Biosynthesis of Glycoprotein by Liver: THE INCORPORATION IN VIVO OF 14C-GLUCOSAMINE INTO PROTEIN-BOUND HEXOSAMINE AND SIALIC ACID OF RAT LIVER SUBCELLULAR FRACTIONS

Abstract

Abstract The kinetics of the incorporation in vivo of 14C-glucosamine into the protein-bound hexosamine and sialic acid of nine rat liver subcellular fractions and of plasma indicates that hexosamine is incorporated into glycoprotein in the channels of both the rough and smooth surfaced endoplasmic reticulum, whereas sialic acid is incorporated primarily within the smooth surfaced endoplasmic reticulum. Puromycin in vitro releases between 13 and 52% of acid-insoluble 14C-glucosamine incorporated into rat liver ribosomes in vivo, indicating that some glucosamine is incorporated into nascent ribosome-bound polypeptide. Evidence is presented that membrane-bound ribosomes are the site of this glucosamine incorporation. The conclusion is drawn that hexosamine is first incorporated into newly synthesized polypeptide while it is still attached to the ribosome on which it is being assembled and that further hexosamine residues are incorporated after detachment from the ribosome as the polypeptide traverses the channels of the rough and smooth surfaced endoplasmic reticulum on its way out of the cell into the plasma. Sialic acid, which is usually found in terminal positions of glycoprotein prosthetic groups, is incorporated primarily during the final stages of this synthetic process while the glycoprotein is within the smooth surfaced endoplasmic reticulum. Morris hepatoma 5123 TC, which is deficient in membrane-bound ribosomes, shows a markedly altered pattern of 14C-glucosamine incorporation into glycoprotein.