Finasteride is employed in treatment of benign prostatic hyperplasia in man, where its target enzyme is steroid 5 alpha-reductase. It is a novel, potent mechanism-based inhibitor of the human prostate (type 2) isozyme. Although it is accepted as an alternate substrate and is ultimately reduced to dihydrofinasteride, this proceeds through an enzyme-bound NADP-dihydrofinasteride adduct. Finasteride is processed with a second-order rate constant, k(i)/K-i = 1 x 10(6) M(-1) s(-1), that approaches k(cat)/K-m for reduction of testosterone, 3 x 10(6) M(-1) s(-1), and essentially every catalytic event is lethal (partition ratio less than or equal to 1.07). The membrane-bound enzyme-inhibitor complex formed from [H-3]finasteride appears to release [H-3]dihydrofinastende with a half-life of 1 month at 37 degrees C (k = (2.57 +/- 0.03) x 10(-7) s(-1)), as identified by mass spectroscopy. The intermediate NADP-dihydrofinasteride adduct can be recovered intact by denaturation of the enzyme-inhibitor complex and has been purified. Free in solution, it likewise decomposes to dihydrofinasteride (half-life 11 days). An extremely potent bisubstrate analog inhibitor, this NADP-dihydrofinasteride adduct binds to the free enzyme with a second-order rate constant equal to k(cat)/K-m for turnover of testosterone and has a dissociation constant K-i less than or equal to 1 x 10(-13) M. Finasteride is also a mechanism-based inhibitor of the human skin (type 1) isozyme, but it is processed with a much smaller second-order rate constant, k(i)/K-i = 3 x 10(3) M(-1) s(-1), which attenuates its activity against this isozyme in vivo. The mechanism explains the exceptional potency and specificity of finasteride in treatment of benign prostatic hyperplasia, and the concept may have application to other pyridine nucleotide-linked enzymes.