8-Azido-Nucleotides as Substrates of Torpedo Electric Organ Apyrase. Effect of Photoactivation on Apyrase Activity

Ecto-apyrase is a widespread enzymatic activity that hydrolyses tri- and diphosphonucleotides and consequently controls the amount of available extracellular ATP and ADP. In the nervous system, purines have important neuromodulatory actions, acting at pre- and postsynaptic sites, and consequently, ecto-apyrase may play an indirect role in the modulation of nucleotide- and nucleoside-mediated processes. The azido-nucleotides have been largely employed to characterize the nucleotide binding sites of several proteins. In the present work the azido-nucleotides are described as putative substrates for apyrase activity in a presynaptic plasma membrane preparation (PSPM) from the Torpedo electric organ. Both 8-N₃-ATP and 8-N₃-ADP were hydrolyzed in a calcium-dependent manner showing V_max of 23.8 ± 4.8 and 14.5 ± 3 U/mg of protein, and K_m values (in μM) of 116 ± 39 and 119 ± 4, respectively. V_max for calcium-dependent hydrolysis of ATP and ADP were significantly higher: 59.2 ± 3.9 and 32.9 ± 3.5 U/mg of protein respectively, while K_m values did not show any significant differences regarding azido-nucleotides: 83.8 ± 12 μM for Ca²⁺-ATP and 121 ± 34 μM for Ca²⁺-ADP. The photoactivation of the PSPM in the presence of the azido-derivatives results in an irreversible inactivation of apyrase activity, showing an IC₅₀ of 10 μM and a maximal inhibitory effect of 38 and 60% on Ca²⁺-ATPase and Ca²⁺-ADPase activities. Apyrase was protected from inactivation by nucleotides that are natural substrates for this enzymatic activity and also by AMP while adenosine did not protect from apyrase inhibition.