MECHANISM-BASED INHIBITION OF HUMAN STEROID 5-ALPHA-REDUCTASE BY FINASTERIDE - ENZYME-CATALYZED FORMATION OF NADP-DIHYDROFINASTERIDE, A POTENT BISUBSTRATE ANALOG INHIBITOR

Finasteride is employed in treatment of benign prostatic hyperplasia i n man, where its target enzyme is steroid 5 alpha-reductase. It is a n ovel, potent mechanism-based inhibitor of the human prostate (type 2) isozyme. Although it is accepted as an alternate substrate and is ulti mately 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 app roaches 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 le ss than or equal to 1.07). The membrane-bound enzyme-inhibitor complex formed from [H-3]finasteride appears to release [H-3]dihydrofinastend e with a half-life of 1 month at 37 degrees C (k = (2.57 +/- 0.03) x 1 0(-7) s(-1)), as identified by mass spectroscopy. The intermediate NAD P-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-dihydrofinast eride adduct binds to the free enzyme with a second-order rate constan t equal to k(cat)/K-m for turnover of testosterone and has a dissociat ion constant K-i less than or equal to 1 x 10(-13) M. Finasteride is a lso a mechanism-based inhibitor of the human skin (type 1) isozyme, bu t 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 th is isozyme in vivo. The mechanism explains the exceptional potency and specificity of finasteride in treatment of benign prostatic hyperplas ia, and the concept may have application to other pyridine nucleotide- linked enzymes.