RESUMO
Lecithin:cholesterol acyltransferase (LCAT), the enzyme that esterifies cholesterol in blood, also esterifies other steroids at the 3beta-hydroxyl. These steroids, like cholesterol, are delta5-3beta-hydroxysteroids, such as pregnenolone and dehydroepiandrosterone. One unusual LCAT substrate is the estrogen, estradiol, which is esterified at the 17beta-hydroxyl. The esterification of estradiol by LCAT has been reported to produce a powerful antioxidant that protects low density lipoprotein (LDL) from oxidation. We investigated the substrate specificity of LCAT, comparing the esterification of four different steroids (estradiol, estriol, testosterone, and 5-androstene-3beta, 17beta-diol) by human LCAT in blood and by acyl-coenzyme A:acyltransferase in tissue (placenta and fat). Estradiol was esterified only at the D ring 17beta-hydroxyl group in both blood and tissue. In contrast, although testosterone has a D ring structure identical to that of estradiol, and it was esterified at the 17beta-hydroxyl by acyl-coenzyme A:acyltransferase in tissue, it was not esterified by LCAT. When 5-androstenediol was the substrate in the tissues, both the 3beta- and 17beta-esters were synthesized, but the major product was the 17beta-ester. Conversely, although 5-androstenediol was an excellent substrate for LCAT, only the 3beta-hydroxyl was esterified. No 17beta-ester was formed. The comparison of the esterification of estriol by acyl-coenzyme A:acyltransferase and LCAT was also surprising. In the tissues, estriol is esterified at both D ring hydroxyls, and both are esterified about equally. Although estriol is an extremely polar estrogen, it is esterified by LCAT, albeit at a very slow rate. Although again both D ring hydroxyls were esterified, the LCAT esterification site was mainly at the 17beta-hydroxyl. Esterification of estriol at the 17beta-hydroxyl in preference to the 16alpha-hydroxyl is especially striking, because the 17beta-hydroxyl group is sterically shielded by the C-18 methyl group, making esterification at this position energetically much more difficult. Furthermore, these studies demonstrate that esterification of the 17beta-hydroxyl 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 antioxidant action that has reported for the estradiol esters formed by LCAT.
