Combined effects of lipid transfers and lipolysis on gradient gel patterns of human plasma LDL

The triglyceride content of the plasma very-low-density lipoprotein (VLDL) fraction is the most important factor affecting the size of low-density lipoprotein (LDL) in humans. Because cholesteryl ester transfer protein (CETP) can influence the size distribution of LDL particles in human plasma, the implication of lipid transfers in the formation of small-sized LDL patterns, which have been associated with elevated plasma triglyceride levels, was investigated. The size distribution of LDL particles in 15 plasma samples was determined by electrophoresis of the plasma LDL fraction on 20 to 160 g/L polyacrylamide gradient gels. The apparent diameter of the major LDL subfraction was shown to correlate negatively with triglyceride concentrations (r = -.706, P < .005) and positively with both high-density lipoprotein cholesterol levels (r = .637, P < .02) and the high-density lipoprotein/VLDL + LDL cholesterol ratio (r = .768, P < .001). In addition, LDL size correlated negatively with both the ability of plasma LDL to donate cholesteryl esters (r = -.79, P < .001) and its ability to acquire triglycerides (r = -.72, P < .005). Whereas these observations indicated that CETP-mediated alterations of the triglyceride/cholesteryl ester ratio of the LDL core would contribute to changes in LDL diameter, they suggested that the formation of small-sized gradient gel LDL patterns would require another biochemical event, such as lipolysis, in addition to neutral lipid transfers. To test this hypothesis, total plasma samples with or without added VLDL (added triglyceride concentration, 2.0 g/L) were preincubated for 24 hours at 37 degrees C. Preincubation mainly induced the replacement of cholesteryl esters by triglycerides in the LDL core, and changes in LDL composition were greater when total plasma was supplemented with VLDL. Subsequently, isolated LDL was incubated in the presence of bovine milk lipoprotein lipase as a source of triglyceride hydrolysis activity. Lipolysis tended to reduce the size of the major LDL subpopulation, and the mean change in LDL diameter was significantly greater when plasma was preincubated with VLDL supplementation than when it was not (-0.6 +/- 0.3 versus -0.2 +/- 0.2 nm, respectively; (P < .01). Moreover, sequential effects of lipid transfer and lipolysis activities induced dramatic changes in the general shape of gradient gel LDL patterns. The largest plasma LDL subpopulations tended to disappear, and the formation of new, small LDL particles could be observed. The combined effects of neutral lipid transfers and triglyceride hydrolysis could account for variations of gradient gel LDL profiles in human plasma.(ABSTRACT TRUNCATED AT 400 WORDS)