Dissection of the Physiological Interconversion of 5α-DHT and 3α-Diol by Rat 3α-HSD via Transient Kinetics Shows That the Chemical Step Is Rate-Determining: Effect of Mutating Cofactor and Substrate-Binding Pocket Residues on Catalysis

Abstract

3α-Hydroxysteroid dehydrogenases (3α-HSDs) catalyze the interconversion between 5α-dihydrotestosterone (5α-DHT), the most potent androgen, and 3α-androstanediol (3α-diol), a weak androgen metabolite. To identify the rate-determining step in this physiologically important reaction, rat liver 3α-HSD (AKR1C9) was used as the protein model for the human homologues in fluorescence stopped-flow transient kinetic and kinetic isotope effect studies. Using single and multiple turnover experiments to monitor the NADPH-dependent reduction of 5α-DHT, it was found that k_lim and k_max values were identical to k_cat, indicating that chemistry is rate-limiting overall. Kinetic isotope effect measurements, which gave ^Dk_cat = 2.4 and ^D₂Ok_cat = 3.0 at pL 6.0, suggest that the slow chemical transformation is significantly rate-limiting. When the NADP⁺-dependent oxidation of 3α-diol was monitored, single and multiple turnover experiments showed a k_lim and burst kinetics consistent with product release as being rate-limiting overall. When NAD⁺ was substituted for NADP⁺, burst phase kinetics was eliminated, and k_max was identical to k_cat. Thus with the physiologically relevant substrates 5α-DHT plus NADPH and 3α-diol plus NAD⁺, the slowest event is chemistry. R276 forms a salt-linkage with the phosphate of 2‘-AMP, and when it is mutated, tight binding of NAD(P)H is no longer observed [Ratnam, K., et al. (1999) Biochemistry38, 7856−7864]. The R276M mutant also eliminated the burst phase kinetics observed for the NADP⁺-dependent oxidation of 3α-diol. The data with the R276M mutant confirms that the release of the NADPH product is the slow event; and in its absence, chemistry becomes rate-limiting. W227 is a critical hydrophobic residue at the steroid binding site, and when it is mutated to alanine, k_cat/K_m for oxidation is significantly depressed. Burst phase kinetics for the NADP⁺-dependent turnover of 3α-diol by W227A was also abolished. In the W227A mutant, the slow release of NADPH is no longer observed since the chemical transformation is now even slower. Thus, residues in the cofactor and steroid-binding site can alter the rate-determining step in the NADP⁺-dependent oxidation of 3α-diol to make chemistry rate-limiting overall.