Structural and functional divergence of class II alcohol dehydrogenase

cDNAs coding for class II alcohol dehydrogenase were isolated from a rabbit‐liver cDNA library. Deduced amino acid sequences show that isozymic forms of rabbit class II alcohol dehydrogenase exist, with a positional identity of 88.4 %. A high variability in structure of class II alcohol dehydrogenase between the species is also reflected in function. The rabbit II‐1 isozyme shows common characteristics with the human enzyme, but has a lower K m value for ethanol, 4.2 mM. The II‐2 isozyme shows restriction for aliphatic alcohols longer than pentanol. For shorter alcohols the II‐2 form has similar K m values as the II‐1 isozyme, 5.5 mM for ethanol, but is a low activity variant with a 10‐fold decrease in k cat values compared with II‐1. Nevertheless, II‐2 has a higher specificity for benzoquinone than II‐1 due to a lower K m value, 80 μM compared with 1 mM, and is in this sense more like the human class II enzyme. In addition a rabbit class III alcohol dehydrogenase cDNA was isolated that encodes a typical class III enzyme/glutathione‐dependent formaldehyde dehydrogenase. The finding of isozymic forms of class II alcohol dehydrogenase is in line with the evolution of the system of medium‐chain alcohol dehydrogenases with different enzymes, different classes and different isozymes and further underline the complexity of the entire mammalian alcohol dehydrogenase system.