Sex differences in plasma cholesterol-esterifying activity in rats.

Esterification of free cholesterol was investigated after incubation at 37 degrees C of plasma from immature and adult rats of both sexes kept on stock, fat-free, or cholesterol-supplemented diets. According to measurements of the decrease in free cholesterol, plasma from the fat-deficient rats showed the highest cholesterol-esterifying activity. Esterification was higher in the mature female rats than in the mature males on stock or cholesterol-containing diets, although no sex differences were observed in the sexually immature young or in the fat-free animals. There were no sex differences in the fatty acid composition of the plasma sterol esters, phospholipids, and triglycerides in the immature animals, but arachidonic acid increased at the expense of linoleic acid in the sterol ester fraction in the adult female (not, however, in the adult male). In the phospholipid fraction the higher ratio of palmitic to stearic acids in the male was confirmed. There was an increase in linoleic acid in all three plasma lipid fractions of the mature male after cholesterol feeding. It is suggested that cholesterol may inhibit the conversion of linoleate to arachidonate. During the incubation of plasma, there was little change in the distribution of fatty acids except for a decrease in palmitoleate, and increases in C(20) tri- and tetraenoic acids, in the sterol esters of mature female rats on the stock ration and the fat-free diet. These C(20) acids decreased concomitantly in the phospholipid fraction, as the transesterification reaction mechanism proposed by earlier workers would predict.