Disposition of the cholesterol moiety of a chylomicron-containing lipoprotein fraction of chyle in the rat*

Abstract

Cholesterol-27-C14 in the form of a chylomicron-containing lipoprotein fraction of chyle was injected intravenously into rats. About 75% of the injected sterol-C14 was in the ester and 25% in the free form. Ten minutes after the injection, about 25% of plasma lipid C14 was recovered in higher-density α- and β-lipoproteins. The transfer of labeled sterol to the higher-density plasma lipoproteins involved both esterified and free forms, but the proportion of free sterol transferred was greater than that in the injected preparation. In 10 minutes, C14 was recovered in the lipid fractions of the nine tissues studied. Liver, adipose tissue, and muscle accounted for about 70% of the injected C14, and liver alone accounted for about 50%. Adipose tissue of glucose-fed rats incorporated about twice as much of the injected sterol-C14 as did the same tissue of fasted rats at this time. During the first 10 minutes, the proportion of sterol-C14 recovered in the free form increased in four of the tissues: adipose, bone, adrenals, and small intestine. At 24 hours, about 90% of the sterol-C14 was present in the free form in all tissues except the adrenal gland. In this gland, the initial increase in the proportion of sterol-C14 recovered in the free form was followed by a decrease, and, after 24 hours, about 90% of the sterol-C14 was in the esterified form. In plasma, an increase in the proportion of labeled sterol recovered in the free form was observed only at an interval between 10 minutes and 2 hours. Among the conclusions drawn are (1) hydrolysis is the principal fate of the injected cholesterol esters within a few hours after injection, and (2) release of labeled free sterol into plasma by liver or exchange of free sterol between liver and plasma occurs at a more rapid rate sometime between 10 minutes and 2 hours after injection than at any other time. To account for the unique observations in the adrenal gland, the suggestion is made that this gland stores specific sterol esters and hydrolyzes chylomicron esters to resynthesize the cholesterol esters characteristic of the gland.