Labeling plasma lipoproteins with radioactive cholesterol.

Abstract

Abstract Cholesterol-4- 14 C suspended in alcoholic physiologic saline was injected intravenously into 13 subjects in order to compare this with other more "physiologic" methods for labeling plasma lipoproteins with radioactive cholesterol. Cholesterol in suspension was removed from the circulation within a few minutes. It was then reincorporated into plasma lipoproteins slowly so that peak incorporation was reached in about 12 hours. Esterification of free cholesterol incorporated into different lipoproteins was similar to those of endogenous cholesterol synthesized in the body. The slopes of the specific activity curves of plasma cholesterol obtained from 1 to 50 days after the injection of particulate cholesterol were identical to those obtained after other more physiologic methods of labeling plasma cholesterol; i.e., (1) injection of radioactive precursors for endogenous synthesis of cholesterol, (2) feeding of radioactive cholesterol, (3) injection of plasma labeled by a modification of Sodhi and Kalant's method, and (4) injection of plasma labeled with Porte and Havel's method. The ratios of specific activity obtained after injection of the plasma labeled with 3 H-cholesterol by the last two methods to the 14 C specific activity obtained after the injection of particulate 14 C-cholesterol were identical to the ratio of the doses of the isotopes initially administered, and so were different parameters of cholesterol kinetics analyzed from the specific activity curves. The values of cholesterol turnover determined from the specific activity curves after the intravenous injection of cholesterol-4- 14 C suspended in physiologic saline were also similar to the values obtained on the same subjects by the cholesterol balance techniques, thus providing additional support for the data obtained from the specific activity curves. Our studies suggest that after having once been incorporated into plasma lipoproteins, the cholesterol injected intravenously as an aqueous suspension is metabolized in the same manner as the cholesterol incorporated into plasma lipoproteins biosynthetically or by other physiologic means.