The homogeneous 3 alpha-hydroxysteroid dehydrogenase of rat liver cytosol binds prostaglandins with low micromolar affinity at its active site and is competitively inhibited by the non-steroidal and steroidal anti-inflammatory drugs [Penning, Mukharji, Barrows & Talalay (1984) Biochem. J. 222, 601-611]. To examine the portion of this binding site that accommodates the glucocorticoid side chain, we have synthesized 17 beta-bromoacetoxy-5 alpha-dihydrotestosterone (BrDHT) and 21-bromoacetoxydesoxycorticosterone (BrDOC) as affinity-labelling agents. Both these agents promote rapid inactivation of the purified enzyme in a time- and concentration-dependent manner. Analyses of the inactivation progress curves gave estimates of Ki for the inactivators and half-life (t1/2) for the enzyme at saturation (tau) as follows: Ki = 33 microM and tau = 18 s for BrDHT, and Ki = 10 microM and tau = 203 s for BrDOC. Under initial-velocity conditions BrDHT and BrDOC act as competitive inhibitors, yielding Ki values identical with those measured in the inactivation experiments. Both indomethacin and prostaglandin E2 can protect the enzyme from inactivation, yielding Ki values for these ligands consistent with those measured independently by competitive-inhibition studies. These data confirm that the bromoacetoxysteroids label the active site, which is coincident with the prostaglandin- and anti-inflammatory-drug-binding site. Neither gel filtration nor extensive dialysis restores activity to the enzyme inactivated with either affinity-labelling agent. Use of radioactive BrDHT or BrDOC, in which either the steroid portion is labelled with 3H or the bromoacetate portion is labelled with 14C, indicates that inactivation is accompanied by a stoichiometric incorporation of 0.7-1.0 molecules of inhibitor per enzyme monomer. The linkage that forms between the dehydrogenase with either [14C]BrDHT or [14C]BrDOC is stable to acid and base treatment. Complete acid hydrolysis of the enzyme inactivated with [14C]BrDHT, followed by amino acid analyses, indicates that 87% of the radioactivity is eluted with carboxymethylcysteine. An almost identical result is obtained with [14C]BrDOC, where at least 75% of the radioactivity is eluted with this amino acid. Thus BrDHT and BrDOC alkylate at least one reactive cysteine residue at the active site that may be of functional importance in binding the glucocorticoid side chain.

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