10 beta-propynyl-substituted steroids. Mechanism-based enzyme-activated irreversible inhibitors of estrogen...

A series of 10 beta-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 Ki of 23 nM and causes time-dependent inactivation (pseudo-first order kinact = 1.11 X 10(-3) s-1). The 10-[(1S)-1-hydroxy-3-propynyl]estr-4-ene-3,17-dione analog of the known enzyme-generated intermediate, 4-androsten-19-ol-3,17-dione, behaved similarly and had an apparent Ki of 27 microM and a psuedo-first order kinact of 2.91 X 10(-3) s-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 Ki of 2.5 microM. 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 Ki of 12 microM and caused time-dependent inactivation with a pseudo-first order kinact of 5.35 X 10(-4) s-1.