Metabolism of ganglioside-amides in cultured human fibroblasts

Metabolism of [3H]ganglioside derivatives GM3-amide and GM2-amide has been investigated in normal human skin fibroblasts. In a cell-free system the ganglioside analogues have been shown to enter biosynthetic pathways, their degradation, however, was curtailed at an early stage, as GM3-amide could not be hydrolysed by sialidase action. GM2-amide was susceptible to beta-hexosaminidase degradation yielding GM3-amide. When incorporated into fibroblasts [3H]GM2-amide was degraded to [3H]GM3-amide presumably in the lysosomes, and at the same time glycosylation to [3H]GD1a-monoamide took place most likely in the Golgi apparatus. [3H]GM3-amide, however, did not seem to reach the glycosylation sites in the Golgi apparatus. It could be detected in the lysosomes, where it was not degraded due to its sialidase resistance. From these results we conclude that in cells exogenously administered [3H]GM3-amide and [3H]GM2-amide both are directed to the lysosomes and that [3H]GM2-amide also reaches the Golgi apparatus. The synthesis of higher [3H]ganglioside-amides from incorporated [3H]GM2-amide can occur by direct glycosylation. [3H]GM3-amide, however, even if it reaches the Golgi compartment, does not enter the biosynthetic pathway.