Net transfer of phosphatidylcholine from plasma low density lipoproteins to sphingomyelin-apolipoprotein A-II complexes by bovine liver and human plasma phospholipid exchange proteins

Cholesteryl ester analogs inhibit cholesteryl ester but not triglyceride transfer catalyzed by the plasma cholesteryl ester-triglyceride transfer protein

A lipid transfer protein complex (LTC), purified from human plasma by immunoaffinity chromatography, catalyzed the interlipoprotein transfer of cholesteryl esters (CE) and triglycerides (TG). The CE transfer activity of LTC was governed by the structure of the CE. Incubation of LTC with long chain CE both activated and stabilized LTC. Short chain CE also enhanced the CE and TG transfer activity of LTC during the initial time of incubation. However, LTC's incubation with short chain CE induced a subsequent and time-dependent loss of CE transfer activity without concomitant loss of TG transfer activity. The data indicate that the CE and TG transfer activity of LTC can be regulated independently.

Isolation and characterization of human plasma lipid transfer proteins

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.

Phospholipid-transfer activities in cytosols from lung, isolated alveolar type II cells and alveolar type II cell-derived adenomas

We have examined phospholipid-transfer activities in cytosols from rat and mouse whole lung, isolated rat alveolar type II cells and alveolar type II cell-derived mouse pulmonary adenomas. We report an enrichment in phosphatidylcholine and phosphatidylglycerol (but not phosphatidylinositol) protein-catalysed transfer in the type II cell and adenoma cytosols compared with the whole-lung cytosols. The activities from these cytosols were resolved using column chromatofocusing, which clearly demonstrated the presence of a phosphatidylcholine-specific transfer protein in each of the four tissues. In addition, two proteins (rat) or three proteins (mouse) catalysing both phosphatidylcholine and phosphatidylglycerol transfer were resolved from whole lung, whereas in both the rat isolated alveolar type II cells and the mouse type II cell-derived adenomas one of these less specific proteins is not present.

Tissue distribution and subcellular localization of phosphatidylcholine transfer protein in rats as determined by radioimmunoassay

A radioimmunoassay for the phosphatidylcholine-transfer protein from rat liver was used to measure levels of PC-transfer protein in rat tissues. The assay as described before (Teerlink, T., Poorthuis, B.J.H.M., Van der Krift, T.P. and Wirtz, K.W.A., Biochim. Biophys. Acta 665 (1981) 74-80) was modified in order to measure PC-transfer protein in tissue homogenates and subcellular membrane fractions. To this end both a detergent (Triton X-100) and a proteolytic enzyme inhibitor (aprotinin) were added to the assay medium. The radioimmunoassay measured levels of PC-transfer protein in the range of 5-50 ng and was specific for PC-transfer protein from rat tissues. Subcellular distribution studies showed that in 10% (w/v) homogenates of liver approximately 60% of the PC-transfer protein was present in the 105000 X g supernatant fraction, the remainder being evenly distributed over the particulate fractions. PC-transfer protein associated with the particulate fractions was almost completely removed by a single washing step, suggesting a dynamic equilibrium between membrane-bound and soluble PC-transfer protein. Both 105000 X g supernatants and homogenates of various rat tissues were assayed. The highest levels of PC-transfer protein were measured in liver and intestinal mucosa. Lower values were found in kidney, spleen and lung, whereas heart and brain contained hardly any PC-transfer protein. PC-transfer protein levels in regenerating rat liver did not differ significantly from levels in normal liver. In fetal lung a change in PC-transfer protein content during development was observed, with a clear maximum 2 days before term, suggesting an involvement of PC-transfer protein in the secretion of lung surfactant.

Transfer of [14C]phosphatidylcholine between liposomes and human plasma high density lipoprotein. Partial purification of a transfer-stimulating plasma factor using a rapid transfer assay

A simple method was developed for the rapid determination of [14C]phosphatidylcholine transfer from small unilamellar liposomes to human plasma HDL, based on the selective precipitation of liposomes by heparin and MnCl2. The assay was utilized to monitor the progress in the partial purification of a phospholipid transfer factor from human plasma. The purification procedure included ultracentrifugation at d = 1.25 g/ml, hydrophobic chromatography on phenyl-Sepharose, affinity chromatography on heparin-Sepharose and gel filtration. The partially purified protein(s) catalyzed the net transfer of phospholipid from small unilamellar phosphatidylcholine liposomes to isolated HDL. The transfer of [14C]phosphatidylcholine from liposomes consisting of phosphatidylcholine/phosphatidylserine/cholesterol (molar ratio, 4:1:5) to HDL was stimulated without affecting the permeability barrier of the liposomal membranes and is, therefore, taken to represent exchange with HDL phospholipid rather than net transfer.

Transfer properties of the bovine brain phospholipid transfer protein. Specificity towards phosphatidylcholine analogs and the inhibitory effect of sphingomyelin

A coupled transport of phosphatidylinositol from the monolayer to phosphatidylcholine vesicles, and a phosphatidylcholine transport in the reverse direction in the presence of bovine brain transfer protein is demonstrated. No significant amounts of protein accumulate at the interface during the transfer reaction. The transfer protein from bovine brain shows a lower specificity for phosphatidylcholine than does the transfer protein from bovine liver. Relative to egg phosphatidylcholine a low transfer rate is found for derivatives with a chain length of 14 carbon atoms and a distance between phosphorus and nitrogen of 6 carbon atoms. The gel state of phosphatidylcholine does not reduce the transfer reaction as catalyzed by the bovine brain protein. The transfer of phosphatidylinositol is inhibited by sphingomyelin. The presence of 200 mM K+ or 1mM Ca2+ does not affect the transfer activity of the bovine brain protein. Divalent ions at concentrations higher than 5 mM cause a fusion of vesicles with monolayers. The pH optimum of the phosphatidylinositol transfer reaction is 8.