Isolation and characterization of human plasma lipid transfer proteins

Abstract

A highly purified protein that facilitates the exchange and net mass transfer of cholesteryl ester (CE), and triacylglycerol (TG), and the transfer of phosphatidylcholine (PC), between plasma lipoproteins was isolated from the d-1.21-1.25 g/ml plasma fraction. Transfer activities showed similar distributions through ultracentrifugation, phenyl-Sepharose, DEAE-Sepharose, CM-cellulose, and hydroxyapatite chromatography. The lipid transfer protein appears to be an acidic protein with an apparent molecular weight of 64,000 +/- 1600 (n = 4) on sodium dodecyl sulfate polyacrylamide gel electrophoresis and an apparent molecular weight of 65,000 by gel filtration chromatography, and has a pI of 5.0 +/- 0.2 (n = 5) by both analytical isoelectricfocusing and chromatofocusing. The purified transfer protein facilitated the net mass transfer of CE from high density lipoprotein (HDL) or low density lipoprotein (LDL) to very low density lipoprotein (VLDL) and the net mass transfer of TG from VLDL to LDL or HDL. Chromatography of lipid transfer protein fractions on a heparin-Sepharose column yielded two separate fractions with PC transfer activity. The first fraction did not bind to heparin column, was relatively resistant to elevated temperature (at 58 degrees C, only 5% activity was lost in 1 hour) and eluted with the CE and TG transfer activities which were also temperature-resistant. The second PC transfer activity bound to the heparin column and was temperature-sensitive (at 58 degrees C, 90% activity was lost in 1 hour). Addition of the temperature-resistant lipid transfer fraction (LTP-1) and purified lecithin cholesterol acyltransferase (LCAT) to whole plasma stimulated the endogenous plasma cholesterol esterification rate by approximately 50%, whereas addition of either LTP-1 or LCAT only slightly enhanced the esterification rate. The transfer of CE, TG, and PC was mediated by a temperature-resistant plasma protein or proteins with very similar properties. Plasma also contained a distinct lipid transfer protein which was temperature-sensitive and facilitated the transfer of PC, but not CE or TG.