In vitro catabolism of human plasma very low density lipoproteins. Effects of VLDL concentration on the interconversion of high density lipoprotein subfractions

The effect of lipolysis of human plasma very low density lipoproteins (VLDL) on the distribution of high density lipoprotein subfractions was studied in an in vitro system consisting of purified bovine milk lipoprotein lipase and albumin. The distribution of lipids and apoproteins (apoC-II and apoC-III) within the lipoprotein fractions corresponding to HDL₂ (d = 1.063-1.120 g/ml) and HDL₃ (d = 1.120-1.210 g/ml) was dependent upon the concentration of VLDL in the incubation mixture. After lipolysis of an incubation mixture containing VLDL-triglyceride (0.6 mg triglyceride/ml) and HDL₃ (0.1 mg protein/ml), most of the lipid and apoproteins were recovered in HDL₃. At higher concentrations of VLDL-triglyceride relative to HDL₃-protein (1.8 or 2.4 mg of VLDL-triglyceride and 0.1 mg of HDL₃-protein) the amount of lipid and apoprotein isolated in the HDL₃ density fraction decreased after lipolysis and there was an increase in the amount isolated between d 1.063-1.120 g/ml. These results provide additional evidence for the conversion of HDL₃ to HDL₂ during lipolysis. Furthermore, they suggest that the relative distribution of plasma HDL₂ and HDL₃ is related to the rate of catabolism of triglyceride-rich lipoproteins.