Substituted pyridines: nonsteroidal inhibitors of human placental aromatase cytochrome P-450.

The binding to human placental aromatase cytochrome P-450 and resulting extent of inhibition was examined for pyridine, pyridines substituted at the 2-, 3-, or 4-positions with phenyl or benzoyl groups, and the nonpyridinic structural analogs biphenyl and benzophenone. Spectral binding studies with partially purified aromatase indicated that pyridine, 3- and 4-phenylpyridines, and 3- and 4-benzoylpyridines interact at the active site via ligation of the pyridinic nitrogen to heme iron, as judged by the type II difference spectra. The apparent dissociation constants for these compounds are 2.1, 0.11, 0.09, 2.6, and 0.27 mM, respectively. Biphenyl, benzophenone, and 2-benzoylpyridine show exclusively a hydrophobic interaction in the presence of the substrate, androst-4-ene-3,17-dione, to give reverse type I difference spectra, with apparent spectral dissociation constants of 0.14, 0.42, and 1.8 mM, respectively. 2-Phenylpyridine shows a unique spectrum, previously not observed with cytochrome P-450, that has components of both type I and type II spectra. As measured by tritium ion release from [1 beta-3H]androst-4-ene-3,17-dione, competitive inhibition of human microsomal aromatase activity was observed with pyridine, 2-, 3-, and 4-phenylpyridines, biphenyl, and benzophenone, with Ki values of 320, 62, 1.48, 0.36, 750, and 130 microM, respectively. In contrast, 2-, 3-, and 4-benzoylpyridines exhibited complex kinetic behavior from which inhibition constants could not be obtained.(ABSTRACT TRUNCATED AT 250 WORDS)