title: The kinetic mechanism catalysed by homogeneous rat liver 3 alpha-hydroxysteroid dehydrogenase. Evidence for binary and ternary dead-end complexes containing non-steroidal anti-inflammatory drugs.
abstract: Rat liver 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD) (EC 1.1.1.50) is an NAD(P)(+)-dependent oxidoreductase that is potently inhibited at its active site by non-steroidal anti-inflammatory drugs (NSAIDs). Initial-velocity and product-inhibition studies performed in either direction at pH 7.0 are consistent with a sequential ordered Bi Bi mechanism in which pyridine nucleotide binds first and leaves last. This mechanism is supported by fluorescence titrations of the E-NADH complex, and by the failure to detect the binding of either [3H]androsterone or [3H]androstanedione to free enzyme by equilibrium dialysis. Dead-end inhibition studies with NSAIDs also support this mechanism. Initial-velocity studies with indomethacin show that this drug is an uncompetitive inhibitor against NAD+, but a potent competitive inhibitor against androsterone, indicating the ordered formation of an E.NAD+.indomethacin complex. Calculation of the individual rate constants reveals that the binding and release of pyridine nucleotide is rate-limiting and that isomerization of the central complex is favoured in the forward direction. Equilibrium dialysis experiments with [14C]indomethacin reveal the presence of two abortive NSAID complexes, a high-affinity ternary complex corresponding to E.NAD+.indomethacin (Kd = 1-2 microM for indomethacin) and a low-affinity binary complex corresponding to E.indomethacin (Kd = 22 microM for indomethacin). Since indomethacin has a low affinity for free enzyme, the formation of this abortive binary complex does not complicate kinetic measurements which are made in the presence of NAD+, but may contribute to the inhibition of the enzyme by NSAIDs. Using either pro-R-[4-3H]NADH or pro-S-[4-3H]NADH as cofactor, radiolabelled androsterone was formed only when the pro-R-[4-3H]NADH was used, confirming that purified 3 alpha-HSD is a Class A dehydrogenase.
authors.affiliation: Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia 19104-6084.
authors.forename: L J
authors.initials: LJ
authors.lastname: Askonas
authors.forename: J W
authors.initials: JW
authors.lastname: Ricigliano
authors.forename: T M
authors.initials: TM
authors.lastname: Penning
mesh_terms: D015096:3-Hydroxysteroid Dehydrogenases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D045002:3-alpha-Hydroxysteroid Dehydrogenase (B-Specific); D000738:Androsterone / Q000378:metabolism; D000818:Animals; D000894:Anti-Inflammatory Agents, Non-Steroidal / Q000378:metabolism* / Q000494:pharmacology; D001665:Binding Sites; D001667:Binding, Competitive; D005043:Etiocholanolone / Q000031:analogs & derivatives / Q000378:metabolism; D007213:Indomethacin / Q000378:metabolism / Q000494:pharmacology; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D009243:NAD / Q000378:metabolism; D051381:Rats; D012984:Software
chemical_list: D000894:Anti-Inflammatory Agents, Non-Steroidal; D009243:NAD; C006576:androstane-3,17-dione; D005043:Etiocholanolone; D000738:Androsterone; D015096:3-Hydroxysteroid Dehydrogenases; D045002:3-alpha-Hydroxysteroid Dehydrogenase (B-Specific); D007213:Indomethacin