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.