Inhibition of rat alpha-reductases by finasteride: Evidence for isozyme differences in the mechanism of inhibition

The mechanism of inhibition of the rat types 1 and 2 5 alpha-reductase by finasteride was investigated using recombinantly expressed enzymes. These studies revealed that finasteride is a potent, reversible inhibitor of the rat type 1 5 alpha-reductase with K-i = 10.2 +/- 1.3 nM. Finasteride is a potent inhibitor of the rat type 2; however, in this case the compound binds to the type 2 isozyme-NADPH complex to form a ternary complex with K-i = 1.19 +/- 0.10 nM, which then rearranges to a high affinity complex (E:I-star) with a pseudo first order rate constant of 1.62 +/- 0.22 x 10(-3)/s. The second order rate constant is k(3)/K-i = 1.37 +/- 0.31 x 10(6) M/s. Heat denaturation of the (type 2 enzyme:inhibitor) complex releases dihydrofinasteride and presumably the NADP(+)-adduct previously identified with the human 5 alpha-reductases. The effects of finasteride were also studied in intact COS cells transiently expressing the rat types 1 and 2 5 alpha-reductase. Results with whole cell assays confirm differences in mechanism of inhibition of rat types 1 and 2 5 alpha-reductase by finasteride. (C) 1997 Elsevier Science Ltd.