Facilitation of phosphatidylcholine transfer into high density lipoproteins by an apolipoprotein in the density 1.20-1.26 g/ml fraction of plasma

Incubation of unilamellar vesicles of egg phosphatidylcholine (PC) in human plasma results in rapid transfer of phospholipid into high density lipoproteins (HDL). A similar but much slower transfer occurs upon incubation of PC vesicles with centrifugally isolated HDL. Experiments were conducted to characterize a plasma factor that might facilitate PC transfer into HDL. Addition of the d greater than 1.21 g/ml fraction of plasma to isolated HDL caused a marked increase in the rate of transfer of PC from vesicles to HDL. Fractionation of plasma by vertical rotor density gradient ultracentrifugation revealed that the factor that facilitated transfer of PC into HDL resided in the density 1.20-1.26 g/ml fraction, associated with a lipoprotein particle of apparent Stokes' diameter 10.2 nm. This fraction caused facilitated transfer of PC mass from vesicles into HDL3, resulting in formation of larger, less dense "HDL2a"-like particles. A partially purified preparation of phospholipid transfer activity was obtained from the d 1.20-1.26 g/ml fraction by a sequence of phenyl-Sepharose, heparin-Sepharose, and carboxymethylcellulose chromatography. The most purified fraction facilitated transfer of 4 micrograms of 14C-labeled PC/microgram protein per 60 min and also promoted transfer of radioactive cholesteryl esters from HDL to LDL. The results suggest that during lipoprotein metabolism the insertion of PC molecules into HDL may be facilitated by a plasma lipid transfer protein.