Effects of Adenylic Acid on the Kinetics of Muscle Phosphorylase a

Abstract

It has long been known that muscle phosphorylase b has a strict requirement for adenylic acid (5’-AMP) (1). Phosphorylase a, on the other hand, is active without 5’-AMP (2) and although 5’-AMP may increase velocity somewhat, the increase according to most reports is not more than 50%. In connection with the use of phosphorylase and glycogen to measure low levels of inorganic orthophosphate, it was noticed that rates at 38” were very slow unless 5’-AMP was present. In exploring this further, it has been found that 5’-AMP can increase phosphorylase a activity 40 times or more when levels of glycogen, Pi, or glucose l-phosphate are low. At 25” the 5’-AMP concentration necessary to produce half-maximal stimulation is very small, less than 1% of that required for phosphorylase b and only 10% of the concentration found by Cori, Cori, and Green (2) to produce halfmaximal increments in phosphorylase a activity at high substrate levels.