Isolation of II-alcohol dehydrogenase of human liver: is it a determinant of alcoholism?

Abstract

Human liver alcohol dehydrogenase (alcohol: NAD+ oxidoreductase, EC 1.1.1.1), homogenous by physicochemical criteria, has been available in quantity only recently. Until now, the biochemical basis of human alcohol metabolism had to be extrapolated from the properties and behaviour of enzymes from other species, primarily horses and yeast. The biological determinants of human alcoholism have remained obscure, although recent evidence indicated a genetic predisposition, requiring relineation. A functionally distinct form of human liver alcohol dehydrogenase (ADH), which we have designated II-ADH, is provocative, since, thus far, it seems to be unique to human beings. It has a high Km for ethanol and is remarkably insensitive (apparent KI 500μM) to pyrazole and its derivatives, which are usually potent ADH inhibitors KI 1 μM), a property that is the basis for the isolation of II-ADH. The affinity resin 4-[3-(N-6-aminocaproyl)aminopropyl)pyrazole sepharose binds all other known forms of ADH but not II-ADH, thereby separating it selectively by affinity chromatography. In turn, this has led to the establishment of its identity with that enzyme form which was previously known as the anodic band and characterized by a high Km for ethanol (20 mM at pH 7.5). The remarkable insensitivity of II-ADH to pyrazole inhibition has also permitted quantitation of its role in hepatic ethanol oxidation. At 5mM ethanol, a saturating concentration for virtually all other forms of ADH, II-ADH contributes less than 15% to total ethanol oxidation. However, at intoxicating concentrations, e.g., 60 mM, it can account for as much as 40% of the total ethanol oxidation of liver indicating a seemingly unique role for this enzyme form in ethanol elimination. Thus far, we have found the amount of II-ADH varies from liver to liver of individuals and is considerably more labile than the other molecular forms, phenomena whose inter- or independence requires further study. The isolation of human II-ADH advances efforts to recognize and understand biochemical mechanisms that may be biological determinants of alcoholism and alcohol-related disease states, now generally approached and managed largely as psychosocial disorders.