Abstract

ATP-diphosphohydrolase (EC 3.6.1.5), also known as apyrase, has been found in rat mammary gland and several other animal tissuesanimal tissues1,2,3,4. The main kinetic characteristics of this enzyme are the broad specificity towards nucleoside di- and triphosphates and the dependence of either Ca2+ or Mg2+. These characteristics differentiate apyrase from the Ca2+transport ATPase and the Golgi UDPase described in mammary gland1,4. The Ca2+ transport enzyme isolated from plasma membrane of mammary carcinoma cells requires Mg2+ in addition to micromolar concentrations of Ca2+, it is calmodulin dependent and has a Mr of 150 kDa by SDS/PAGE5. The UDPase activity6, considered a Golgi apparatus marker, is specific for UDP, GDP and IDP and the optimum pH (below 7.0) is lower than the optimum for apyrase from animal tissues1. We have proposed that the function of this plasma membrane enzyme in mammary gland might be related to the extracellular nucleotide metabolism7. The action of apyrase, which removes Pi from ATP and ADP, and 5′-nucleotidase which finally produces adenosine, could play a role in the scavenging of purine ring ensuring its reincorporation into the cell. Furthermore this enzyme might have an important function in the modulation of extracellular ATP-induced changes of intracellular calcium activity, mediating a P2-type purinergic receptor, recently described in mammary gland epithelial cells8. If these proposed functions are correct, apyrase location in the plasma membrane should be as an ecto-enzyme. It has been shown by Carraway et al.9 in mammary gland and its tumours the presence of an ecto-ATPase activity stimulated by either Mg2+ or Ca2+.