Genetically influenced metabolic factors have been implicated in the etiology of alcoholism in a number of ethnic groups. The conversion of alcohols to the corresponding aldehydes is catalyzed by ADHs. This is the rate-limiting step in the elimination of ethanol in humans and experimental animals (18). Seven ADH-encoding genes (ADH1A, ADH1B, ADH1C, ADH4, ADH5, ADH6, and ADH7) are located as a cluster on human chromosome 4q22–23 (33). The class 1 enzymes---encoded by ADH1A, ADH1B, and ADH1C (previously termed ADH3) in humans---have high affinity for ethanol and contribute the most to its conversion to acetaldehyde, particularly during the elimination phase. This class of ADH enzymes includes the most important ADH isoforms for oxidizing ethanol in humans (33). ADH7 acts early in the time course of alcohol metabolism in stomach mucosa that is exposed to high concentrations of alcohol (42).
