Interactions of ganglioside GM1 with human and fetal calf sera. Formation of ganglioside-serum albumin complexes.

The interactions of ganglioside GM1 with human and fetal calf sera were studied, the following main results being obtained: (a) GM1, upon incubation with both sera gave origin to two GM1-protein complexes, which also occurred after interaction of GM1 with the albumin fractions prepared from the same sera. Instead no complex formation occurred using the albumin-free fractions. Therefore GM1 appeared to specifically bind serum albumin and to form GM1-albumin complexes. (b) GM1 binding to serum albumin started at ganglioside concentrations surely micellar (above 10(-6) M), was time and concentration dependent, and resulted in a relevant degree of GM1 complexation (up to 80% of total GM1 in human serum and up to 18% in fetal calf serum). (c) the binding kinetics appeared, in both serum and the correspondent albumin fraction, to be biphasic: in the first phase, occurring till about 2 . 10(-4) M GM1, the ratio between bound and total GM1 increased linearly with increasing GM1 concentration; in the second phase, occurring above 2 . 10(-4) M, the ratio remained practically constant. After these findings it should be expected that GM1, when present in serum containing systems, forms complexes with albumin. This should be approximately considered when studying the effects of exogeneous GM1 in in vivo and in vitro (tissue cultures) systems.