Biological aromatization of Δ 4,6- and Δ 1,4,6-androgens and their 6-alkyl analogs, potent inhibitors of aromatase

Abstract

Enzymic aromatization of Δ 6- and Δ 1,6-derivatives of the natural substrate androstenedione with human placental aromatase was first studied using gas-chromatography-mass spectrometry. The two steroids were aromatized with apparent K m and V max values of 62 nM and 32 pmol/min/mg protein for the Δ 6-steroid and 167 nM and 10 pmol/min/mg protein for the Δ 1,6-steroid, respectively. We next explored the aromatization of a series of 6-alkyl (methyl, ethyl, n-propyl, and n-pentyl)-substituted Δ 6-androstenediones and their Δ 1,6-analogs, potent competitive inhibitors of aromatase, to gain insight into the relationships between the inhibitory activity of the 6-alkyl-C 19 steroids and their ability to serve as a substrate of aromatase. In a series of the Δ 1,6-androstenediones, all the 6-alkyl steroids were more efficient substrates than the parent Δ 1,6-steroid in which the aromatization rates of the alkyl steroids were about 2-fold that of the parent steroid, in contrast, all of the 6-alkyl-substituted Δ 6-androstenediones were converted into the corresponding 6-alkyl-Δ 6-estrogens with the rates of less than about a half that of the parent steroid. These results indicate that the 6-alkyl function decreases the aromatization rate of the Δ 6-steroid but enhances that of the Δ 1,6-steroid. The relative apparent K m values for the C 19 steroids obtained in this study are different from the relative K i values obtained previously, indicating that a good inhibitor is not essentially a good substrate in the 6-alkyl-substituted Δ 6- and Δ 1,6-androstenedione series.