CRYSTALLINE MUSCLE PHOSPHORYLASE

Abstract

It has been shown that glycogen is a highly branched molecule and that a 1: 6 glucosidic linkage is present at the point of branching (1). Recent fractionation studies of natural starches have shown that two components can be separated (24). One (which is called amylopectin by Meyer (5)) has a branched structure and is similar to glycogen; the other, amylose, is made up of long, unbranched chains of glucose residues in 1:4 glucosidic linkage. Potato starch and corn-starch contain about 20 per cent amylose, the remainder being amylopectin. The polysaccharides formed in vitro by brain, heart, and liver phosphorylase preparations give a brown color with iodine and resemble glycogen (6), while those formed by potato (7) and by muscle phosphorylase (8) have been shown to possess an unbranched structure and to resemble closely the amylose fraction of natural starch. If one assumes that muscle phosphorylase differs from the phosphorylases of other tissues, the fact that muscle contains glycogen and not amylose cannot be explained. Another possibility is to assume that all phosphorylases are of the type of the muscle enzyme; i.e., that they can only form polysaccharides with 1:4 linkages and that another enzyme (or factor) is necessary to make the 1: 6 linkages at the point of branching. The latter enzyme would be present in relatively high concentration in heart, liver, and brain extracts, while muscle extract (for an unknown reason) would contain very little of it.1 Attempts to free liver or heart phosphorylase of the second enzyme have not been successful. The best heart phosphorylase preparations had a purity level of only 20 per cent, if one assumes that pure muscle and heart