One would be found in the region coding for amino acid residues 64 - 79; a similar secondary structure could be responsible for the existing truncated cDNAs from the bovine-cDNA libraries. Comparisons with the amino acid sequences of other aldehyde dehydrogenases support the suggestion that the NAD-binding domain may be located in the middle portion of the mature enzyme.

Some comparisons of pig and sheep liver cytosolic aldehyde dehydrogenases

1. The pig enzyme was purified to homogeneity and was found to be a tetramer of apparently identical subunits. 2. The pig enzyme was found to contain 1 mol NADH/mol enzyme which is tightly bound, which is not directly involved in catalysis and which so far has not been removed from the enzyme so as to produce an active apoenzyme. 3. The pig enzyme seems to contain only one functioning active site/tetramer. 4. The pig and sheep enzymes are compared in respect of NADH binding, substrate specificity, immunological response and surface charge.