Over the past decade it has become increasingly clear that steroid hormones are enzymatically esterified with fatty acids. These steroidal esters are the natural analogs of synthetic esters that are used therapeutically. One such family of pharmacological steroids is the synthetic alkyl esters of testosterone, androgens with great hormonal potency. We have investigated whether testosterone esters exist naturally by using the rat as a model. Most tissues of male rats, including blood, have very little if any ester (quantified by immunoassay as a nonpolar saponifiable metabolite), but fat and testes have sizable quantities, -3 ng of testosterone equivalents per g of tissue. Testosterone in fat averages 9 ng/g. The fat from female rats and long-term (>2 weeks) castrated males has no detectable testosterone ester. The presence of testosterone esters was confirmed by GC/MS, which clearly showed the presence of testosterone in the hydrolyzed ester fraction of fat from intact males but not long-term castrates.Upon castration, testosterone levels in the fat completely disappear within 6 hr. To the contrary, it is not until 48 hr after castration that a measurable fall in the testosterone ester fraction was observed; even after 10 days a small amount of ester is still present in the fat. These experiments demonstrate the existence of a previously unknown androgen with a potentially important physiological impact; testosterone esters, natural analogs of potent therapeutic agents, occur in the fat where they can serve as a reservoir of preformed androgen to stimulate neighboring target tissues.Although fatty acid esters of sterols, such as cholesterol, have been known for decades, the existence of naturally occurring fatty acid esters of steroids is a much more recent discovery. In 1979, putative steroidal esters of the A5-3(3-hydroxysteroids, pregnenolone (1), dehydroisoandrosterone, and 17a-hydroxypregnenolone (2), were discovered in the adrenal gland. They were named lipoidal derivatives to convey their nonpolar nature and their ability to be converted back to the parent steroid as a result of mild hydrolytic procedures. They were subsequently identified as fatty acid esters (3). These findings raised the possibility of the existence of similar esters of biologicaLly active steroids. Such compounds would be the natural analogs of synthetic steroidal esters that have been used pharmacologically as extremely potent and long-lived hormones. The well known therapeutic use of one such family of pharmacological steroid hormone esters, the estrogens (4), led to experiments which showed that estradiol (E2) is biosynthetically converted into a lipoidal derivative, LE2 (5), a nonpolar metabolite, which was identified as a family of C-17 fatty acid esters of E2 (6). Although LE2 is not estrogenic when esterified (7)-i.e., it does not bind to the estrogen receptor directly (8)-it is converted to E2 by esterase action. The fatty acid esters comprising LE2 are extremely longlived (9) and, thus, act as a reservoir of E2. They ...
