ECTO-ATP-Diphosphohydrolase from Normal and Abnormal Placenta

Vasoactive properties of adenosine and ATP on placenta has been described.1 Adenosine has been associated with vasodilation and ATP has a dual effect (vasodilation and vasoconstriction) depending on the receptors to which it binds. ADP is a powerful platelet stimulant, whereas ATP and adenosine inhibits ADP-induced platelet aggregation2. Mechanims of inactivation of circulating nucleotides, which imply the sequential degration of nucleoside di- and triphosphates to adenosine, are important in the control of their vascular actions. The scavenging of extracellular nucleotides is only possible in the dephosphorylated form3. We propose that ATP-diphosphohydrolase or apyrase (E. C. 3.6.1.5), characterized by its ATPase and ADPase activities, could be involved in the regulation of the extracellular nucleotide levels acting together with 5′-nueleotidase4. This enzyme has been found localized as an ectoenzyme in erythrocytes and platelets5,6, as well as in endothelial and smooth muscle cells7. Apyrase is different from the high affinity Ca2+-ATPase involved in the Ca2+ transport8 and its function has been associated to platelet aggregation inhibition. The placental tissue does not produce significant amounts of PGI2, a potent platelet antiaggregant and ADPase activity could be the most important part of the anti-platelet activity9.