ATP-DIPHOSPHOHYDROLASE ACTIVITY (APYRASE, EC-3.6.1.5) IN HUMAN BLOOD-PLATELETS

Human platelets contain an ATP diphosphohydrolase activity (apyrase, E C 3.6.1.5) that is Ca2+ dependent, hydrolyses ATP and ADP and also GTP , ITP, CTP, GDP, IDP, CDP, The enzyme does not hydrolyse AMP, p-nitrop henylphosphate, inorganic phosphate or glucose-6-phosphate. Contaminan t activities were ruled out because the enzyme was not inhibited by 2 mu g/ml ouabain, 1.0 mM levamisole, 10 mu M Ap5A or 1.0 mM azide, The enzyme was sensitive to 100 mu M orthovanadate, 100 mu M Ap5A and 10 m M azide, reagents that have been described as inhibitors of some other apyrases, A strong inhibition by 1.0 mM NEM was observed, indicating that sulphydryl groups are involved in the enzyme activity, The parall el behaviour of ATPase and ADPase activities and the competition plot presented suggest that ATP and ADP hydrolysis occurs at the same activ e site, ATP diphosphohydrolase from human platelets may be involved in the modulation of nucleotide concentration in the circulation and thu s in vascular tonus.