Kinetics of plasma protein-catalyzed exchange of phosphatidylcholine and cholesteryl ester between plasma lipoproteins.

A lipid transfer complex (LTC) isolated from human plasma catalyzes equimolar exchange of cholesteryl ester and phosphatidylcholine between low density (LDL) and high density (HDL) plasma lipoproteins. Activation parameters for LTC-catalyzed exchange of neutral and polar lipid are equal and are not influenced by the degree of purity of the catalyst. Activation parameters for exchange of both cholesteryl ester and phosphatidylcholine are influenced by the extent of saturation of phosphatidylcholine fatty acyl groups. The activation parameters also depend on the amount of HDL present in the assay. The flux rates of lipid exchange depend on the concentration of both LDL and HDL. At constant HDL concentration, flux rates become independent of LDL concentration when the ratio of [LDL]:[HDL] exceeds 9:1 (based on cholesteryl ester); at constant LDL concentration, facilitated LDL,HDL lipid exchange is inhibited at high HDL concentration, suggesting preferential HDL,HDL exchange. Analysis of the dependence of initial lipid exchange rate on LDL concentration at two constant HDL concentrations suggests that, in the reaction pathway, LTC mediates a productive collision (ternary complex) between LDL and HDL. A kinetic mechanism consistent with the data is one in which lipid exchange occurs in a ternary complex consisting of LTC, HDL and LDL. At low HDL concentration, this complex is formed by a random sequential route; at high HDL concentration, the mechanism is ordered sequential since the reactants are an LTC-HDL complex and LDL.