The metabolic role of human ADH3 functioning as ethanol dehydrogenase.

Human class III alcohol dehydrogenase (ADH3), also known as glutathione-dependent formaldehyde dehydrogenase, exhibited non-hyperbolic kinetics with ethanol at a near physiological pH 7.5. The S(0.5) and k(cat) were determined to be 3.4+/-0.3 M and 33+/-3 min(-1), and the Hill coefficient (h) 2.21+/-0.09, indicating positive cooperativity. Strikingly, the S(0.5) for ethanol was found to be 5.4 x 10(6)-fold higher than the K(m) for S-(hydroxymethyl)glutathione, a classic substrate for the enzyme, whereas the k(cat) for the former was 41% lower than that for the latter. Isotope effects on enzyme activity suggest that hydride transfer may be rate-limiting in the oxidation of ethanol. Kinetic simulations using the experimentally determined Hill constant suggest that gastric ADH3 may highly effectively contribute to the first-pass metabolism at 0.5-3 M ethanol, an attainable range in the gastric lumen during alcohol consumption. The positive cooperativity mainly accounts for this metabolic role of ADH3.