DIRECT UTILIZATION OF MANNOSE FOR MAMMALIAN GLYCOPROTEIN-BIOSYNTHESIS

Direct utilization of mannose for glycoprotein biosynthesis has not be en studied because cellular mannose is assumed to be derived entirely from glucose. However, animal sera contain sufficient mannose to force uptake through glucose-tolerant, mannose-specific transporters. Under physiological conditions this transport system provides 75% of the ma nnose for protein glycosylation in human hepatoma cells despite a 50- to 100-fold higher concentration of glucose. This suggests that direct use of mannose is more important than conversion from glucose. Consis tent with this finding the liver is low in phosphomannose isomerase ac tivity (fructose-6-P<->mannose-6-P), the key enzyme for supplying gluc ose-derived mannose to the N-glycosylation pathway, [2-H-3] Mannose is rapidly absorbed from the intestine of anesthetized rats and cleared from the blood with a t(1/2) of 30 min. After a 30 min lag, label is i ncorporated into plasma glycoproteins, and into glycoproteins of all o rgans during the first hour. Most (87%) of the initial incorporation o ccurs in the liver, but this decreases as radiolabeled plasma glycopro teins increase. Radiolabel in glycoproteins also increases 2- to 6-fol d in other organs between 1-8 h, especially in lung, skeletal muscle, and heart. These organs may take up hepatic-derived radiolabeled plasm a glycoproteins. Significantly, the brain, which is not exposed to pla sma glycoproteins, shows essentially no increase in radiolabel. These results suggest that mammals use mannose transporters to deliver manno se from blood to the liver and other organs for glycoprotein biosynthe sis. Additionally, contrary to expectations, most of the mannose for g lycoprotein biosynthesis in cultured hepatoma cells is derived from ma nnose, not glucose. Extracellular mannose may also make a significant contribution to glycoprotein biosynthesis in the intact organism.