Inter-relationship of lipids transferred by the lipid-transfer protein isolated from human lipoprotein-deficient plasma.

In a previous study we demonstrated that highly purified lipid-transfer protein facilitated the transfer of triglyceride, cholesteryl ester, and phosphatidylcholine between plasma lipoproteins. It remained unclear, however, whether these lipids were transferred by independent sites on the lipid-transfer protein. To address this point, we have studied the protein-mediated transfer of triglyceride, cholesteryl ester, and phosphatidylcholine as a function of the concentration and lipid composition of donor and acceptor lipoproteins. Lipoproteins labeled in vitro, reconstituted lipoproteins of defined lipid composition, and phosphatidylcholine liposomes with or without triglyceride and/or cholesteryl ester have been used to investigate the inter-relationships of lipids transferred by the lipid-transfer protein. In studies of initial (less than or equal to 10-13%) transfer, we found that, although absolute transfer rates were affected, the ratio of cholesteryl ester to triglyceride transferred was independent of donor and acceptor lipoprotein concentrations and acceptor lipoprotein lipid composition. With reconstituted lipoproteins as donor, we demonstrated that this ratio was linearly related to the ratio of cholesteryl ester to triglyceride in the donor particle; the sum of triglyceride and cholesteryl ester transferred remained constant and independent of the lipid composition of the donor. Experiments with intact lipoproteins labeled in vitro and with small unilamellar vesicles in the presence and absence of p-chloromercuriphenylsulfonate, confirmed the interdependence of triglyceride and cholesteryl ester transfer. In contrast, under all assay conditions, no correlation was found between the amount of phosphatidylcholine transferred and the transfer of triglyceride and/or cholesteryl ester. We conclude that triglyceride and cholesteryl ester compete for transfer and that the extent of transfer for each lipid is determined by its relative concentration in the donor particle, whereas phosphatidylcholine transfer is independent of triglyceride and cholesteryl ester transfer. The data also strongly support the conclusion that lipid transfer protein promotes both the exchange and net transfer of triglyceride and cholesteryl ester and that the net transfer process proceeds by a reciprocal exchange of triglyceride and cholesteryl ester without net transfer of core lipid between lipoproteins.