SUBSTRATE-SPECIFICITY OF ALPHA-2-]3-SIALYLTRANSFERASES IN GANGLIOSIDE BIOSYNTHESIS OF RAT-LIVER GOLGI

The acceptor specificities of four sialyltransferases (I, II, IV and V) involved in ganglioside biosynthesis were studied in Golgi vesicles derived from rat liver. The activities of these sialyltransferases were strongly detergent-dependent. Competition experiments with different detergent concentrations using LacCer (Gal-beta-1 --> 4Glc-beta-1 --> 1Cer), G(M1a) [Gal-beta-1 --> 3GalNAc-beta-1 --> 4(NeuAc-alpha-2 --> 3)Gal-beta-1 --> 4Glc-beta-1 --> 1Cer] and G(D1b) [Gal-beta-1 --> 3GalNAc-beta-1 --> 4(NeuAc-alpha-2 --> 8Neu-alpha-2 --> 3)Gal-beta-1 --> 4Glc-beta-1 --> 1Cer] as substrates, and as mutual inhibitors for ganglioside sialyltransferase activity, suggested that sialyltransferase IV was able to catalyze the sialyltransfer in alpha-2 --> 3 linkage to the galactose residues of LacCer as well as of G(M1a) and G(D1b). The other three sialyltransferases (I, II and V) seemed to be quite specific for their respective glycolipid acceptors, LacCer, G(M3) and G(M1b), G(D1a) and G(T1b). Futhermore the kinetic data showed that sialyltransferase I was inactive at higher detergent concentrations (> 75-mu-g Triton CF-54); under these conditions, formation of G(M3) and G(D1a) was catalyzed only by sialytransferase IV. These results have been integrated into a model for ganglioside biosynthesis and its regulation.