THE MAJOR PISCINE LIVER ALCOHOL-DEHYDROGENASE HAS CLASS-MIXED PROPERTIES IN RELATION TO MAMMALIAN ALCOHOL DEHYDROGENASES OF CLASS-I AND CLASS-III

The major alcohol dehydrogenase of cod liver has been purified, enzymatically characterized, and structurally analyzed in order to establish original functions and relationships among the deviating classes of the enzyme in mammalian tissues. Interestingly, the cod enzyme exhibits mixed properties-many positional identities with a class III protein, but functionally a class I enzyme-blurring the distinction among the classes of alcohol dehydrogenase. The two domain interfaces, affected by movements upon coenzyme binding, both exhibit substitutions in a manner thus far unique to the cod enzyme. In contrast, coenzyme-binding residues are highly conserved. At the active site, inner and outer parts of the substrate pocket show different extents of amino acid replacement. In total, no less than 7-10 residues of 11 in the substrate binding pocket differ from those of all the mammalian classes, explaining the substrate specificities. However, the inner part of the substrate pocket is very similar to that of the class I enzymes, which is compatible with the observed characteristics of the cod enzyme: ethanol is an excellent substrate (K(m) = 1.2 mM) and 4-methylpyrazole is a strong inhibitor (K(i) = 0.1-mu-M). These values are about as low as those typical for the ethanol-active class I mammalian enzyme and do not at all resemble those for class III, for which ethanol is hardly a substrate and pyrazole is hardly an inhibitor. Further out in the substrate pocket, several residues differ from the mammalian classes, affecting large substrates. Exchanges at positions 57 (His in the cod enzyme, Asp in human class III), 294 (Trp versus Val, but partly compensated by a gap versus Ala at 295), and 318 (Met versus Ala) narrow the middle part of the substrate-binding pocket, giving it a restricted shape, like that of the class I enzyme, and unlike that of the distally wide pocket of the class III enzyme. Overall, the piscine form exhibits both divergence within the enzyme family, with large structural changes at the active site, and apparent conservation of overall interactions, with hybrid properties illustrating class evolution and functional interactions.