A COMPARISON OF THE ESTERIFICATION OF STEROIDS BY RAT LECITHIN-CHOLESTEROL ACYLTRANSFERASE AND ACYL-COENZYME A-CHOLESTEROL ACYLTRANSFERASE

Although fatty acid esters of several steroids have been found in both blood and tissues, their biosynthetic origins are uncertain. For example, the fatty acid esters of Delta(5)-3 beta-hydroxysteroids pregnenolone and dehydroepiandrosterone (DHEA) are synthesized in tissues by an acyl coenzyme A:acyltransferase. These esters are not secreted, and the circulating esters are formed in blood by lecithin:cholesterol acyltransferase (LCAT). Fatty acid esters of corticosterone (B) and estradiol (E(2)) are also present in both blood and tissues, but unlike the Delta(5)-3 beta-hydroxysteroids, their structures are so different from cholesterol that it would not necessarily follow that they are esterified by the same enzyme. We have examined the esterification of the steroids DHEA, B, and E(2) in blood and tissue, in comparison to the esterification of cholesterol, using as a model plasma and hepatic microsomes from the rat. All of the steroids were esterified in plasma, but at very different rates: cholesterol > DHEA >> E(2) = B. The LCAT inhibitor, 5.5'dithiobis-(2-nitrobenzoic acid), inhibited the esterification of all of the substrates. DHEA inhibited the esterification of cholesterol, albeit only at high concentration. The fatty acid compositions of the cholesterol and DHEA esters were analyzed, and they were found to be identical, with arachidonate the predominant ester, greater than 60%. In hepatic microsomes, the rate of esterification was different than plasma: cholesterol > E(2) greater than or equal to DHEA >> B. Although B was esterified in both plasma and hepatic microsomes, the rate was exceedingly slow in both. The acyl coenzyme A:cholesterol acyltransferase inhibitor, N'-(2,4-difluorophenyl)-N-[[4-(2,2-dimethylpro- pyl)phenyl]-methyl]-N-heptylurea, blocked the esterification of cholesterol almost completely, but surprisingly, it had no effect on the esterification of the other steroids. The fatty acid esters of cholesterol, E(2), and DHEA synthesized in the hepatic microsomes were analyzed. The composition of the cholesterol esters from the microsomes was very different than the esters of DHEA and E(2). These results show that all of the steroids tested are esterified by LCAT, and consequently that blood LCAT is the probable source of the circulating steroidal esters. Most interesting are the studies of microsomal esterification. It has been presumed that similar to blood, the esterification of steroids in tissues is carried out by the same enzyme that esterifies cholesterol. However, the specificity of the acyl coenzyme A:cholesterol acyltransferase inhibitor and the difference in the fatty acid composition of the esters of cholesterol from the other steroids indicates that the enzyme that esterifies cholesterol in tissue is different from the one(s) that esterifies the other steroids. The presence of an enzyme system(s) that esterifies the steroids, distinct from the one that esterifies cholesterol, emphasizes that the esterification of steroids is not merely fortuitous, and it is a further indication that the formation of steroidal fatty acid esters serves important biological functions.