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 exam ple, the fatty acid esters of Delta(5)-3 beta-hydroxysteroids pregneno lone and dehydroepiandrosterone (DHEA) are synthesized in tissues by a n acyl coenzyme A:acyltransferase. These esters are not secreted, and the circulating esters are formed in blood by lecithin:cholesterol acy ltransferase (LCAT). Fatty acid esters of corticosterone (B) and estra diol (E(2)) are also present in both blood and tissues, but unlike the Delta(5)-3 beta-hydroxysteroids, their structures are so different fr om cholesterol that it would not necessarily follow that they are este rified 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 e sterification of cholesterol, using as a model plasma and hepatic micr osomes from the rat. All of the steroids were esterified in plasma, bu t at very different rates: cholesterol > DHEA >> E(2) = B. The LCAT in hibitor, 5.5'dithiobis-(2-nitrobenzoic acid), inhibited the esterifica tion of all of the substrates. DHEA inhibited the esterification of ch olesterol, albeit only at high concentration. The fatty acid compositi ons of the cholesterol and DHEA esters were analyzed, and they were fo und to be identical, with arachidonate the predominant ester, greater than 60%. In hepatic microsomes, the rate of esterification was differ ent 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 ac yltransferase inhibitor, N'-(2,4-difluorophenyl)-N-[[4-(2,2-dimethylpr o- pyl)phenyl]-methyl]-N-heptylurea, blocked the esterification of cho lesterol almost completely, but surprisingly, it had no effect on the esterification of the other steroids. The fatty acid esters of cholest erol, E(2), and DHEA synthesized in the hepatic microsomes were analyz ed. The composition of the cholesterol esters from the microsomes was very different than the esters of DHEA and E(2). These results show th at all of the steroids tested are esterified by LCAT, and consequently that blood LCAT is the probable source of the circulating steroidal e sters. Most interesting are the studies of microsomal esterification. It has been presumed that similar to blood, the esterification of ster oids in tissues is carried out by the same enzyme that esterifies chol esterol. However, the specificity of the acyl coenzyme A:cholesterol a cyltransferase inhibitor and the difference in the fatty acid composit ion of the esters of cholesterol from the other steroids indicates tha t the enzyme that esterifies cholesterol in tissue is different from t he one(s) that esterifies the other steroids. The presence of an enzym e system(s) that esterifies the steroids, distinct from the one that e sterifies cholesterol, emphasizes that the esterification of steroids is not merely fortuitous, and it is a further indication that the form ation of steroidal fatty acid esters serves important biological funct ions.