Manganese sulfate-dependent glycosylation of endogenous glycoproteins in human skeletal muscle is catalyzed by a nonglucose 6-P-dependent glycogen synthase and not glycogenin

Glycogenin, a Mn2+-dependent, self-glucosylating protein, is considered to catalyze the initial glucosyl transfer steps in glycogen biogenesis. To study the physiologic significance of this enzyme, measurements of glycogenin mediated glucose transfer to endogenous trichloroacetic acid precipitable material (protein-bound glycogen, i.e., glycoproteins) in human skeletal muscle were attempted. Although glycogenin protein was detected in muscle extracts, activity was not, even after exercise that resulted in marked glycogen depletion. Instead, a MnSO4-dependent glucose transfer to glycoproteins, inhibited by glycogen and UDP-pyridoxal (which do not affect glycogenin), and unaffected by CDP (a potent inhibitor of glycogenin), was consistently detected. MnSO4-dependent activity increased in concert with glycogen synthase fractional activity after prolonged exercise, and the MnSO4-dependent enzyme stimulated glucosylation of glycoproteins with molecular masses lower than those glucosylated by glucose 6-P-dependent glycogen synthase. Addition of purified glucose 6-P-dependent glycogen synthase to the muscle extract did not affect MnSO4-dependent glucose transfer, whereas glycogen synthase antibody completely abolished MnSO4-dependent activity. It is concluded that: (1) MnSO4-dependent glucose transfer to glycoproteins is catalyzed by a nonglucose 6-P-dependent form of glycogen synthase; (2) MnSO4-dependent glycogen synthase has a greater affinity for low molecular mass glycoproteins and may thus play a more important role than glucose 6-P-dependent glycogen synthase in the initial stages of glycogen biogenesis; and (3) glycogenin is generally inactive in human muscle in vivo. (C) 1999 Elsevier Science B.V, All rights reserved.