Possible role of ceramide in defining structure and function of membrane glycolipids.

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Abstract
A possible role of ceramide in defining the carbohydrate structure of glycolipids and the expression of glycolipid function has been proposed, on the basis of the finding that the ceramide composition of lacto-series glycosphingolipid isolated from human erythrocytes shows a remarkable correlation with the terminal carbohydrate structure. The ceramides of 3 glycosphingolipids with Lex (or x) determinant [Gal.beta.1 .fwdarw. 4(Fuc.alpha.1 .fwdarw. 3)GlcNac] had almost exclusively 16:0 fatty acid; in contrast, the ceramide of its positional isomer H determinant had mainly 20-24:0 fatty acids. The ceramide of 2 glycosphingolipids with NeuAc.alpha.2 .fwdarw. 6Gal structure was predominantly of 16:0 fatty acid, in contrast to that of its positional isomer NeuAc.alpha.2 .fwdarw. 3Gal residue, in which the ceramide had 20-24:0 fatty acids. These results, together with the previous observation that ceramide composition of mouse lymphoma and myelocytic leukemia M1 cells affects their antigenicity, suggest that ceramide structure may define the organization of glycosyltransferases for synthesis of the carbohydrate determinants and may affect the organization and orientation of the carbohydrate chain in membranes, eliciting or suppressing the reactivity to its ligand. Because these glycolipids with Lex and NeuAc.alpha.2 .fwdarw. 6Gal structures are developmentally regulated and are highly expressed in certain tumors, ceramide composition may affect development, differentiation and oncogenesis.