The conversion of phosphorylase b (1,4-α-d-glucan:orthophosphate α-glucosyltransferase, EC 126.96.36.199) to phosphorylase a has been investigated in rabbit and frog skeletal muscle in vivo. It was found that hybrid phosphorylase (containing phosphorylated and non-phosphorylated subunits) is formed in vivo through the effects of hormonal and electrical stimulation. The stimulation in all cases leads to the conversion of the total amount of phosphorylase b to hybrid phosphorylase, i.e. phosphorylase b does not remain in the stimulated muscle. Hybrid phosphorylase is an allosteric sensitive enzyme species, it may be activated by AMP in the absence and in the presence of ATP, too. Therefore AMP and ATP may play a regulatory role, because ATP inhibits the activity of phosphorylase b in resting muscle but it does not inhibit the activity of hybrid phosphorylase. The possibility of this mechanism will be also discussed in this paper.
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