Aromatase and testosterone fatty acid esters: the search for a cryptic biosynthetic pathway to estradiol esters.

The estradiol fatty acid esters (lipoidal derivatives, LE2) are extremely potent estrogens that accumulate in fat, including fat of menopausal women. These steroidal esters are protected from metabolism and are converted to the free, biologically active steroid through the action of esterases. Previous studies have shown that biosynthetic pathways in the adrenal gland exist in which steroid fatty acid esters are substrates. This led us to determine whether a cryptic aromatase pathway exists in which testosterone esters could be converted directly into LE2. We tested a representative fatty acid ester, testosterone stearate, both as an inhibitor and as a substrate for the aromatase enzyme from human placental microsomes. This ester had neither activity. In addition, we tested 1 beta-3H]testosterone acetate as a substrate for this enzyme complex, measuring the production of 3H2O as evidence of aromatization. Although the rate of reaction was considerably slower than that of testosterone, 3H2O was produced. However, when 2, 4, 6, 7-3H]testosterone acetate was incubated and the steroidal products isolated, we found that hydrolysis of the substrate had occurred. Both 3H]-labeled testosterone and estradiol were found, and very little if any 3H]estradiol acetate was formed. Thus, we conclude that an aromatase pathway involving testosterone esters does not exist and that the sole source of LE2 is through direct esterification of estradiol.