Regiospecific esterification of estrogens by lecithin : cholesterol acyltransferase

Lecithin:cholesterol acyltransferase (LCAT), the enzyme that esterifies cho lesterol in blood, also esterifies other steroids at the 3 beta-hydroxyl. T hese steroids, like cholesterol, are Delta(5)-3 beta-hydroxysteroids, such as pregnenolone and dehydroepiandrosterone. One unusual LCAT substrate is t he estrogen, estradiol, which is esterified at the 17 beta-hydroxyl. The es terification of estradiol by LCAT has been reported to produce a powerful a ntioxidant that protects :Low density lipoprotein (LDL) from oxidation. We investigated the substrate specificity of LCAT, comparing the esterificatio n of four different steroids (estradiol, estriol, testosterone, and 5-andro stene-3 beta, 17 beta-diol) by human LCAT in blood and by acyl-coenzyme A:a cyltransferase in tissue (placenta and fat). Estradiol was esterified only at the D ring 17 beta-hydroxyl group in both blood and tissue. In contrast, although testosterone has a D ring structure identical to that of estradio l, and it was esterified at the 17 beta-hydroxyl by acyl-coenzyme A:acyltra nsferase in tissue, it was not esterified by LCAT. When 5-androstenediol wa s the substrate in the tissues, both the 3 beta- and 17 beta-esters were sy nthesized, but the major product was the 17 beta-ester. Conversely, althoug h 6-androstenediol was an excellent substrate for LCAT, only the 3 beta-hyd roxyl was esterified. No 17 beta-ester was formed. The comparison of the es terification of estriol by acyl-coenzyme A:acyltransferase and LCAT was als o surprising. In the tissues, estriol is esterified at both D ring hydroxyl s, and both are esterified about equally. Although estriol is an extremely polar estrogen, it is esterified by LCAT, albeit at a very slow rate. Altho ugh again both D ring hydroxyls were esterified, the LCAT esterification si te was mainly at the 17 beta-hydroxyl. Esterification of estriol at the 17 beta-hydroxyl in preference to the 16 alpha-hydroxyl is especially striking , because the 17 beta-hydroxyl group is sterically shielded by the C-18 met hyl group, making esterification at this position energetically much more d ifficult. Furthermore, these studies demonstrate that esterification of the 17 beta-h ydroxyl group by LCAT is unique to estrogens. It suggests that this unusual regiospecific esterification of C-17 of the estrogens underlies a distinct stereochemical requirement for the powerful anti-oxidant action that has r eported for the estradiol esters formed by LCAT.