Substrate Specificity and Kinetic Mechanism of Tetrahymena 20α-Hydroxysteroid Dehydrogenase
20α-Hydroxysteroid dehydrogenase (20HSD) is distributed in mammalian tissues (Gower, 1984) and micro-organisms (Dorfman and Ungar, 1965). 20HSDs purified from mammalian tissues are NADP+-dependent monomeric proteins with Mr values of 35,000-40,000 (Shikita et al., 1967; Sato et al., 1972;Nakajin et al., 1989;Noda et al., 1991), whereas the bacteral enzymes are NAD+-dependent tetramers of Mr 162,000 (Krafft and Hylemon, 1989) and monomers of Mr 48,000 (Rimsay et al., 1988). Recently, the cDNAs encoding the enzymes of bovine testis (Warren et al., 1993), rabbit ovary (Lacy et al., 1993) and rat ovary (Miura et al., 1994) have been cloned, and the enzymes have been shown to be members of the aldo-keto reductase superfamily which includes monomeric NADPH-de-pendent oxidoreductases (Flynn and Green, 1993; bifunctional enzymes: Aldehyde and aldose reductases exhibit dihydrodiol dehydrogenase activity (Matsuura et al., 1987; Hara et al., 1985, 1991) rat and human liver 3a-hydroxysteroid dehydrogenases associate with both carbonyl reductase and dihydrodiol dehydrogenase activities (Penning et al., 1986; Pawlowski et al., 1991; Deyashiki et al., 1992, 1994), bovine liver prostaglandin F synthase shows carbonyl reductase activity (Chen et al., 1992), and bovine testicular 20HSD possesses aldose reductase activity (Warren et al., 1993).
KeywordsAldose Reductase Kinetic Mechanism Ethyl Acetoacetate Ethyl Pyruvate Lithocholic Acid
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