EARLY LABELING WITH GLUCOSAMINE-14C OF GRANULAR AND AGRANULAR ENDOPLASMIC RETICULUM AND FREE RIBOSOMES FROM RAT LIVER

The initiation of carbohydrate prosthetic group attachment onto liver glycoprotein apopolypeptides occurs while the latter are still bound nascent to their sites of synthesis on endoplasmic reticulum-bound ribosomes. Free ribosomes or free polysomes appear to be inactive in glycoprotein initiation. Though initial incorporation of glucosamine-14C is fastest into agranular reticulum glycoproteins, the granular reticulum soon attains a similar specific activity by 20-30 minutes after an intraperitoneal injection of glucosamine. The most active, early-labeled components of the agranular fraction are low-density membranes that differ from plasma membrane fragments and the Dallner subfractions of agranular reticulum. © 1968.