On the nature of rabbit liver glycogen. I. Branching characteristics.

Both glycogen and amylopectin are branched chain polyglucosides. A differentiation has most often been made on the basis of differences in the degree of branching of the two polysaccharides. For large molecular weight, highly ramified structures like glycogen and amylopectin, the number of branch points may be considered to be equal to the number of terminal, non-reducing glucose residues (end-groups). End-group analysis has thus provided a means for determining the average frequency or degree of branching for such polysaccharides. The end-group assay value (EAV), here defined as the average number of glucose units per end-group, approximates 11 to 12 for glycogen (2-5). The corresponding value for amylopectin ranges from 20 to 30 (2, 3, 5), depending on the source. From these figures it is seen that glycogen is more highly branched than amylopectin since, as defined, the EAV is inversely related to the degree of branching. Indeed, glycogen has been described in this sense (6) as resembling the limit dextrin obtained from amylopectin by the action of @-amylase. In this light the isolation of an “abnormal” glycogen from rabbit liver by Haworth et al. (3) is of considerable interest. The abnormal glycogen yielded an EAV of 18, a value close to the range commonly associated with the amylopectins. A study of the conditions which give rise to the ‘Labnorma1” glycogen led Bell and coworkers (7,s) to conclude that rabbit liver glycogen may occur in either of two forms, the “normal” 12 unit structure or the “abnormal” 18 unit structure. Of the several carbohydrates tested as glycogen precursors only sucrose and galactose were found to give rise to the “abnormal” glycogen. Glycogen synthesis is generally assumed to be the resultant of the action of at least two enzymes, phosphorylase and the “branching” enzyme. It is not unreasonable to believe that these enzymes differ in their Michaelis-