A comparison of the structure of class II human liver alcohol dehydrogenase (alcohol:NAD+ oxidoreductase, EC 1.1.1.1) (containing nT subunits) with those of the human class I isozymes (containing a, (3, and y subunits) reveals differences at about 40% of all positions. Variations are large for active-site regions, the segment around the second zinc atom, and for segments involved in subunit interactions. The two classes of alcohol dehydrogenase have diverged to exhibit structural differences to about half the extent of those between alcohol and polyol dehydrogenases. Hence, the two classes of alcohol dehydrogenase represent steps in enzyme rather than isozyme divergence. An evolutionary scheme that relates different types of zinc-containing mammalian dehydrogenases to one another encompasses at least three levels of gene duplication subsequent to the early step(s) of assembly of building unit(s). The first level of duplication results in the formation of now clearly different enzymes. The second level concerns the various classes of alcohol dehydrogenase, forming steps between typical enzymes and isozymes. The third level encompasses recent and multiple duplications in isozyme evolution of alcohol dehydrogenases. This scheme, linking zinc-containing dehydrogenases at different levels, resembles that in other protein families and reflects general patterns in protein relationships.Isozyme differences of mammalian alcohol dehydrogenases (alcohol:NAD' oxidoreductase, EC 1.1.1.1) were initially defined structurally for the horse liver enzyme (1). The enzyme from this source is still the only alcohol dehydrogenase for which the tertiary structure has been analyzed directly by x-ray crystallography (ref. 2; recent review in ref.3). However, progress in the knowledge of alcohol dehydrogenase in general has been rapid. The yeast, plant, and mammalian enzymes are related to the horse enzyme (4-16), all depending on functional zinc at the active site. A completely different type of alcohol dehydrogenase in insects has been differentiated in a scheme that separates alcohol dehydrogenases into two types that differ in zinc content, size, and enzymatic mechanisms (17). Furthermore, sorbitol dehydrogenases are known to be related to the group of large, zinc-containing alcohol dehydrogenases (17,18), while bacterial ribitol and glucose dehydrogenases are related to the line of small alcohol dehydrogenases that lack zinc (17,19).Knowledge about the human alcohol dehydrogenase has increased particularly rapidly. This enzyme constitutes a complex system composed ofthree classes, I, II, III (20). The class I enzymes constitute the "typical" alcohol dehydrogenases composed of three types of subunit, a, 13, and -y, and originating from three loci (21) located on chromosome 4 (22).All of these class I structures have been characterized (summaries in refs. 14 and 23) at both the protein and cDNA levels. Genomic structures (24) and their organizational relationships to other alcohol dehydrogenases (25) have also been studied. Fu...