Repair of BHK cell surface ganglioside GM3 after its degradation by extracellular sialidase

Treatment of BHK fibroblasts with V, cholerae sialidase for 20 min caused t he breakdown of about 70% of total cellular ganglioside GM3 and the product ion of an approximately equivalent amount of lactosylceramide. On removal o f the enzyme, a slow resynthesis of GM3 from lactosylceramide was observed, equivalent to about 5-6%/h of the degraded GM3. Resynthesis of degraded su rface ganglioside has not previously been observed, but its magnitude is si milar to previous measurements of the rate of protein resialylation after s ialidase treatment. This suggests that resialylation of both lipid and prot ein is limited by vesicular transport of plasma membrane components through the trans-Golgi network [TGN] where sialyltransferase is thought to be loc alized. In contrast, resynthesis of sphingomyelin which has been degraded a t the cell surface by exogenous sphingomyelinase is about five times faster than resynthesis of GM3 and may involve non-vesicular transport of ceramid e.