Enzymatic Synthesis of Disialogangliosides from Monosialogangliosides by Sialyltransferases from Embryonic Chicken Brain

A particulate preparation from embryonic chicken brain was previously described that catalyzed the transfer of sialic acid from CMP-N-acetylneuraminic acid to galactosyl → glucosyl → ceramide to give a monosialoganglioside, sialyl → galactosyl → glucosyl → ceramide (Reaction A). In the present studies, two monosialogangliosides served as acceptors in similar reactions, giving the corresponding disialogangliosides (Reactions B and C). Reaction B yields the major disialoganglioside of human brain; the acceptor is a ceramide-pentasaccharide containing a terminal galactose residue, and the sialic acid is transferred to this galactose unit. Reaction C involves the synthesis of N-acetylneuraminyl → N-glycolylneuraminyl → galactosyl → glucosyl → ceramide, where the terminal sialic acid is linked at C-8 to the N-glycolylneuraminic acid. The sialyl → sialyl unit occurs in brain tri- and tetrasialogangliosides, and in the minor disialogangliosides. Reaction B represents the last step in the synthesis of the major disialogangliosides from ceramide according to a proposed pathway, while Reaction C represents a branch in the pathway, and may be a key step in the synthesis of the minor disialogangliosides, and of the tri- and tetrasialogangliosides. Kinetic studies indicate that Reactions A, B, and C are catalyzed by different sialyltransferases in the particulate fraction.