Interaction between rat prostatic steroid 5α-reductase and 3-Carboxy-17β-substituted steroids: Novel mechanism of enzyme inhibition☆

Abstract Efforts to identify novel compounds capable of blocking the steroid 5α-reductase (SR) catalyzed conversion of testosterone (T) to 5α-dihydrotestosterone have resulted in the development of 17β-substituted-3-androstene-3-carboxylic acids as potent inhibitors of the rat prostatic enzyme. The dead-end inhibition patterns of one of these steroidal acrylates, 17β- N -(2-methyl-2-propyl)-carbamoyl-androst-3,5-diene-3-carboxylic acid were best evaluated with a linear uncompetitive kinetic model vs both T ( K ii = 11 ± 1 nM ) and NADPH ( k ii = 22 ± 2 nM ). To interpret these observations, the kinetic mechanism of the rat prostatic SR was shown to involve the binding of NADPH prior to that of T through a series of dead-end and product inhibition experiments. Within the context of this preferentially ordered kinetic mechanism, it is proposed that the uncompetitive inhibition patterns result from the association of the steroidal acrylate to an enzyme complex containing NADP + in formation of a dead-end ternary complex of enzyme, NADP + , and inhibitor.