Biosynthesis of bovine thyroid gangliosides

Abstract

Homogenates of bovine thyroid gland contained the glycosyltransferases required for the in vitro formation of the disialoganglioside N-acetylneuraminylgalactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]-galactosylglucosylceramide (G& and the trisialoganglioside N-acetylneuraminylgalactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl-N-acetylneuraminyl]galactosylglucosylceramide (&lb) from lactosylceramide (GL-2). These enzyme activities were constituted into two separate biosynthetic pathways which culminated in the formation of Gnls and GJTlb, respectively. Based on substrate competition experiments, the last three reactions in each pathway appeared to be catalyzed by the same three glycosyltransferases. These three enzymes as well as CMP-AcNeu:GL-2 sialyltransferase were highly enriched in the Golgi apparatus fraction from bovine thyroid. The reaction product of the latter enzyme is Nacetylneuraminylgalactosylglucosylceramide (Gm), the first ganglioside intermediate in both pathways. In contrast, CMP-AcNeu:Gm sialyltransferase, which catalyzes the formation of N-acetylneuraminyl-Nacetylneuraminylgalactosylglucosylceramide (G&, the branch point between the two pathways, was not localized in the Golgi apparatus. Bovine thyroid, which contains a complex pattern of gangliosides, is capable of synthesizing these important membrane constituents. In addition, these results suggest that the separate subcellular localization of Gn3 synthesis may be involved in the regulation of ganglioside biosynthesis by the two pathways.