Radiochemical evidence for estradiol-17-fatty acid esters in human blood

The C-17 fatty acid esters of estradiol, known as the lipoidal derivatives of estradiol, LE2, are metabolites of estradiol that were originally isolated from various tissues after in vitro incubations with estradiol. These steroidal esters are active estrogens with extremely prolonged potencies. The present study investigates the existence of LE2 in human blood using a radiochemical isotope dilution technique. The LE2 fraction from blood was isolated, saponified, and the hydrolyzed estradiol was then acetylated with [3H]acetic anhydride. It was found that 3H was incorporated into estradiol diacetate, demonstrating that LE2 is present in human blood. Thus these steroidal esters represent a new class of endogenous estrogens which have not been previously considered in the physiology of the female sex steroids.