Unusual gangliosides of eggs and embryos of the sea urchin Strongylocentrotus intermedius. Structure and density-dependence of surface localization.

From eggs and embryos of the sea urchin Strongylocentrotus intermedius two gangliosides, provisionally named G-1 and G-2, were isolated in the pure state. Both gangliosides contained glucose, N-glycoloylneuraminic acid and sphingosines in a 2:2:1 ratio; G-2 contained also a sulfate group, and yielded G-1 on desulfation. By periodate oxidation/borohydride reduction, permethylation analysis, neuraminidase degradation, analysis of the aldohexitol acetates and mass-spectrometry G-1 and G-2 were shown to have hitherto unknown structures: G-1 was identified as N-glycoloylneuraminosyl-(alpha 2 leads to 6)-glucosyl-(1 leads to 8)-N-glycoloylneuraminosyl-(2 leads to 6)-glucosyl-(1 leads to 1)-ceramide, and G-2 as sulfated G-1, carrying a sulfate ester group at C-8 of the terminal sialic acid. Antisera against the two gangliosides were prepared in rabbits by immunization with ganglioside G-1 or G-2. The specificity of the antisera was revealed by immunoelectrophoresis and immunodiffusion. The antisera did not react with bovine-brain and rat-liver gangliosides, with glucosylceramide and with various hydrolytic fragments of G-1 and G-2. The surface localization of the gangliosides in embryos incubated at different cell densities was studied by immunofluorescence microscopy. The intensity of the immunofluorescence was found to increase with decreasing cell density, indicating a different surface organization in sparse and dense embryos. In the sparse embryos immunofluorescence was seen mainly in the contact regions between the blastomers.