Reversible exchange of glycosphingolipids between human high and low density lipoproteins

Both human serum low density lipoprotein (LDL) and high density lipoprotein (HDL) can acquire [3H]glycosphingolipids from glycosphingolipid-coated hydrophobic glass beads, but the process produces variable denaturation of LDL. However, endogenous LDL glycosphingolipid can be 3H-labeled by the galactose oxidase/NaB[3H]4 technique without structural modification. We have now demonstrated that 3H-labeled neutral glycosphingolipids can be reversibly exchanged under physiological conditions between HDL and LDL. Maximal exchange was achieved following 4 to 8 h of incubation at 37 degrees C when the ratio of HDL to LDL concentration was 1:1 by weight. Only a small fraction, 10-15%, of the total glycosphingolipid contents of both HDL and LDL was available for exchange, indicating that at least two separate pools of glycosphingolipid exist on the surface of lipoprotein particles. When lipoprotein-deficient serum was added, the amount of glycolipid exchanged was not stimulated significantly. The level of phosphoglyceride exchange was 2-fold greater, and that of neutral lipids 4-fold greater, than neutral glycosphingolipids. Based on this and on our previous observations, we propose that high density lipoprotein can be used to modify the glycosphingolipid content and thus the biological properties of membranes.