Cholesterol turnover in human plasma lipoproteins: studies with stable and radioactive isotopes

The kinetics of cholesterol labeling was studied in the plasma lipoproteins of three subjects who had received an oral dose of octadeuterated cholesterol and an intravenous administration of 3H-cholesterol and 14C-mevalonate or 13C-acetate. After each labeled cholesterol pulse into the plasma (absorption, exchange, or synthesis), the isotopic concentrations of free and esterified cholesterol became identical after 120 h. Before this time, very low-density lipoprotein free cholesterol appeared more easily exchangeable than high-density and low-density lipoprotein free cholesterol, high-density lipoproteins were shown to be a source for very low-density lipoprotein cholesterol esters and the role of very low-density lipoproteins associated with chylomicrons was demonstrated in the initial transport of dietary cholesterol. The rates of the various processes involved in cholesterol turnover were calculated. The total cholesterol inflow into the plasma by absorption and synthesis, determined by long-term kinetic data (18 or 28 wk) was consistent with the result obtained by sterol balance for the total cholesterol outflow from the plasma (fecal excretion and conversion into bile acids).