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 relative 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 associated with the albumin fraction and 85% were distributed on the lipoproteins in the order HDL > LDL > VLDL. To compare the relative affinities of serum proteins for gangliosides, the levels of exchange of exogenous gangliosides between preloaded serum proteins were determined. Although albumin had the highest binding capacity for gangliosides, 85% of the albumin-loaded gangliosides were transferred to the total lipoprotein fraction and this exchange was reversible. The transfer rate from albumin to isolated lipoproteins was higher to LDL (90%) and HDL (85%) whereas only 55% of albumin-loaded gangliosides were transferred to VLDL. The study of exchanges of preloaded gangliosides between isolated lipoproteins showed that the extent of transfer of gangliosides from a given lipoprotein fraction onto other lipoproteins was inversely correlated with its endogenous ganglioside content. Moreover, in the absence of albumin from the incubation medium, the final lipoprotein distribution of remaining exogenous gangliosides was similar to the normal distribution of endogenous gangliosides in serum lipoproteins. The formation of unexchangeable complexes between albumin and micellar exogenous gangliosides could be a possible explanation for the observed differences in the distribution of exogenous and endogenous gangliosides in serum proteins.