To understand the relative importance of alcohol dehydrogenase (ADH) isoenzymes in gastric ethanol metabolism, a stomach-specific ADH (sigma-ADH) was purified to homogeneity from human transplant donor and surgical tissues, and its activity for ethanol oxidation was examined. The enzyme from these tissues had a specific activity at pH 10 of approximately 70 units/mg, about 10 times that reported by Moreno and Pares (J. Biol. Chem. 266:1128-1133, 1991). The enzyme exhibited a high K(m) for ethanol at pH 7.5 and 10 (29 and 5.2 mM, respectively). This high-activity sigma-ADH isoenzyme migrated on starch and isoelectric focusing gels to a position slightly anodic to the liver pipi isoenzyme. It was subjected to digestion by endoproteinases, and approximately 40% of the protein was sequenced. The sigma-ADH exhibited 75%, 68%, and 62% sequence identity to the human class I (beta1), II (pi), and III (chi) isoenzymes, respectively, and 61% identity to the deduced ADH6 amino acid sequence. Phylogenetic analysis indicated that precursors to this high-activity sigma-ADH and the class I isoenzymes diverged more recently than precursors to the class II and III isoenzymes, after reptilian and avian divergence. The high-activity sigma-ADH isoenzyme therefore represents a distinct class of ADH (class IV), more closely related in evolution to the class I isoenzymes than to the other known human isoenzymes.
