Enzymatic and structural differences between usual and atypical human liver alcohol dehydrogenases.

It has been known that human cytosol alcohol dehydrogenase (alcoho1:NAD' oxidoreductase, EC 1.1.1.1), controlled by the ADHz locus, is polymorphic. Most Caucasians have enzyme Containing the usual PI subunit, while nearly 90% of Orientals have an enzyme containing the atypical / I 2 subunit. At physiological pH, the cytosol enzyme activity of atypical liver is much higher than that of typical liver. It has been suggested that a high frequency of acute alcohol intoxication among Orientals could be related to the rapid accumulation of acetaldehyde due to the atypical enzyme. The usual homodimer PlPl and the atypical homodimer f12P2 isozymes were purified to homogeneity, The specific activity of the atypical enzyme was several times higher at pH 10, and 80 times higher at pH 8.5, than that of the usual enzyme. K , values for ethanol, NAD, acetaldehyde, and NADH were several times higher in the atypical enzyme than in the usual enzyme. The usual enzyme was rapidly inactivated by iodoacetate, indicating the existence of an "active site cysteine" in the molecule. In contrast, the atypical enzyme was resistant to iodoacetate inactivation. Peptide mapping analysis revealed that the active site cysteine in the usual /I1 subunit is replaced by histidine in the atypical /Iz subunit. A remarkable structural homology exists at the active sites of the horse and human enzymes. In the usual &PI enzyme, as in the horse enzyme, the catalytic Zn is expected to link to the sensitive cysteine at position 47, histidine at position 67, and cysteine, presumably at position 174, thus making up the active site. In contrast, the active site of the atypical Pz& enzyme is expected to consist of the catalytic Zn linked to histidine at position 47, histidine at position 67, and cysteine, presumably at position 174. The resistance of the atypical f lz& against inactivation by iodoacetate is a direct consequence of the replacement of the sensitive cysteine ab position 47 by histidine.