PHOSPHOLIPID TRANSFER PROTEIN MEDIATES TRANSFER OF NOT ONLY PHOSPHATIDYLCHOLINE BUT ALSO CHOLESTEROL FROM PHOSPHATIDYLCHOLINE CHOLESTEROL VESICLES TO HIGH-DENSITY-LIPOPROTEINS

Phospholipid transfer protein (PLTP) purified from human plasma was fo und to enhance the transfer of cholesterol from single bilayer vesicle s containing phosphatidylcholine and cholesterol to high density lipop rotein-3. The rate of cholesterol transfer was greatly influenced by t he cholesterol content of the donor vesicles. The maximal rate was obs erved with the vesicles containing 20-25 mol % cholesterol. This was i n contrast to a progressive decline in the rate of phosphatidylcholine transfer with an increase in the cholesterol content. To determine th e binding of cholesterol and phosphatidylcholine to PLTP, the mixtures of PLTP and the vesicles containing H-3-labeled phosphatidylcholine a nd C-14-labeled cholesterol were incubated and subjected to sucrose de nsity gradient centrifugation. Determination of the label profiles sho wed that cholesterol as well as phosphatidylcholine were transferred f rom the vesicles to PLTP. The reversible nature of the binding was sho wn by the transfer of labeled cholesterol and phosphatidylcholine boun d to PLTP to the acceptor vesicles or low density lipoprotein. Isother mal equilibrium binding of PLTP for cholesterol and phosphatidylcholin e showed that PLTP possessed a considerably higher affinity and bindin g capacity for phosphatidylcholine than for cholesterol. The phosphati dylcholine binding affinity and capacity were greater when PLTP was in cubated with phosphatidylcholine vesicles without cholesterol. A possi ble importance of PLTP-mediated cholesterol transfer in the circulatio n was described.