THE VERTEBRATE ALCOHOL-DEHYDROGENASE SYSTEM - VARIABLE CLASS-II TYPE FORM ELUCIDATES SEPARATE STAGES OF ENZYMOGENESIS

A mixed-class alcohol dehydrogenase has been characterized from avian liver, Its functional properties resemble the classical class I type e nzyme in livers of humans and animals by exhibiting low K-m and k(cat) values with alcohols (K-m = 0.7 mM with ethanol) and low K-i values w ith 4-methylpyrazole (4 mu M). These values are markedly different fro m corresponding parameters of class II and III enzymes, In contrast, t he primary structure of this avian liver alcohol dehydrogenase reveals an overall relationship closer to class II and to some extent class I II (69 and 65% residue identities, respectively) than to class I or th e other classes of the human alcohol dehydrogenases (52-61%), the pres ence of an insertion (four positions in a segment close to position 12 0) as in class II but in no other class of the human enzymes, and the presence of several active site residues considered typical of the cla ss II enzyme, Hence, the avian enzyme has mixed-class properties, bein g functionally similar to class I, yet structurally similar to class I I, with which it also clusters in phylogenetic trees of characterized vertebrate alcohol dehydrogenases, Comparisons reveal that the class I I enzyme is approximate to 25% more variable than the ''variable'' cla ss I enzyme, which itself is more variable than the ''constant'' class III enzyme, The overall extreme variability, the activity unexpected for a class II enzyme, and the unusual chromatographic behavior may ex plain why the class II enzyme has previously not been found outside ma mmals, The properties define a consistent pattern with apparently repe ated generation of novel enzyme activities after separate gene duplica tions.