Cholesterol metabolism in the macrophage. I. The regulation of cholesterol exchange

The cholesterol metabolism of homogeneous populations of mouse peritoneal macrophages was evaluated under in vitro conditions. Macrophages are rich in free cholesterol and maintain a constant cholesterol to protein ratio (12 microg cholesterol/mg protein). No detectable cholesterol ester was present within the cell. More than 95% of total cholesterol was membrane associated and the majority was present in subcellular fractions containing lysosomes and plasma membrane. Less than 0.1% of cell cholesterol was synthesized from acetate-1-(14)C. During in vitro cultivation, macrophages rapidly exchanged their membrane cholesterol with that of lipoproteins of calf serum. About 30% of the cell cholesterol was exchanged per hour in 20% serum medium, and exchange was nearly complete by 5 hr. Exchange proceeded in a rapid exponential phase followed by a slower phase. Calculations based on a two compartment model indicated that the rapidly exchanging cholesterol compartment represented 60-70% of the total cell cholesterol, and the slowly exchanging compartment accounted for 30-40%. The relationship between serum lipoprotein concentration and exchange rate exhibited first-order kinetics. The rate was determined by thermal energy, in keeping with a Q(10) of 2, and an activation energy of 12 kcal/mole. Exchange was independent of bulk transport of lipoproteins by pinocytosis and phagocytosis, and was not linked to energy metabolism. The alpha-lipoproteins were the major class of proteins of calf serum participating in exchange.