Inhibition of human steroid 5alpha reductases type I and II by 6-aza-steroids: structural determinants of one-step vs two-step mechanism.

We have discovered that 17 beta-[N,N-(diethyl)carbamoyl]-6-azaandrost-4-en-3-one is a time-dependent inhibitor of type II 5 alpha-reductase, as is the drug finasteride. Unlike finasteride, the 6-aza-steroid is not a time-dependent inhibitor of type I 5 alpha-reductase. Finasteride inhibition of type Il enzyme proceeds in a two-step mechanism. At pH 6 and 37 degrees C, an initial finasteride-reductase complex is formed with a K-i(app) of 11.9 +/- 4.1 nM. In a second step, an irreversible complex is formed with a rate constant of inactivation of 0.09 +/- 0.01 s(-1). In contrast, the 6-aza-steroid is a reversible inhibitor. From the results of a simplified mathematical analysis, based on the rapid equilibrium approximation, the inhibitor and the enzyme form an initial complex with a K-i of 6.8 +/- 0.2 nM, The reversible formation of a final complex, with an overall K-i of 0.07 +/- 0.02 nM, is characterized by a first-order isomerization rate constant 0.0035 +/- 0.0001 s(-1) for the forward step and 0.00025 +/- 0.00006 s(-1) for the backward step. All rate constants for the two-step mechanism were obtained by using a general numerical integration method. The best fit values for the association and dissociation rate constants were 5.0 mu M(-1) s(-1) and 0.033 +/- 0.008 s(-1), respectively, and the isomerization rate constants were 0.0035 +/- 0.0007 s(-1) and 0.000076 +/- 0.000019 s(-1). These values correspond to an initial K-i of 6.5 nM and an overall dissociation constant of 0.14 nM. The data presented here show that both finasteride and the 6-aza-steroid analogs are potent against type II 5 alpha-reductase, although their mechanisms of inhibition are different.