The Mechanism of Carbohydrase Action 8 . STRUCTURES OF THE MUSCLE - PHOSPHORYLASE LIMIT DEXTRINS OF GLYCOGEN AND AMYLOPECTIN BY GWEN

Cori & Larner (1951) described the preparation from rabbit muscle of an enzyme splitting the x-1:6-bonds of glycogen and amylopectin. This enzyme, amylo-1:6-glucosidase, did not act on the native polysaccharides but on the muscle-phosphorylase limit dextrins (#-dextrins), from which glucose was liberated, the dextrins then becoming susceptible to further phosphorolysis. They concluded that in the #-dextrin the side chains (A chains; Peat, Whelan & Thomas, 1952) were reduced in length to single glucose units, and that the outer portions of the main chains (B chains) were five to six units long, an estimate which was revised to six or seven units by Larner, Illingworth, Cori & Cori (1952) (Fig. 1). The unequal attenuation of the A and B chains by muscle phosphorylase is in contrast with the effect of ,-amylase, since the ,-limit dextrin (f-dextrin) of amylopectin contains A and outer B chains of equal length (two or three units) (Peat,Whelan& Thomas, 1952; Summer & French, 1956). Accordingly the s-dextrins of amylopectin and glycogen were reinvestigated to find a reason for these differences. The evidence suggests that the structure of the 4-dextrin proposed by Cori & Lamer (1951) is incorrect, in that theA chain is not a single glucose unit and that there is no great disparity, if any, between the lengths of the attenuated A and outer B chains. A preliminary account of this work has already b3en published (Walker & Whelan, 1959a).