Substrate specificity of α2↔3‐sialyltransferases in ganglioside biosynthesis of rat liver golgi^*

Abstract

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 (Galβ1↔4Glcβ1↔1 Cer), G M1a [Galβ1↔3GalNAcβ1↔4(NeuAcα2↔3)Galβ1↔4Glcβ1↔1 Cer] and G D1b [Galβ1↔3GalNAcβ1↔4(NeuAcα2↔8NeuAcα2↔3)Galβ1↔4Glcβ1↔ 1 Cer] as substrates, and as mutual inhibitors for ganglioside sialyltransferase activity, suggested that sialyltransferase IV was able to catalyze the sialyltransfer in α2↔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 . Furthermore the kinetic data showed that sialyltransferase I was inactive at higher detergent concentrations (> 75 μg Triton CF‐54); under these conditions, formation of G M3 and G D1a was catalyzed only by sialyltransferase IV. These results have been integrated into a model for ganglioside biosynthesis and its regulation.