Studies on the Biosynthesis of Glyco‐sphingolipids in Cultured Mouse Neuroblastoma Cells: Characterization and Acceptor Specificities of N‐acetylneuraminyl‐ and N‐acetylgalactosaminyltransferases

The pathway of biosynthesis of N‐acetylgalactosamine‐containing gangliosides in mouse neuroblastoma has been studied using NB41A cells grown in monolayer tissue culture. Cell‐free enzyme preparations catalyzed the transfer of NeuNAc from CMP‐NeuNAc to lactosylceramide (GL‐2a), to form GM3. Asialo‐GM2 was neither an acceptor nor a competitive inhibitor of the sialyltransferase (CMP‐NeuNAc: GL‐2a N‐acetylneuraminyltransferase, EC 2.4.99.‐) under a variety of experimental conditions. Enzyme preparations also contained an N‐acetylgalactosaminyltransferase (UDP‐GalNAc. GM3N‐acetylgalactosaminyltransferase, EC 2.4.1.‐) which catalyzed the conversion of GM3 to GM2. No significant transfer of N‐acetylgalactosamine to GL‐2a could be demonstrated. The results of the glycosyltransferase assays support the concept that the first NeuNAc of brain gangliosides is introduced into GL‐2a. The present data suggests that the occurrence of asialo‐GM2 in NB41A cells under some culture conditions is a consequence of the catabolism of higher gangliosides.