Rapidly metabolized glycoproteins in a neuroblastoma cell line

The metabolism of neuroblastoma cell glycoproteins was examined using L-E13H]fucose. Incubation of monolayer cultures with [3H]fucose resulted in a rapid uptake of the radioactive precursor and its incorporation into acid-insoluble macromolecules. Less than 3% of the [3H]fucose that was isolated from neuroblastoma cells by trichloroacetic acid precipitation was associated with glycolipids. The metabolism of fucosylated macromolecules was studied in cells which were labelled to a steady state, and then reincubated under conditions which limited reutilization of the radioactive precursor (40 mM unlabelled fucose). During reincubation of the cells, we observed a rapid metabolism (27% by 2 h) of the prelabelled macromolecules which stabilized within a cell generation time to give an overall rate of turnover of 9%. This rapid loss of radioactivity from the cells was not due to exocytosis since less than 4% of the [3H]-fucose was lost into the media as macromolecules during a 5 h reincubation period. The presence of 40 mM fucose in the media did not affect cell growth until after 24 h of incubation or cellular protein synthesis until after 15 h of incubation. When the metabolism of neuroblastoma cell glycoproteins was measured in the presence of 1.8 - 10(-4) M cycloheximide, there appeared to be a less rapid decrease in cell-associated specific activity, and an increased reutilization of [3H]fucose. Although the major proportion of the radioactivity remained as e13H]fucose, extensive incubation of neuroblastoma cells with this radioactive precursor led to increased amounts of tritium associated with other cellular components; However, a rapid rate of glycoprotein metabolism could also be demonstrated with cells incubated with [14C]fucose. This eliminated the possibility that the above results were restricted to the tritiated precursor and merely a reflection of hydrogen-tritium exchange.