Alcohol dehydrogenase (ADH, EC 126.96.36.199) enzymes are zinc-containing dimeric enzymes, found in the cytosolic fraction of the cell, that catalyze the NAD(H)-dependent 'reversible' oxidation of low-molecular-weight alcohols to aldehydes. ADH enzymes perform numerous important metabolic functions, especially with endogenous substrates, but the prototypic reaction is the oxidation of ethanol to acetaldehyde, during which they transfer the pro-R hydrogen from NADH to substrate. Globally, these enzymes constitute part of the medium-chain dehydrogenase/reductase superfamily.70 In humans, five classes (classes I-V) of ADHs are known: members of different classes share less than 70% amino-acid sequence identity within a species.71 Three genes found within the human class 1 enzymes - ADH1, ADH2, and ADH3 are expressed in most adult tissue, with the exception of brain, kidney, and placenta. The absence of ADH1 enzymes, the most efficient ethanol-metabolizing enzymes among the ADH family, in the placenta and brain argues against a substantial contribution of this enzyme in local ethanol developmental central nervous system toxicity. On the other hand, polymorphisms in the human ADH2 gene have been associated with alcoholism.72
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