Lecithin: cholesterol acyltransferase of human plasma. Role of chylomicrons, very low, and high density lipoproteins in the reaction.

The effects of chylomicrons, VLDL (very low density lipoprotein), HDL (high density lipoprotein), and its subfractions HDLz and HDLs, on lecithin: cholesterol acyltransferase reaction in total plasma have been studied. Plasma cholesterol esteritication was measured by the decrease in unesterified cholesterol which was determined by gas-liquid chromatography. Addition of HDL or HDLI had no effect on lecithin: cholesterol acyltransferase reaction in plasma, but HDLz was inhibitory to the reaction. On the contrary, both chylomicrons and VLDL significantly increased the initial rate as well as the net yield of the reaction in plasma. When increasing concentrations of chylomicrons or VLDL were added to normal plasma, the initial rate of cholesterol esterification increased progressively and the response curve was that of a sigmoid. When normal subjects were given an oral fat load, the initial velocity of 1ecithin:cholesterol acyltransferase reaction in their plasma increased significantly together with the plasma concentration of chylomicrons and VLDL, which supports the assumption that the results described above have a physiological role. The mechanism of this stimulation of lecithin: cholesterol acyltransferase reaction by triglyceriderich lipoproteins may result from the transfer of lipid substrates from these lipoproteins to the HDL, or from the activation of this reaction by transfer of peptides between HDL and the triglyceride-rich lipoproteins, or both.