INHIBITION OF ATP-DIPHOSPHOHYDROLASE (APYRASE) OF TORPEDO ELECTRIC ORGAN BY 5'-P-FLUOROSULFONYLBENZOYLADENOSINE

It has been shown previously that ATP is released into extracellular s pace from pre- and postsynaptic sources in peripheral synapses, The ex tracellular metabolism of ATP is likely to affect nucleotide- and nucl eoside-mediated regulation of neurotransmission. The enzymes responsib le for ATP breakdown are nucleotidases whose active site faces the ext racellular space. ATPase and ADPase Ca2+-dependent activities were cha racterized in presynaptic plasma membrane preparation from the electri c organ of Torpedo. Features described were in accordance with the pre sence of an ATP-diphosphohydrolase (apyrase EC 3.6.1.5) in this fracti on. Active site studies using the affinity label 5'-fluorosulfonylbenz oyladenosine were performed on Torpedo apyrase. ATPase and ADPase Ca2-dependent activities were inhibited with 5'-fluorosulfonylbenzoyladen osine. From this study it is concluded: (1) 5'-fluorosulfonylbenzoylad enosine binds specifically to the active site of apyrase, (2) Divalent cations accelerate the apyrase inactivation rate. (3) Divalent cation s are not required for the binding of either the substrate or the inhi bitor to the active site, (4) The apyrase active site is more specific for highly phosphorylated nucleotides. The results presented may be e xtrapolated to apyrases from other sources. The importance of this enz yme and its regulation are discussed.