Adenosine 5′-0(3-thiotriphosphate) in the control of phosphorytase activity

Rabbit muscle phosphorylase b (EC 2.4.1.1) is converted to a thio-analog of phosphorylase a by phosphorylase kinase, Mg2+ and adenosine 5′-O(3-thiotriphosphate)(ATPγS). Conversion proceeds at one-fifth the rate obtained with ATP though the extent of reaction and final level of activation of the enzyme are the same. However, the thiophosphorylase a produced is resistant to phosphorylase phosphatase and, therefore, behaves as a competitive inhibitor with a KI of 3 μM, similar to the KM obtained with normal phosphorylase a . ATPγS can also be utilized by protein kinase in the activation of phosphorylase kinase at a rate similar to that obtained with ATP. It is hydrolyzed at 5 to 10 times the normal rate by the sarcoplasmic reticulum ATPase. When added to a muscle glycogen-particulate complex in the presence of Ca2+ and Mg2+, ATPγS triggers an activation of phosphorylase with simultaneous inhibition of phosphorylase phosphatase as previously observed with ATP.