Glycogen metabolism in glycogen rich erythrocytes

High concentrations of red blood cell glycogen were visualized by electron microscopy and demonstrated biochemically in amyl 1,6 glucosidase and phosphorylase deficient red blood cells. Glycogen concentration decreased as a function of cell age. Similar incorporation rates of ¹⁴C U glucose into glycogen were observed in amylo 1,6 glucosidase deficient and normal erythrocytes, characterized by an initial rise, followed by a plateau formation reflecting a steady state between glycogen synthesis and breakdown. A different pattern of kinetics was observed in phosphorylase deficient cells, which in view of the lack of the degradative enzyme showed a continuous linear increase in radioactive glycogen formation leveling off only after exhaustion of substrate. Evidence that in amylo 1,6 glucosidase deficient red blood cells the main metabolic activity affects the outer branches of the glycogen molecule was obtained directly by β amylolytic degradation of the radioactive glycogen molecule and indirectly by a chase experiment substituting radioactive with nonlabeled glucose. Normal glycogen synthetase activity was found in all cases of amylo 1,6 glucosidase examined except in one family in which an unexpected low affinity of the enzyme to glycogen was found. The observation that both amylo 1,6 glucosidase and phosphorylase deficient red blood cells retain the capacity to incorporate glucose into glycogen indicates that glycogen synthesis in erythrocytes proceeds along the UDPG glycogen synthetase pathway and is not a result of reverse activity of any of the degradative enzymes.