Site-Directed Mutagenesis of Mammalian Alcohol and Sorbitol Dehydrogenases Map Functional Differences within the Enzyme Family

Abstract

Mammalian alcohol dehydrogenases (ADH) and sorbitol dehydrogenase (SDH) belong to the same protein super-family of zinc-containing ADHs (Jörnvall et al., 1989). ADHs have two zinc atoms per subunit, one catalytic and one structural, while SDH only harbors the catalytic zinc atom (Jeffery et al., 1983). The human class I ADH consists of several isozymes that are formed from three different types of subunit, α, β, and γ (Smith et al., 1971; cf. Jörnvall et al., 1989). They combine to form active homo- and heterodimers, which constitute the enzymes responsible for the main ethanol metabolism in liver but exhibit differences in substrate specificity and catalytic efficiency. Steroid dehydrogenase activity has only been shown for isozymes containing the γ subunit (McEvily et al., 1988), and these isozymes are the only ones that can be inhibited by testosterone (Mårdh et al., 1986). Moreover, ββ but not γγ ADH has a large pH dependence when ethanol is the substrate (Bosron et al., 1983). In addition to these class I ADHs, at least four more classes of mammalian ADH exist, differing enzymatically, structurally and in organ distribution (Jörnvall et al., 1989; Parés et al., 1992).