The alcohol dehydrogenase system.

Alcohol dehydrogenases constitute a complex system of enzymes, classes, isozymes, and allelic variants. The zinc containing, well-known liver enzyme is a class I medium-chain alcohol dehydrogenase. Other classes of this family include the class II protein, the glutathione-dependent formaldehyde dehydrogenase (the class III enzyme), the stomach-expressed class IV form, and the recently defined class V protein. Characterized forms suggest that the glutathione-dependent formaldehyde dehydrogenase is the original ancestor, defining a role for the whole protein family in cellular defense mechanisms. The isozyme-multiple class I protein is derived from an early gene duplication, allowing sub-specialization in vertebrates. Class IV is the one most ethanol-active and appears to be derived from the class I line. Allelic variants within class I, in association with aldehyde dehydrogenase variants, correlate with population differences in ethanol metabolism and hence with susceptibility to develop alcohol-related diseases. The structures also correlate with functional properties and define molecular building units for the whole family.