Abstract
Human alcohol dehydrogenases of class I and class II but not class III catalyse NAD +-dependent aldehyde oxidation in addition to the NADH-dependent aldehyde reduction. The two reactions are coupled, i.e. the enzymes display dismutase activity. Dismutase activity of recombinantly expressed human class I isozymes β 1 β 1 and γ 2 γ 2, class II and class III alcohol dehydrogenases was assayed with butanal as substrate by gas chromatographic-mass spectrometric quantitations of butanol and butyric acid. The class I γ 2 γ 2 isozyme showed a pronounced dismutase activity with a high k cat, 1300 min −1, and a moderate K m, 1.2 mM. The class I β 1 β 1 isozyme and the class II alcohol dehydrogenase showed moderate catalytic efficiencies for dismutase activity with lower k cat values, 60–75 min −1. 4-Methylpyrazole, a potent class I ADH inhibitor, inhibited the class I dismutation completely, but cyanamide, an inhibitor of mitochondrial aldehyde dehydrogenase, did not affect the dismutation. The dismutase reaction might be important for metabolism of aldehydes during inhibition or lack of mitochondrial aldehyde dehydrogenase activity.
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