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

Abstract The mechanism of inhibition of the rat types 1 and 2 5α-reductase by finasteride was investigated using recombinantly expressed enzymes. These studies revealed that finasteride is a potent, reversible inhibitor of the rat type 1 5α-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 ★ ) with a pseudo first order rate constant of 1.62 ± 0.22 × 10 −3 /s. The second order rate constant is k 3 K i = 1.37 ± 0.31 × 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 5α-reductases. The effects of finasteride were also studied in intact COS cells transiently expressing the rat types 1 and 2 5α-reductase. Results with whole cell assays confirm differences in mechanism of inhibition of rat types 1 and 2 5α-reductase by finasteride.