ABSTRACT
The cells of blood vessel walls and the external surface of all blood cells have an ecto-ATPase which hydrolyzes ATP to ADP and also ADP to AMP. This enzyme has also been called apyrase or ATP-diphosphohydrolase. The enzyme hydrolyzes a broad range of tri-and diphosphate nucleosides such as UTP and UDP, GTP and GDP in additon to the adenine nucleotides and because of that it has also been called a nucleoside triphosphate hydrolase. The possible physiological roles for this ecto-ATPase involve the control of vascular tone by modulation of the levels of ATP and ADP binding to purino-receptors of the vasculature, the modulation of thrombogenesis by controlling the extracellular level of ADP which is known to activate platelet aggregation, and the protection from cytolytic effects of extracellular ATP. An ATP-diphosphohydrolase activity has been characterized on the external surface of Schistosoma mansoni, a parasite that lives in the circulation of the human host, and on the outer surface of Entamoeba histolytica, a parasite that may enter the circulation of the host through ulceration in the intestinal mucosa. The endoparasite Toxoplasma gondii also exhibits a nucleoside triphosphate hydrolase of high activity, although in this case the ecto-localization is still not documented. We raise the possibility that the endoparasites have evolved in a way to possibly mimic some of the conditions on the surface of cells normally present in the host circulation, thus escaping hemostatic defense responses of the host which require extracellular ADP or ATP.