Turnover of plasma membrane glycoproteins and glycolipids of hepatoma tissue culture cells.

Tritiated borohydride reduction of cells treated in situ with galactose oxidase and neuraminidase was used to label externally oriented glycoproteins and glycolipids of the plasma membrane. The tritium was incorporated into galactose and galactosamine residues of both glycoproteins and glycolipids of two rat hepatoma cell types-monolayer H-35 cells and suspension cultures of HTC cells. The glycoproteins with accessible externally oriented galactose residues also were iodinated when cells in situ were subjected to lactoperoxidase-catalyzed iodination. Thus, tyrosine residues as well as galactose residues of the plasma membrane glycoproteins could be labeled in situ in these hepatoma cells. The label incorporated into the carbohydrate residues of externally oriented membrane glycoproteins was lost during subsequent cell culture in a biphasic manner. Two-thirds of the label incorporated into glycoprotein was lost with a half-life of about 1 day. The rest of the label was lost more slowly corresponding to a half-life of 100 h. Iodinated membrane glycoproteins isolated by concanavalin A chromatography also showed biphasic turnover with nearly identical kinetics to that shown by the total galactose-labeled plasma membrane glycoproteins. Contributing significantly but not solely to the early rapid phase of glycoprotein degradation was a rapidly turning-over trypsin-sensitive plasma membrane glycoprotein(s) in each of the two different hepatoma cells. Fragments of these glycoproteins were found in serumfree medium and were similar to fragments released from cells treated with trypsin. Thus, some surface glycoproteins are degraded by membrane proteases or cell proteases released into the medium. Most of the other labeled glycoproteins were degraded at similar rates to small molecular weight material. The glycolipids of the plasma membrane also were degraded with a half-life of 100 h. These results suggest that the main pathway for plasma membrane degradation is by interiorization and degradation of structural membrane units.