Synthesis and turnover of membrane glycoconjugates in monolayer culture of pig and human epidermal cells

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Abstract
The regeneration and turnover of the surface glycoconjugates of trypsin-prepared pig and human cultured epidermal cells were determined using the glycoprotein precursors N-acetyl-D-(I-3H) glucosamine (3H-NAG) and N-(3H)-acetyl-D-mannosamine (3H-NAM). Sialic acid assays were performed on similar unlabeled cells. Trypsin-damaged cell surfaces are rapidly repaired, probably by normal membrane turnover. There was a 12% regeneration of sialic acid within 2 h and total resynthesis occurred within 24 h. The presence of an internal membrane system, part of which also demonstrates turnover, probably contributes to the speed of surface membrane repair. Some of the glycoprotein/sialic acid of this internal membrane system (30%) remains bound for a considerable length of time. The membrane turnover maintains the cell in equilibrium so that total loss equals the synthesis of glycoprotein. The equilibration of 3H-NAG or 3H-NAM uptake between 24 and 48 h is limited by the relative concentrations of glucose and labeled sugar in the medium at this time. 3H-NAM was a more specific marker of glycoprotein than 3H-NAG. The results for human epidermal cells closely matched those for pig epidermal cells, indicating that pig cells can be used as a model for human cells.