The N-acetylgalactosamine residue of exogenous GM2 ganglioside is recycled for glycoconjugate biosynthesis in rat liver.

A metabolic recycling of N-acetylgalactosamine (GalNAc), liberated from exogenous GM2 ganglioside [nomenclature of Svennerholm (1964) J. Lipid Res. 5, 145-155; IUPAC-IUB recommendations (1977) Lipids 12, 455-468], is demonstrated in rat liver. After the injection of a GM2 ganglioside isotopically radiolabelled on the terminal GalNAc residue ([GalNAc-3H]GM2), the liver retained a large amount of radioactivity distributed among: (1) a glycoprotein/glycosaminoglycan fraction, (2) a ganglioside fraction; and (3) a free-sugar fraction. Furthermore, volatile radioactivity was also found. The relative incorporation in the above fractions was time-dependent. The glycoprotein/glycosaminoglycan fraction contained radioactivity that was located on the GalNAc and GlcNAc residues. The ganglioside fraction was composed of two main families: gangliosides formed by a recycling of the liberated GalNAc, and gangliosides derived by direct utilization of the administered GM2. The free-sugar fraction contained mainly GalNAc. We suggest that GalNAc, after being released in the course of intra-lysosomal ganglioside catabolism, crosses the lysosomal membrane and passes into the cytosol, where the part not degraded is re-utilized for the biosynthesis of the different glycoconjugate classes.