Inhibition of glycoprotein synthesis by cycloheximide in liver and Ehrlich tumor cells

Abstract Cycloheximide caused a rapid inhibition of [ 14 C]glucosamine incorporation into liver and plasma glycoproteins in rats and mice with a concomitant accumulation of UDP- N -acetyl hexosamine. In rat liver only a very small amount of radioactivity was present in lipid components but in mouse liver the lipid labeling was more substantial. The formation of a glucosamine-phospholipid, the main product in experiments in vitro and assumed to participate in glycosylation of proteins, could be demonstrated in vivo too. Cycloheximide had no significant effect on lipid labeling in these systems. When cycloheximide treatment of rats was made 30 min after the administration of [ 14 C]glucosamine the radioactivity of liver microsomal and ribosomal proteins decreased due to completion and discharge of glycoproteins into the plasma. Treatment of [ 14 C]glucosamine labeled purified ribosomes with puromycin resulted in the release of [ 14 C]glucosamine labeled nascent protein. In the ribosomes of cycloheximide treated rats much less nascent proteins was present then in the controls. These observations were consistent with one of the proposed pathways of glycoprotein synthesis. Cycloheximide, under conditions inhibitory to protein synthesis, had no effect on [ 14 C]glucosamine incorporation into proteins of Ehrlich cells in vitro . But if mice were treated with the drug and subsequently cells were isolated and tested for [ 14 C]-glucosamine uptake a decrease in the rate of incorporation was observed which was more pronounced as the duration of drug treatment increased. These observations could be attributed to the depletion of sugar-acceptor proteins prior to isolation of the cells. Using high concentrations of cycloheximide (1–5 mg/ml) caused strong inhibition of [ 14 C]glucosamine incorporation in vitro . Under such conditions a rapid loss of cytoplasmic nucleotides and radioactive components was observed. These results were due to the strong inhibitory action of cycloheximide on oxygen utilization. Supplementing the system with glucose, under conditions where glucose did not inhibit glucosamine incorporation, prevented the leakage of nucleotides from the cells and decreased the inhibitory effect of cycloheximide on glucosamine incorporation. In spite of extensive loss of nucleotides the cells retained viability in vivo .