Hydrolysis of P-2-purinoceptor agonists by a purified ectonucleotidase from the bovine aorta, the ATP-diphosphohydrolase

Pharmacologists are becoming more and more aware of the possibility that certain ATP analogues currently used to classify the P-2-purinoceptors are dephosphorylated by ectonucleotidases. In this study, we provide evidence that in the vascular system, these purine analogues are hydrolysed by an ATP-diphosphohydrolase (ATPDase). This enzyme is known as the major plasma membrane nucleotidase of endothelial and smooth muscle cells, and is believed to dephosphorylate extracellular triphospho- and diphosphonucleosides. Assays were conducted with a purified ATPDase from smooth muscle cells of bovine aorta. At a concentration of 250 mu M, adenosine 5'-(alpha,beta-methylene) triphosphonate (alpha,beta-metATP), adenosine 5'-(beta,gamma-methylene) triphosphonate (beta,gamma-metATP), adenosine 5'-(alpha,beta-methylene) diphosphonate (alpha,beta-metADP), adenylyl 5'-(beta,gamma-imido) diphosphonate (beta,gamma-imidoATP) and adenosine 5'-O-(2-thiodiphosphate) (ADP beta S) all resisted dephosphorylation, whereas 2-chloroadenosine triphosphate (2-chloroATP), 2-methylthioadenosine triphosphate (2-MeSATP) and 8-bromoadenosine triphosphate (8-bromoATP) were hydrolysed at 99, 63, and 20% of the rate of ATP hydrolysis, respectively. All the non-hydrolysable analogues tested, except alpha,beta-metADP, competed with ATP and ADP for the ATPDase catalytic site, reducing their hydrolysis by 35-50%. Apparent K-m values for ATP and ADP were estimated at 14.1 and 12.0 mu M, respectively, whereas apparent K-m and K-i values for the purine analogues ranged from 12 to 28 mu M These results strongly support the view that (1) the ATPDase is expected to reduce substantially the P-2-response induced by ATP, ADP, and some hydrolysable agonists; and (2) by competing with the hydrolysis of endogenously released ATP and ADP, non-hydrolysable analogues could alter the amplitude or direction of the cellular response induced by these natural substrates.