Kinetics and specificity of human liver aldehyde dehydrogenases toward aliphatic, aromatic, and fused polycyclic aldehydes

Human aldehyde dehydrogenases: potential pathological, pharmacological, and toxicological impact

Aldehyde dehydrogenases catalyze the pyridine nucleotide-dependent oxidation of aldehydes to acids. Seventeen enzymes are currently viewed as belonging to the human aldehyde dehydrogenase superfamily. Summarized herein, insofar as the information is available, are the structural composition, physical properties, tissue distribution, subcellular location, substrate specificity, and cofactor preference of each member of this superfamily. Also summarized are the chromosomal locations and organization of the genes that encode these enzymes and the biological consequences when enzyme activity is lost or substantially diminished. Broadly, aldehyde dehydrogenases can be categorized as critical for normal development and/or physiological homeostasis (1). even when the organism is in a friendly environment or (2). only when the organism finds itself in a hostile environment. The primary, if not sole, evolved raison d'être of first category aldehyde dehydrogenases appears to be to catalyze the biotransformation of a single endobiotic for which they are relatively specific and of which the resultant metabolite is essential to the organism. Most of the human aldehyde dehydrogenases for which the relevant information is available fall into this category. Second category aldehyde dehydrogenases are relatively substrate nonspecific and their evolved raison d'être seems to be to protect the organism from potentially harmful xenobiotics, specifically aldehydes or xenobiotics that give rise to aldehydes, by catalyzing their detoxification. Thus, the lack of a fully functional first category aldehyde dehydrogenase results in a gross pathological phenotype in the absence of any insult, whereas the lack of a functional second category aldehyde dehydrogenase is ordinarily of no consequence with respect to gross phenotype, but is of consequence in that regard when the organism is subjected to a relevant insult.

Ziprasidone metabolism, aldehyde oxidase, and clinical implications

Ziprasidone (Geodon, Zeldox), a recently approved atypical antipsychotic agent for the treatment of schizophrenia, undergoes extensive metabolism in humans with very little (<5%) of the dose excreted as unchanged drug. Two enzyme systems have been implicated in ziprasidone metabolism: the cytosolic enzyme, aldehyde oxidase, catalyzes the predominant reductive pathway, and cytochrome P4503A4 (CYP3A4) is responsible for two alternative oxidation pathways. The involvement of two competing pathways in ziprasidone metabolism greatly reduces the potential for pharmacokinetic interactions between ziprasidone and other drugs. Because CYP3A4 only mediates one third of ziprasidone metabolism, the likelihood of interactions between ziprasidone and CYP3A4 inhibitors/ substrates is low. Furthermore, aldehyde oxidase activity does not appear to be altered when drugs or xenobiotics are coadministered. Aldehyde oxidase, a molybdenum-containing enzyme, catalyzes the oxidation of N-heterocyclic drugs such as famciclovir and zaleplon, in addition to reducing some agents such as zonisamide. Both reactions can occur simultaneously. Although in vitro inhibitors of aldehyde oxidase have been identified, there are no reported clinical interactions with aldehyde oxidase inhibitors or inducers. There is no evidence of genetic polymorphism in aldehyde oxidase, and thus it not surprising that ziprasidone exposure demonstrates unimodality in humans. Aldehyde oxidase is unrelated to the similarly named enzyme aldehyde dehydrogenase, which is predominantly responsible for the oxidation of acetaldehyde during ethanol metabolism. Consequently, it is unlikely that there would be any pharmacokinetic interaction between ethanol and ziprasidone.

Modulating carbonyl cytotoxicity in intact rat hepatocytes by inhibiting carbonyl metabolizing enzymes. II. Aromatic aldehydes

The molecular cytotoxic mechanisms of dietary benzaldehydes towards hepatocytes and its modulation by metabolizing enzymes were compared. Salicylaldehyde was found to be the most cytotoxic followed by cinnamaldehyde and both rapidly depleted some glutathione before an inhibition of respiration occurred, which preceded cell lysis. Reactive oxygen species were formed, but lipid peroxidation was induced with cinnamaldehyde, but not salicylaldehyde. Glutathione depleted hepatocytes were more susceptible to cytotoxicity. Mitochondrial toxicity and cytotoxicity were prevented by glycolytic substrates (e.g. fructose), citric acid cycle substrates (e.g. glutamine) or cyclosporin, the mitochondrial permeability transition inhibitor. Inhibition of mitochondrial ALDH with chloral hydrate, crotonaldehyde or citral or decreasing mitochondrial NAD+ with rotenone increased cinnamaldehyde induced cytotoxicity with a much smaller effect on salicylaldehyde induced cytotoxicity. Cyanamide was the most effective ALDH inhibitor for increasing cinnamaldehyde induced cytotoxicity, presumably because cyanamide also inhibits microsomal ALDH. Although cinnamaldehyde was a better substrate than salicylaldehyde for ADH1, cytosolic NADH generators (e.g. xylitol) prevented salicylaldehyde and cinnamaldehyde cytotoxicity similarly. This could be explained as salicylaldehyde was not a substrate for the ALDHs and would then be more dependent on ADH for detoxification.

Functional specialization of maize mitochondrial aldehyde dehydrogenases

The maize (Zea mays) rf2a and rf2b genes both encode homotetrameric aldehyde dehydrogenases (ALDHs). The RF2A protein was shown previously to accumulate in the mitochondria. In vitro import experiments and ALDH assays on mitochondrial extracts from rf2a mutant plants established that the RF2B protein also accumulates in the mitochondria. RNA gel-blot analyses and immunohistolocation experiments revealed that these two proteins have only partially redundant expression patterns in organs and cell types. For example, RF2A, but not RF2B, accumulates to high levels in the tapetal cells of anthers. Kinetic analyses established that RF2A and RF2B have quite different substrate specificities; although RF2A can oxidize a broad range of aldehydes, including aliphatic aldehydes and aromatic aldehydes, RF2B can oxidize only short-chain aliphatic aldehydes. These two enzymes also have different pH optima and responses to changes in substrate concentration. In addition, RF2A, but not RF2B or any other natural ALDHs, exhibits positive cooperativity. These functional specializations may explain why many species have two mitochondrial ALDHs. This study provides data that serve as a basis for identifying the physiological pathway by which the rf2a gene participates in normal anther development and the restoration of Texas cytoplasm-based male sterility. For example, the observations that Texas cytoplasm anthers do not accumulate elevated levels of reactive oxygen species or lipid peroxidation and the kinetic features of RF2A make it unlikely that rf2a restores fertility by preventing premature programmed cell death.

Mechanisms of idiosyncratic drug reactions: the case of felbamate

Idiosyncratic drug reactions (IDR) are a specific type of drug toxicity characterized by their delayed onset, low incidence and reactive metabolite formation with little, if any, correlation between pharmacokinetics or pharmacodynamics and the toxicological outcome. As the name implies, IDR are unpredictable and often result in the post marketing failure of otherwise useful therapies. Examples of drugs, which have failed as a result of IDR in recent years, include trovafloxacin, zileuton, troglitazone, tolcapone and felbamate. To date there exists no pre-clinical model to predict these adverse drug reactions and a mechanistic understanding of these toxicities remains limited. In an attempt to better understand this class of drug toxicities and gain mechanistic insight, we have studied the IDR associated with a model compound, felbamate. Our studies with felbamate are consistent with the theory that compounds which cause IDR undergo bioactivation to a highly reactive electrophilic metabolite that is capable of forming covalent protein adducts in vivo. In additon, our data suggest that under normal physiological conditions glutathione plays a protective role in preventing IDR during felbamate therapy, further emphasizing a correlation between reactive metabolite formation and a toxic outcome. Clinical studies with felbamate have been able to demonstrate an association between reactive metabolite formation and a clinically relevant toxicity; however, additional research is required to more fully understand the link between reactive metabolite formation and the events which elicit toxicity. Going forward, it seems reasonable that screening for reactive metabolite formation in early drug discovery may be an important tool in eliminating the post-marketing failure of otherwise useful therapies.

Chaperonin GroESL mediates the protein folding of human liver mitochondrial aldehyde dehydrogenase in Escherichia coli

An efficient bacterial expression system for the human mitochondrial aldehyde dehydrogenase (ALDH2) was developed using co-overexpression of heat shock chaperone gene GroESL. On the basis of the ALDH2 amino acid sequence and cDNA sequences a full-length cDNA encoding wild-type ALDH2 was cloned from a human liver library. A mutant-type ALDH2 (ALDH2(2)) was developed using site-directed mutagenesis of the ALDH2 cDNA and also cloned. Both types of ALDH2 cDNA were subcloned for expression in Escherichia coli (E. coli), recombinant ALDH2 and ALDH2(2) were successfully expressed as soluble active enzymes following co-expression with a second plasmid construct producing GroES and GroEL, E. coli chaperonin proteins. Purified wild-type ALDH2 and mutant ALDH2(2) had a K(m) for acetaldehyde of 0.65 and 25.73 microM, respectively. Co-expression of ALDH2 with ALDH2(2) in the presence of E. coli chaperonins produced a soluble enzyme with a K(m) for acetaldehyde of 8.79 microM, suggesting that the product was a heteromer. Mitochondrial matrix hsp60 and hsp10 chaperonins are then thought to act on imported ALDH2 and are essential for accurate protein folding and multisubunit formation. Protein-protein interactions between ALDH2s and various chaperones were investigated using the yeast two-hybrid system. The wild-type and mutant-type enzymes strongly interacted with each other and GroEL and ALDH2s also interacted but only weakly. Chaperone hsp10 also interacted with hsp60 and ALDH2(1) and ALDH2(2), but again the interactions were weak ones.

Characterization of the amino acids involved in substrate specificity of nonphosphorylating glyceraldehyde-3-phosphate dehydrogenase from Streptococcus mutans

In order to address the molecular basis of the specificity of aldehyde dehydrogenase for aldehyde substrates, enzymatic characterization of the glyceraldehyde 3-phosphate (G3P) binding site of non-phosphorylating glyceraldehyde-3-phosphate dehydrogenase (GAPN) from Streptococcus mutans has been undertaken. In this work, residues Arg-124, Tyr-170, Arg-301, and Arg-459 were changed by site-directed mutagenesis and the catalytic properties of GAPN mutants investigated. Changing Tyr-170 into phenylalanine induces no major effect on k(cat) and K(m) for d-G3P in both acylation and deacylation steps. Substitutions of Arg-124 and Arg-301 by leucine and Arg-459 by isoleucine led to distinct effects on K(m), on k(cat), or on both. The rate-limiting step of the R124L GAPN remains deacylation. Pre-steady-state analysis and substrate isotope measurements show that hydride transfer remains rate-determining in acylation. Only the apparent affinity for d-G3P is decreased in both acylation and deacylation steps. Substitution of Arg-459 by isoleucine leads to a drastic effect on the catalytic efficiency by a factor of 10(5). With this R459L GAPN, the rate-limiting step is prior to hydride transfer, and the K(m) of d-G3P is increased by at least 2 orders of magnitude. Binding of NADP leads to a time-dependent formation of a charge transfer transition at 333 nm between the pyridinium ring of NADP and the thiolate of Cys-302, which is not observed with the holo-wild type. Accessibility of Cys-302 is shown to be strongly decreased within the holostructure. The substitution of Arg-301 by leucine leads to an even more drastic effect with a change of the rate-limiting step similar to that observed for R459I GAPN. Taking into account the three-dimensional structure of GAPN from S. mutans and the data of the present study, it is proposed that 1) Tyr-170 is not essential for the catalytic event, 2) Arg-124 is only involved in stabilizing d-G3P binding via an interaction with the C-3 phosphate, and 3) Arg-301 and Arg-459 participate not only in d-G3P binding via interaction with C-3 phosphate but also in positioning efficiently d-G3P relative to Cys-302 within the ternary complex GAPN.NADP.d-G3P.

Inhibition effects of furfural on alcohol dehydrogenase, aldehyde dehydrogenase and pyruvate dehydrogenase

The kinetics of furfural inhibition of the enzymes alcohol dehydrogenase (ADH; EC 1.1.1.1), aldehyde dehydrogenase (AlDH; EC 1.2.1.5) and the pyruvate dehydrogenase (PDH) complex were studied in vitro. At a concentration of less than 2 mM furfural was found to decrease the activity of both PDH and AlDH by more than 90%, whereas the ADH activity decreased by less than 20% at the same concentration. Furfural inhibition of ADH and AlDH activities could be described well by a competitive inhibition model, whereas the inhibition of PDH was best described as non-competitive. The estimated K(m) value of AlDH for furfural was found to be about 5 microM, which was lower than that for acetaldehyde (10 microM). For ADH, however, the estimated K(m) value for furfural (1.2 mM) was higher than that for acetaldehyde (0.4 mM). The inhibition of the three enzymes by 5-hydroxymethylfurfural (HMF) was also measured. The inhibition caused by HMF of ADH was very similar to that caused by furfural. However, HMF did not inhibit either AlDH or PDH as severely as furfural. The inhibition effects on the three enzymes could well explain previously reported in vivo effects caused by furfural and HMF on the overall metabolism of Saccharomyces cerevisiae, suggesting a critical role of these enzymes in the observed inhibition.

Effects of aldehyde dehydrogenase-2 genetic polymorphisms on metabolism of structurally different aldehydes in human liver

Genotype analysis of the aldehyde dehydrogenase (ALDH)-2 gene was performed using an improved simplified method, and effects of the genotype on the metabolism of a variety of aldehydes in different fractions of human liver cells were investigated. The effects of sex, aging, smoking, drinking alcohol, liver function, and various drugs on ALDH activity were also analyzed. Of the 39 subjects, eight were heterozygotes of the wild (ALDH2*1) and mutant (ALDH2*2) alleles, and the others were homozygotes of the wild allele. ALDH activity toward acetaldehyde in liver mitochondria from subjects with a mutant allele was less than 10% of that with two alleles of wild-type, and the activities toward formaldehyde, propionaldehyde, n-butyraldehyde, capronaldehyde, and heptaldehyde were also significantly lower in the ALDH2*1/*2 rather than ALDH2*1/*1 group. However, the metabolism of octylaldehyde, decylaldehyde, retinaldehyde, benzaldehyde, 3-hydroxybenzaldehyde, and 2,5-dihydroxybenzaldehyde was similar in the two genotypes. Changes in activity in the cytosolic fraction were similar to those in mitochondria. There was no significant difference in ALDH activity in microsomes between the two groups. Total activities of ALDH toward acetaldehyde and other short-chain aliphatic aldehydes in supernatant fractions of homogenized liver were affected in a manner similar to that in mitochondria. Our results suggest that the single nucleotide polymorphisms of the ALDH2 gene only alter the metabolism of aldehydes with a short aliphatic chain. Furthermore, sex, drinking alcohol, and smoking had little effect on ALDH activity, although the activity in elderly individuals tended to be lower albeit statistically insignificant.

Aldehyde dehydrogenase (ALDH) 2 associates with oxidation of methoxyacetaldehyde; in vitro analysis with liver subcellular fraction derived from human and Aldh2 gene targeting mouse

A principal pathway of 2-methoxyethanol (ME) metabolism is to the toxic oxidative product, methoxyacetaldehyde (MALD). To assess the role of aldehyde dehydrogenase (ALDH) in MALD metabolism, in vitro MALD oxidation was examined with liver subcellular fractions from Japanese subjects who carried three different ALDH2 genotypes and Aldh2 knockout mice, which were generated in this study. The activity was distributed in mitochondrial fractions of ALDH2*1/*1 and wild type (Aldh2+/+) mice but not ALDH2*1/*2, *2/*2 subjects or Aldh2 homozygous mutant (Aldh2-/-) mice. These data suggest that ALDH2 is a key enzyme for MALD oxidation and ME susceptibility may be influenced by the ALDH2 genotype.

Mechanism of inhibition of aldehyde dehydrogenase by citral, a retinoid antagonist

Low concentrations of citral (3,7-dimethyl-2,6-octadienal), an inhibitor of retinoic acid biosynthesis, inhibited E1, E2 and E3 isozymes of human aldehyde dehydrogenase (EC1.2.1.3). The inhibition was reversible on dilution and upon long incubation in the presence of NAD+; it occurred with simultaneous formation of NADH and of geranic acid. Thus, citral is an inhibitor and also a substrate. Km values for citral were 4 microM for E1, 1 microM for E2 and 0.1 microM for E3; Vmax values were highest for E1 (73 nmol x min-1 x mg-1), intermediate for E2 (17 nmol x min-1 x mg-1) and lowest (0.07 nmol x min-1 x mg-1) for the E3 isozyme. Citral is a 1 : 2 mixture of isomers: cis isomer neral and trans isomer, geranial; the latter structurally resembles physiologically important retinoids. Both were utilized by all three isozymes; a preference for the trans isomer, geranial, was observed by HPLC and by enzyme kinetics. With the E1 isozyme, both geranial and neral, and with the E2 isozyme, only neral obeyed Michaelis-Menten kinetics. With the E2 isozyme and geranial sigmoidal saturation curves were observed with S0.5 of approximately 50 nM; the n-values of 2-2.5 indicated positive cooperativity. Geranial was a better substrate and a better inhibitor than neral. The low Vmax, which appeared to be controlled by either the slow formation, or decomposition via the hydride transfer, of the thiohemiacetal reaction intermediate, makes citral an excellent inhibitor whose selectivity is enhanced by low Km values. The Vmax for citral with the E1 isozyme was higher than those of the E2 and E3 isozymes which explains its fast recovery following inhibition by citral and suggests that E1 may be the enzyme involved in vivo citral metabolism.

Urinary excretion of biomarkers for radical-induced damage in rats treated with NDMA or diquat and the effects of calcium carbimide co-administration

The urinary excretion of seven aldehydes, acetone, coproporphyrin III and 8-hydroxy-2'-deoxyguanosine (8-OH-dG) as non-invasive biomarkers of oxidative damage was measured in rats treated with diquat or N-nitrosodimethylamine (NDMA), two compounds causing hepatic damage by different mechanisms. Furthermore, the effect of co-administration of the aldehyde dehydrogenase inhibitor, calcium carbimide (CC) on the urinary excretion of the aldehydes was determined. Slight hepatotoxicity was found at the end of the experiment after treatment with NDMA (0.5, 4 and 8 mg/kg at t = 0, 48 and 96 h, respectively) or diquat (6.8 and 13.6 mg/kg at t = 0 and 48 h, respectively). In diquat treated rats slight nephrotoxicity was also found. Urinary excretion of aldehydes, acetone and coproporphyrin III remained largely unchanged in rats treated with NDMA. In the rats treated with diquat, the urinary excretion of several aldehydes was several-fold increased. An increase was also found in the urinary excretion of 8-OH-dG after the second dose of diquat. Treatment of rats with CC did not significantly influence the urinary excretion of aldehydes in control and NDMA rats. However, in rats treated with diquat, CC caused a potentiating effect on the excretion of acetaldehyde, hexanal and malondialdehyde (MDA), indicating that oxidation of aldehydes to carbonylic acids by aldehyde dehydrogenases (ALDHs) might be an important route of metabolism of aldehydes. In conclusion, increased urinary excretion of various aldehydes, acetone, coproporphyrin III and 8-OH-dG was observed after administration of diquat, probably reflecting oxidative damage induced by this compound. No such increases were found after NDMA administration, which is consistent with a different toxicity mechanism for NDMA. Therefore, excretion of aldehydes, acetone, coproporphyrin III and 8-OH-dG might be used as easily accessible urinary biomarkers of free radical damage.

Sheep liver cytosolic aldehyde dehydrogenase: the structure reveals the basis for the retinal specificity of class 1 aldehyde dehydrogenases

BACKGROUND

. Enzymes of the aldehyde dehydrogenase family are required for the clearance of potentially toxic aldehydes, and are essential for the production of key metabolic regulators. The cytosolic, or class 1, aldehyde dehydrogenase (ALDH1) of higher vertebrates has an enhanced specificity for all-trans retinal, oxidising it to the powerful differentiation factor all-trans retinoic acid. Thus, ALDH1 is very likely to have a key role in vertebrate development.

RESULTS

. The three-dimensional structure of sheep ALDH1 has been determined by X-ray crystallography to 2.35 A resolution. The overall tertiary and quaternary structures are very similar to those of bovine mitochondrial ALDH (ALDH2), but there are important differences in the entrance tunnel for the substrate. In the ALDH1 structure, the sidechain of the general base Glu268 is disordered and the NAD+ cofactor binds in two distinct modes.

CONCLUSIONS

. The submicromolar Km of ALDH1 for all-trans retinal, and its 600-fold enhanced affinity for retinal compared to acetaldehyde, are explained by the size and shape of the substrate entrance tunnel in ALDH1. All-trans retinal fits into the active-site pocket of ALDH1, but not into the pocket of ALDH2. Two helices and one surface loop that line the tunnel are likely to have a key role in defining substrate specificity in the wider ALDH family. The relative sizes of the tunnels also suggest why the bulky alcohol aversive drug disulfiram reacts more rapidly with ALDH1 than ALDH2. The disorder of Glu268 and the observation that NAD+ binds in two distinct modes indicate that flexibility is a key facet of the enzyme reaction mechanism.

Glyoxylate synthesis, and its modulation and influence on oxalate synthesis

PURPOSE

We define the major pathways of hepatic oxalate synthesis in humans, examine the association with other metabolic pathways and identify ways that oxalate synthesis may be modified. In addition, we suggest what is required for further progress in this area.

MATERIALS AND METHODS

We consolidated relevant data primarily from recently published literature, considered new pharmacological approaches to decrease oxalate synthesis, and formulated an overview of the regulation and modification of oxalate synthesis pathways.

RESULTS

Experiments with animals, including humans, animal cells and in vitro preparations of cellular components, support the existence of a major metabolic pathway linking the amino acids serine, glycine and alanine. Oxalate synthesis is a minor, secondary reaction of a cascade of reactions termed the glyoxylate pathway, which has a prominent role in gluconeogenesis and ureagenesis. The enzymatic steps and effectors which regulate glyoxylate and oxalate synthesis are not well characterized. Pharmacological approaches can reduce oxalate synthesis by diminishing the glyoxylate pool and possibly modifying enzymatic reactions leading to glyoxylate synthesis.

CONCLUSIONS

The individual steps associated with glyoxylate and oxalate synthesis can be identified. The glyoxylate pathway has a significant functional role in intermediary liver metabolism but the way it is regulated is uncertain. Oxalate synthesis can be modified by drugs, indicating that primary and idiopathic hyperoxaluria may respond to pharmacological intervention.

Thermodynamics and molecular simulation analysis of hydrophobic substrate recognition by aminotransferases

Aromatic amino acid aminotransferase (AroAT) and aspartate aminotransferase (AspAT) are known as dual-substrate enzymes, which can bind acidic and hydrophobic substrates in the same pocket (Kawaguchi, S., Nobe, Y., Yasuoka, J., Wakamiya, T., Kusumoto, S., and Kuramitsu, S. (1997) J. Biochem. (Tokyo) 122, 55-63). In order to elucidate the mechanism of hydrophobic substrate recognition, kinetic and thermodynamic analyses using substrates with different hydrophobicities were performed. They revealed that 1) amino acid substrate specificity (kmax/Kd) depended on the affinity for the substrate (1/Kd) and 2) binding of the hydrophobic side chain was enthalpy-driven, suggesting that van der Waals interactions between the substrate-binding pocket and hydrophobic substrate predominated. Three-dimensional structures of AspAT and AroAT bound to alpha-aminoheptanoic acid were built using the homology modeling method. A molecular dynamic simulation study suggested that the outward-facing position of the Arg292 side chain was the preferred state to a greater extent in AroAT than AspAT, which would make the hydrophobic substrate bound state of the former more stable. Furthermore, AroAT appeared to have a more flexible conformation than AspAT. Such flexibility would be expected to reduce the energetic cost of conformational rearrangement induced by substrate binding. These two mechanisms (positional preference of Arg and flexible conformation) may account for the high activity of AroAT toward hydrophobic substrates.

Involvement of glutamate 399 and lysine 192 in the mechanism of human liver mitochondrial aldehyde dehydrogenase

Mutation to the conserved Glu399 or Lys192 caused the rate-limiting step of human liver mitochondrial aldehyde dehydrogenase (ALDH2) to change from deacylation to hydride transfer (Sheikh, S., Ni, L., Hurley, T. D., and Weiner, H. (1997) J. Biol. Chem. 272, 18817-18822). Here we further investigated the role of these two NAD+-ribose-binding residues. The E399Q/K/H/D and K192Q mutants had lower dehydrogenase activity when compared with the native enzyme. No pre-steady state burst of NADH formation was found with the E399Q/K and K192Q enzymes when propionaldehyde was used as the substrate; furthermore, each mutant oxidized chloroacetaldehyde slower than propionaldehyde, and a primary isotope effect was observed for each mutant when [2H]acetaldehyde was used as a substrate. However, no isotope effect was observed for each mutant when alpha-[2H]benzaldehyde was the substrate. A pre-steady state burst of NADH formation was observed for the E399Q/K and K192Q mutants with benzaldehyde, and p-nitrobenzaldehyde was oxidized faster than benzaldehyde. Hence, when aromatic aldehydes were used as substrates, the rate-limiting step remained deacylation for all these mutants. The rate-limiting step remained deacylation for the E399H/D mutants when either aliphatic or aromatic aldehydes were used as substrates. The K192Q mutant displayed a change in substrate specificity, with aromatic aldehydes becoming better substrates than aliphatic aldehydes.

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.