DISTRIBUTION OF EXOGENOUSLY ADDED GANGLIOSIDES IN SERUM-PROTEINS DEPENDS ON THE RELATIVE AFFINITY OF ALBUMIN AND LIPOPROTEINS

Gangliosides in normal serum are found only in lipoproteins and the re lative content of the three major lipoprotein fractions is low density lipoprotein > high density lipoprotein > very low density lipoprotein (LDL > HDL > VLDL). Upon in vitro incubation of labeled gangliosides with human serum, about 15% of the exogenous gangliosides became assoc iated with the albumin fraction and 85% were distributed on the lipopr oteins in the order HDL > LDL > VLDL. To compare the relative affiniti es of serum proteins for gangliosides, the levels of exchange of exoge nous gangliosides between preloaded serum proteins were determined. Al though albumin had the highest binding capacity for gangliosides, 85% of the albumin-loaded gangliosides were transferred to the total lipop rotein fraction and this exchange was reversible. The transfer rate fr om albumin to isolated lipoproteins was higher to LDL (90%) and HDL (8 5%) whereas only 55% of albumin-loaded gangliosides were transferred t o VLDL. The study of exchanges of preloaded gangliosides between isola ted lipoproteins showed that the extent of transfer of gangliosides fr om a given lipoprotein fraction onto other lipoproteins was inversely correlated with its endogenous ganglioside content. Moreover, in the a bsence of albumin from the incubation medium, the final lipoprotein di stribution of remaining exogenous gangliosides was similar to the norm al distribution of endogenous gangliosides in serum lipoproteins. The formation of unexchangeable complexes between albumin and micellar exo genous gangliosides could be a possible explanation for the observed d ifferences in the distribution of exogenous and endogenous ganglioside s in serum proteins.