The d> 1.21 g/ml fraction of human plasma stimulates mass transfer of phosphatidylcholine (PC) from egg PC vesicles into plasma high density lipoproteins (HDL) and also exchange of PC and cholesterol esters (CE) between HDL and low density lipoproteins (LDL). By measuring the facilitated transfer of PC from egg PC vesicles into HDL, and the facilitated exchange of cholesterol esters between HDL and LDL, PC transfer and CE exchange activities were monitored through several purification steps. After application of the d> 1.19 fraction to phenyl-Sepharose, both activities were eluted with HzO. However, they were subsequently partially separated by a linear NaCl gradient on a carboxymethylcellulose column. After further chromatography on diethylaminoethylcellulose, fractions showing highest PC transfer or CE exchange activities showed, respectively, no CE exchange or PC transfer activities. Further purification of the PC transfer activity was achieved by adsorption from active ion exchange fractions to PC vesicles, followed by isolation of vesicles by agarose chromatography and ultracentrifugal flotation. Sodium dodecyl sulfate-polyacrylamide gradient gel electrophoresis showed that the CE exchange and PC transfer activities were associated with proteins of apparent M,= 63,000 and 41,000, respectively. In further experiments, double-labeled HDL ([14C] PC,[3H] CE) were used to measure PC and CE exchange between HDL and LDL. In contrast to the results obtained in the PC transfer assay, the PC and CE exchange activities showed identical distribution through the ion exchange steps. The results indicate that the mass transfer ofPC into HDL is facilitated by a different plasma protein to that enhancing exchange of CE and PC between LDL and HDL.

Proteins which stimulate the exchange of cholesterol esters between plasma lipoproteins have recently been described (1-5). Preparations obt. ained from the d> 1.21 g/ml fraction stimulate both cholesterol ester and phospholipid exchange between LDL'and HDL, with cholesterol ester and phospholipid exchange occurring in an approximate 1: l molar ratio (3-5). Using a sequence of ultracentrifugation, phenyl-Sepharose,