Glycogen Turnover 293 



It implies perfect mixing of newly introduced with preformed mole- 

 cules; it requires complete randomization of isotopic and non-isotopic 

 molecules and random selection of material from this pool for destruc- 

 tion. For want of a method of approach, this fourth assumption was 

 not explored at that time. Our more recent studies have led to a 

 realization that this assumption is incorrect. 



Since our more recent exploration of this assumption has depended 

 upon the newer knowledge of glycogen structure and of the several 

 enzymes related to glycogen breakdown, it may be well to digress 

 sufficiently to review the pertinent aspects of these developments. It is 

 generally agreed that glycogen is a branched polysaccharide in which 

 the major glucosidic linkage is a-1,4' and the minor linkage is a-1,6'. 

 The ratio of 1,4' to 1,6' linkages in typical glycogen samples range 

 from 10 to 18. 3 Of the three possible branching patterns that have 

 come into serious consideration the arboreal pattern first advocated 

 for amylopectin by Meyer 4 is almost certainly the dominant one. It 

 is in the nature of an arboreally branched polyglucosidc containing n 

 glucose residues that there must be, per molecule, one free reducing 

 end and n — 1 glucosidic bonds. In glycogen the number of branch 

 points should equal the number of a-1,6' glucosidic bonds and will be 

 one less than the number of non-reducing ends. Discrepancies between 

 experimentally secured values and the above expectations have resulted 

 in the suggestion that the structure of glycogen may not be of a simple 

 arboreal nature, 5 but, for the purposes of the present discussion, the 

 arboreal structure will be taken for granted. 



The experimental basis for the above assignment of structure rests 

 largely upon the examination of products secured when glycogen is 

 subjected to various enzymic degradations. AVhen exhaustively treated 

 with /^-amylase, glycogen was found by Meyer to yield maltose and a 

 limit dextrin which represented approximately 50% of the glucose 

 residues initially present. This dextrin was totally refractory to further 

 attack by /3-amylase. 6 In Cori's laboratory this product was further 

 studied, and, in addition, a limit dextrin was secured from glycogen 

 by the action of purified phosphorylase and inorganic orthophosphate. 7 

 This latter limit dextrin, LD (glycogen, phosphorylase), insensitive to 

 further attack by phosphorylase, could be rendered phosphorylase- 

 sensitive again by the action of a specific amylo-l,6-glucosidase, which 

 yielded glucose by the selective hydrolytic cleavage of a-1,6' glucoside 

 bonds of exposed glucose residues. 8 The exposure of glycogen to simul- 

 taneous attack by phosphorylase and by amylo-l,6-glucosidase yielded 



