Relations between glycogen phosphorylase activity and activities of amylolytic enzymes in rabbit skeletal muscle

In previous studies an inverse relation was found between glycogen phosphorylase activity and activities of amylolytic enzymes (a-amylase, neutral and acid glucoamylase) in extracts from various muscles. The present study was carried out in an attempt to explain this phenomenon in enzyme systems isolated from rabbit skeletal muscles: glycogen phosphorylase--a-amylase, glycogen phosphorylase--neutral glucoamylase, and a-amylase--neutral glucoamylase. Inhibition of a-amylase activity in presence of glycogen phosphorylase, as previously observed in muscle extracts, was absent in the system of purified enzymes, but was restored by addition of proteins of the muscle extract, particularly proteins of the mitochondrial fraction which form a complex with glycogen. The mechanism of inhibition of a-amylase activity depends on competition for the altered substrate: glycogen phosphorylase degrades the glycogen-protein complex as well as free glycogen, whereas a-amylase acts mainly on the free polysaccharide. In presence of glucoamylase, activity of glycogen phosphorylase decreases, mainly because of the inhibitory influence of glucose liberated by glycoamylase. In the a-amylase--glucoamylase system, activity of glucoamylase increases due to its greater affinity to the breakdown products of glycogen by a-amylase compared with affinity to intact molecules of the polysaccharide. These results explain the antagonism between glycogen phosphorylase and amylolytic enzymes and permit its schematic presentation.