Cell surface sialoglycopeptide metabolism and surface glycosyl transferase activity in the Ehrlich ascites cell

1. The in vivo incorporation of radioactivity from [3H]glucosamine into a trypsin labile, cell surface sialoglycopeptide fraction (SGP) of Ehrlich ascites cells was studied in the presence and absence of puromycin pretreatment. The results indicated a much more complete inhibition of incorporation into the surface SGP than in the average intracellular acid insoluble glycoproteins. No evidence of turnover of the carbohydrate portion of the surface SGP independent of protein synthesis could be obtained. 2. However, when intact cells were incubated with labelled uridine 5'-diphosphate-N-actely glucosamine or cytidine 5'-monophosphate (CMP)-sialic acid there was some incorporation largely into acid insoluble material, suggesting the presence of glycosyl transferase activity in the surface. Further evidence for surface activity was obtained when neuraminidase pretreatment of intact cells stimulated incorporation of labelled CMP-sialic acid sixfold and almost all of the incorporated counts could be released by subsequent neuraminidase treatment. Furthermore, a much greater proportion of the incorporated counts could be released by papain than by trypsin treatment of the intact cells. These results suggest that the surface acceptor for exogenously added CMP-sialic acid is not identical to the endogenously synthesized trypsin labile surface SGP.