Effect of very low-density lipoproteins on lipid transfer in incubated serum

Incubation of human serum solutions low in glyceride content for 16 hr resulted in a net increase of cholesterol esters in all three major human serum lipoprotein classes, namely the very low-density lipoproteins of d < 1.006 (VLDL), low-density lipoproteins of d 1.006-1.063 (LDL) and high-density lipoproteins of d 1.063-1.21 (HDL). Transfer of glycerides from the VLDL to the LDL and HDL was also observed during incubation. Reductions in lipoprotein unesterified cholesterol and phospholipids occurred during incubation and indicated the presence of serum fatty acid transferase activity in these solutions.When serum was incubated in the presence of high concentrations of VLDL lipoproteins, the increase in cholesterol esters was found only in the VLDL fraction. The VLDL apparently took up newly-formed cholesterol esters produced by serum fatty acid transferase activity, and accepted some cholesterol esters initially bound to the LDL and HDL. When the transferase was inhibited with sodium p-hydroxymercuribenzoate, some of the cholesterol esters initially bound to the LDL and HDL were still transferred to the VLDL. The HDL class was then the principal contributor of the cholesterol esters. These observations indicate that reciprocal transfer of cholesterol esters for glycerides in human serum lipoproteins can occur.