Glycogen metabolism in the normal red blood cell.

Evidence for active glycogen metabolism in normal mature red blood cells (RBC) is presented. Initial rates of ‘4C-U-glucose incorporation into erythrocyte glycogen were found to be independent of substrate concentration over a range of 3.3-16.6 mM. Incorporation of label into glycogen was initially linear but reached a plateau after a variable period of time that was Inversely related to RBC concentration in the medium. The major part of the incorporated radioactivity resided in the outer branches of the glycogen molecule. The optimum pH for 14C-Uglucose incorporation into glycogen was pH 7.6. Replacing the radioactive glucose employed for incorporation after 1 hr of incubation with non labeled glucose resulted in a gradual loss of radioactivity from erythrocyte glycogen. In normal cells, glycogen synthesis and breakdown do not result in any significant accumulation of glycogen, whereas in erythrocytes with enzyme defects affecting glycogen breakdown, substantial deposition of glycogen may be observed. T HE MATURE HUMAN ERYTHROCYTE depends on environmental glucose to meet its energy requirements.’ 2 The normal red blood cell (RBC) has no significant glycogen stores,3 although enzymes catalyzing glycogen synthesis and breakdown have been shown to be present.48 In contrast, glycogen deposition has been demonstrated in erythrocytes of patients affected with certain types of glycogen-storage disease (type III and type VI).9 ’#{176}Sidbury et al. assumed that the glycogen in affected red blood cells is a vestigial remnant from an early stage in the development of the erythrocyte that, in view of the absence of an active glycogen metabolism, is not broken down to any significant degree in the mature cell.9 It is also possible that the normal mature erythrocyte maintains an active glycogen metabolism in which the steady state favors, in the presence of normal enzyme activities, glycogen breakdown, whereas glycogen accumulates unless either amylo-1,6-glucosidase or phosphorylase is missing. The present study was undertaken to investigate glycogen metabolism in the mature red blood cell, measuring rates of glycogen synthesis and degradation under varying incubation conditions and determining the distribution of radioactivity within the glycogen molecule after incorporation of radioactive glucose into glycogen.