Alcohol dehydrogenases: a family of isozymes with differential functions.

Human alcohol dehydrogenases (ADHs) are encoded by at least 7 genes, and comprise at least 5 Classes. The isozymes are differentially distributed in tissues, with most Classes exhibiting highest activity in liver. Class IV (mu or sigma) ADH exhibits high activity in stomach and cornea. Class I ADHs have a wide range of physiological substrates, in addition to ethanol, involving metabolism of the following: bile compounds; testosterone; neurotransmitters; congeners; retinol; peroxidic aldehydes; and mevalonate. Class II (pi) ADH is involved in peroxidic aldehyde, norepinephrine, mevalonate and congener metabolism, but apparently plays a minor role in ethanol oxidation. Class III (chi) ADH is inactive with ethanol under physiological conditions, but functions in formaldehyde and omega-hydroxy fatty acid metabolism. Class IV (mu or sigma) is apparently involved in first-pass metabolism of gastric ethanol and other dietary alcohols, and in peroxidic aldehyde metabolism.