Plasma cholesteryl ester-triglyceride transfer protein. The catalytic domain is a low molecular weight proteolipid.

The lipid transfer protein complex (LTC) isolated from human plasma by immunoaffinity chromatography transfers cholesteryl esters (CE), triglycerides, and phosphatidylcholine (PC) between lipoproteins in vitro. The molecular weight of this lipid transfer catalyst in sodium dodecyl sulfate-polyacrylamide gels was 65,000. When resolved on a gel filtration column by high performance liquid chromatography (HPLC), LTC was composed of fractions of high (greater than 150,000) to low (18,000) molecular weight, although sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of each fraction revealed bands at Mr 65,000 (major) and 52,000 (minor). The CE and triglyceride transfer activity of the low Mr HPLC fraction (1049 nmol of triglyceride/mg/h and 244 nmol of CE/mg/h) was significantly greater than that of the high Mr HPLC fraction (15-27 nmol of triglyceride/mg/h and 20-30 nmol of CE/mg/h). The PC transfer activity of the HPLC fractions was not determined. LTC proteins were separated by dialysis in acidified chloroform:methanol solution into dialysand and dialysate proteins. The dialysate contained a low Mr proteolipid, designated the catalytic domain Cd, which catalyzed CE and triglyceride transfer at equivalent rates (11.0 versus 9.5 mumol/mg/h, respectively). PC transfer activity was approximately 10% of these levels (1.5 mumol/mg/h). The dialysand consisted of a protein, designated the transfer protein TP, which facilitated CE (3.4 mumol/mg/h) preferentially over triglyceride and PC (1.0 mumol/mg/h) transfer, and a catalytically inactive protein, designated the heparin-binding domain Hd. We propose a model of the LTC protein (based on catalytic activities, monoclonal antibody reactivities, and heparin-binding capacities of the isolated proteins) in which both Hd (approximately 13 kDa) and Cd (approximately 3 kDa) originate from a single lipid transfer protein, TP.