10β-Propynyl-substituted steroids. Mechanism-based enzyme-activated irreversible inhibitors to estrogen biosynthesis

A series of 10β-propynyl-substituted steroids are reported as mechanism-based enzyme-activated irreversible inhibitors of the human placental aromatase which converts 4-androstene-3,17-dione to 1,3,5(10)-estratrien-3-ol-17-one. Thus 10-propargylestr-4-ene-3,17-dione binds to the enzyme with an apparent K(i) of 23 nM and causes time-dependent inactivation (pseudo-first order k(inact) = 1.11 x 10^-3 5^-1). The 10-[(1S)-1-hydroxy-2-propynyl]estr-4-ene-3,17-dione analog of the known enzyme-generated intermediate, 4-androsten-19-ol-3,17-dione, behaved similary and had an apparent K(i) of 27 μM and a psuedo-first order k(inact) of 2.91 x 10^-35^-1. The stereoisomeric 10-[(1R)-1-hydroxy-2-propynyl]estr-4-ene-3,17-dione did not cause time-dependent inactivation, but bound in a competitive manner with an apparent K(i) of 2.5μM. Finally, 10-(1-oxo-2-propynyl)-estr-4-ene-3,17-dione, the proposed enzyme-generated affinity label and analog of the second intermediate (3,17-dioxoandrost-4-en-19-al) in the enzymatic reaction, bound to the enzyme with an apparent K(i) of 12 μM and caused time-dependent inactivation with a pseudo-first order k(inact) of 5.35 x 10^-45^-1.