EFFECTS OF ATP ON THE ACTIVITY OF NUCLEOSIDE 3′,5′‐CYCLIC MONOPHOSPHATE PHOSPHODIESTERASE FROM BRAIN

ATP is known to inhibit the phosphodiesterase activity in the supernatant fraction of the brain homogenate. Results showed that, when enzyme activity was assayed by determining the change in the concentration of substrate, the magnitude of the inhibition by 2 ∼ 3 mm-ATP was not more than 20% and this effect of ATP can be explained mainly, if not entirely, on the basis of chelation of ATP with Mg2+ and Ca2+in vitro, both of which are necessary for enzyme activity. When brain phosphodiesterase was assayed by measuring 5′-AMP (product), the effect of ATP was erroneously exaggerated. This is due to ATP-dependent conversion of 5′-AMP to inosinic acid catalysed by adenylate deaminase in the crude preparation.