In vitro studies of human liver alcohol dehydrogenase variants using a variety of substrates
Kassam, J.P.; Tang, B.K.; Kadar, D.; Kalow, W.
Drug Metabolism and Disposition the Biological Fate of Chemicals 17(5): 567-572
1989
ISSN/ISBN: 0090-9556 PMID: 2573502 Document Number: 328254
Alcohol dehydrogenase (ADH) is genetically polymorphic, and large differences in allele frequencies exist between the major human races. Genetic variants at the ADH2 gene locus include the .beta.2-ADH ("atypical" ADH) present in 85% of Orientals and the .beta.1-ADH ("normal ADH) present in 85 to 95% of whites. Although the presence of one or the other of these ADH variants does not significantly affect the rate of ethanol oxidation in the living subject, it may affect that of other substrates. The overall objective of this work was to screen in vitro for ADH substrates which might be differentially metabolized by these ADH2 variants in living subjects. In an in vitro screening method using autopsy livers at pH 8.5, the formation or disappearance of NADH at 340 nm was measured before and after exposure to 4-methylpyrazole, and ADH-specific competitive inhibitor. The screening test revealed three new substrates and suggested that alcohol substrates fall into two groups. The majority of substrates belonged to a group which was oxidized at a significantly lower rate by the .beta.2-ADH as compared to the .beta.1-ADH, but this was not the case for a small group which included ethanol. Subsequent kinetic studies of selected alcohols tended to indicate a uniqueness of ethanol kinetics in that both KM and Vmax favored its oxidation by .beta.2-ADH rather than by the .beta.1 variant. None of the other seven tested alcohols showed a similar differential. Also, reduction of aldehydes and ketones tended to be moderately slower by .beta.2- than by .BETA.1-ADH. Since the group of discriminating substrates included compounds of toxicological interest such as common industrial solvents, the relevance of the ADH2 phenotypes as potential modifiers of certain occupational hazards deserves to be investigated. Ethanol cannot be considered to be a model substrate.