Expression, activities, and kinetic mechanism of human stomach alcohol dehydrogenase. Inference for first-pass metabolism of ethanol in mammals

Single-nucleotide polymorphisms interact to affect ADH7 transcription

BACKGROUND

The class IV alcohol dehydrogenase (ADH7, μ-ADH, σ-ADH) is important in the metabolism of ethanol and retinol. ADH7 is the only ADH not expressed in liver, instead being expressed mainly in the upper gastrointestinal tract. Genome-wide studies have identified significant associations between single-nucleotide polymorphisms in ADH7 and alcoholism and cancer, but the causative variants have not been identified.

METHODS

In vitro studies of gene expression by transient transfection into cell lines that express endogenous ADH7 (CP-A cells) and that do not (HepG2 cells).

RESULTS

We have identified transcriptional regulatory elements of ADH7 and observed differences in the effects of variants on gene expression in CP-A cells and HepG2 cells. Two haplotypes of the proximal promoter that differ in a single nucleotide at rs2851028, A7P-G and A7P-A, have different transcriptional activities. There is an interaction between variants farther upstream and these proximal variants: Upstream regulatory sequences generally showed a greater increase or smaller reduction in activity when combined with the A7P-A promoter than with the A7P-G promoter. A sequence located 12.5-kb upstream (7P10) can function as an enhancer. In CP-A cells, both haplotypes of 7P10 increased A7P-A activity by 2.5-fold while having only 1.2-fold effect on A7P-G. In HepG2 cells, the 7P10-TTT haplotype had no effect on the A7P-A promoter but decreased A7P-G promoter activity by 50%, whereas the CTT haplotype increased A7P-A activity by 50%, but had no effect on A7P-G.

CONCLUSIONS

These complex interactions indicate that the effects of variants in the ADH7 regulatory elements depend on both sequence and cellular context and should be considered in interpretation of the association of variants with alcoholism and cancer.

Functional Assessment of Human Alcohol Dehydrogenase Family in Ethanol Metabolism: Significance of First-Pass Metabolism

BACKGROUND

Alcohol dehydrogenase (ADH) is the principal enzyme responsible for ethanol metabolism in mammals. Human ADH constitutes a unique complex enzyme family with no equivalent counterpart in experimental rodents. This study was undertaken to quantitatively assess relative contributions of human ADH isozymes and allozymes to hepatic versus gastric metabolism of ethanol in the context of the entire family.

METHODS

Kinetic parameters for ethanol oxidation for recombinant human class I ADH1A, ADH1B1, ADH1B2, ADH1B3, ADH1C1, and ADH1C2; class II ADH2; class III ADH3; and class IV ADH4 were determined in 0.1 M sodium phosphate at pH 7.5 over a wide range of substrate concentrations in the presence of 0.5 mM NAD+. The composite numerical formulations for organ steady-state ethanol clearance were established by summing up the kinetic equations of constituent isozymes/allozymes with the assessed contents in livers and gastric mucosae with different genotypes.

RESULTS

In ADH1B*1 individuals, ADH1B1 and ADH1C allozymes were found to be the major contributors to hepatic-alcohol clearance; ADH2 made a significant contribution only at high ethanol levels (> 20 mM). ADH1B2 was the major hepatic contributor in ADH1B*2 individuals. ADH1C allozymes were the major contributor at low ethanol (< 2 mM), whereas ADH1B3 the major form at higher levels (> 10 mM) in ADH1B*3 individuals. For gastric mucosal-alcohol clearance, the relative contributions of ADH1C allozymes and ADH4 were converse as ethanol concentration increased. It was assessed that livers with ADH1B*1 may eliminate approximately 95% or more of single-passed ethanol as inflow sinusoidal alcohol reaches approximately 1 mM and that stomachs with different ADH1C genotypes may remove 20% to 30% of single-passed alcohol at the similar level in mucosal cells.

CONCLUSIONS

This work provides just a model, but a strong one, for quantitative assessments of ethanol metabolism in the human liver and stomach. The results indicate that the hepatic-alcohol clearance of ADH1B*2 individuals is higher than that of the ADH1B*1 and those of the ADH1B*3 versus the ADH1B*1 vary depending on sinusoidal ethanol levels. The maximal capacity for potential alcohol first-pass metabolism in the liver is greater than in the stomach.

Considerable haplotype diversity within the 23kb encompassing the ADH7 gene

BACKGROUND

Of the seven known human alcohol dehydrogenase (ADH) genes, the non-liver expressed ADH7 gene codes for the enzyme with the highest maximal activity for ethanol. Previous study from our laboratory has suggested that ADH7 has an epistatic role for protection against alcoholism based on a single ADH7 SNP.

METHODS

We have now studied seven SNPs, additional populations for the SNP previously examined, and six more new SNPs, across 23 kb of ADH7 in 38 population samples originating from different geographical regions of the world.

RESULTS

The overall linkage disequilibrium is moderate to strong across this region even though considerable 7-SNP haplotype diversity is observed. This uncommonly high haplotype diversity is explained by high LD within each "half," the three upstream SNPs and the four downstream SNPs, but near randomization between the "halves." This division significantly simplified the haplotype pattern: only four major haplotypes account for almost all chromosomes in all populations in each "half."

CONCLUSIONS

The low linkage disequilibrium between these two "halves" suggests multiple recombination(s) have occurred in this region, specifically, within intron 7. The absence of strong LD between the functional variation in ADH1B that is strongly associated with alcoholism and any of the variation in ADH7 supports the genetic independence of ADH7 in association studies. Thus, the previously observed epistatic effect of ADH7 cannot be explained by its linkage disequilibrium with a causative factor in ADH1B.

Human class IV alcohol dehydrogenase: kinetic mechanism, functional roles and medical relevance

Human alcohol dehydrogenase (ADH) constitutes a complex family. Class IV ADH (ADH4) is characteristic in its epithelial expression in the aerodigestive tract and high V(max) and K(m) for oxidation of ethanol. ADH4 exhibits the highest catalytic efficiency for retinol oxidation in human ADH family. Initial velocity, product inhibition, and dead-end inhibition studies indicate that ADH4, when functioning as ethanol dehydrogenase, conforms to an ordered sequential mechanism with coenzyme binding first and releasing last in catalytic cycle. When functioning as retinol dehydrogenase, the mechanism of ADH4 deduced from steady-state kinetic and equilibrium-binding studies is best described as a rapid equilibrium random mechanism with two dead-end ternary complex for retinol oxidation and a rapid equilibrium ordered mechanism with one dead-end ternary complex for retinal reduction, a unique mechanistic form for zinc-containing ADHs in the medium chain dehydrogenase/reductase superfamily. Kinetic and genetic studies support the proposal that ADH4 may play two important physiological roles, i.e., as a major contributor to first-pass metabolism of ethanol in stomach as well as involvement in the synthesis of retinoic acid, a hormonal ligand controlling a nuclear receptor signaling pathway that regulates growth, development, and epithelial maintenance. Quantitative simulation studies indicate that retinol metabolism through ADH pathway can be inhibited to a significant extent during alcohol consumption. The perturbation of retinoic acid synthesis by ethanol may underlie the pathogenesis of fetal alcohol syndrome and alcohol-related upper digestive tract cancer.

Kinetic mechanism of human class IV alcohol dehydrogenase functioning as retinol dehydrogenase

Molecular genetic studies have indicated that alcohol dehydrogenase may be involved in the synthesis of retinoic acid, a hormonal molecule regulating diverse cellular functions at the transcriptional level. Class IV alcohol dehydrogenase (ADH) has been reported to be the most efficient enzyme catalyzing oxidation of retinol in human ADH family. Initial velocity, product inhibition, and dead-end inhibition experiments were performed with the recombinant human class IV ADH to elucidate kinetic mechanism with all-trans-retinol and all-trans-retinal as natural substrates. Fluorescence quenching was titrated in formation of the binary and abortive ternary enzyme complexes. The minimal mechanism deduced from steady-state kinetic and equilibrium binding studies is best described as an asymmetric rapid equilibrium random mechanism with two dead-end ternary complexes for retinol oxidation and a rapid equilibrium ordered mechanism with one dead-end ternary complex for retinal reduction, a unique mechanistic form for zinc-containing ADHs in the medium chain dehydrogenase/reductase superfamily. Dissociation constants for the binary complexes as well as the productive and abortive ternary complexes determined from different experimental approaches are in reasonable agreement. Kinetic isotope effect studies suggest rate-limiting isomerization of the central ternary complexes in both reaction directions. The potential interference of retinol metabolism by ethanol through the ADH pathway may play a significant role in the pathogenesis of fetal alcohol syndrome and alcohol-related upper digestive tract cancer.

No sex and age influence on the expression pattern and activities of human gastric alcohol and aldehyde dehydrogenases

BACKGROUND

Alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) are the principal enzymes responsible for ethanol metabolism in humans. The stomach is involved in the metabolism of alcohol during absorption. Conflicting reports exist with regard to the influence of sex and age on the activity of ADH in the human gastric mucosa. The purpose of the present study was to determine the effects of age and sex on the expression pattern and activities of stomach ADH and ALDH.

METHODS

A total of 115 endoscopic gastric biopsy specimens were investigated from Han Chinese men (n = 70) and women (n = 45) aged 20-79 years with approximately even distribution among 10-year age intervals. The expression patterns of ADH and ALDH were identified by isoelectric focusing, and the activities were assayed spectrophotometrically.

RESULTS

The expression patterns of gastric ADH and ALDH remained unchanged with respect to sex and age. At 33 mM or 500 mM ethanol, pH 7.5, the ADH activities did not differ significantly among the various age groups or between men and women. At 200 microM or 20 mM acetaldehyde, the ALDH activities did not differ significantly in relation to sex and age. No correlations were found between the ADH or ALDH activities at both the high and low substrate concentrations and the ages in men and women.

CONCLUSIONS

The results indicate that there is no significant effect of either sex or age on the expression pattern and activity of ADH and ALDH in the human gastric mucosa. The stomach ADH seems unlikely to account for possible variations in the first-pass metabolism of alcohol with regard to sex and age.

Regulation of the mammalian alcohol dehydrogenase genes

This review focuses on the regulation of the mammalian medium-chain alcohol dehydrogenase (ADH) genes. This family of genes encodes enzymes involved in the reversible oxidation of alcohols to aldehydes. Interest in these enzymes is increased because of their role in the metabolism of beverage alcohol as well as retinol, and their influence on the risk for alcoholism. There are six known classes ADH genes that evolved from a common ancestor. ADH genes differ in their patterns of expression: most are expressed in overlapping tissue-specific patterns, but class III ADH genes are expressed ubiquitously. All have proximal promoters with multiple cis-acting elements. These elements, and the transcription factors that can interact with them, are being defined. Subtle differences in sequence can affect affinity for these factors, and thereby influence the expression of the genes. This provides an interesting system in which to examine the evolution of tissue specificity. Among transcription factors that are important in multiple members of this gene family are the C/EBPs, Sp1,USF, and AP1, HNF-1, CTF/NF-1, glucocorticoid, and retinoic acid receptors, and several as-yet unidentified negative elements, are important in at least one of the genes. There is evidence that cis-acting elements located far from the proximal promoter are necessary for proper expression. Three of the genes have upstream AUGs in the 5' nontranslated regions of their mRNA, unusual for mammalian genes. The upstream AUGs have been shown to significantly affect expression of the human ADH5 gene.