A novel ATP-diphosphohydrolase from human term placental mitochondria

This report describes an ATP-diphosphohydrolase activity associated with the inner membrane of human term placental mitochondria. An enriched fraction containing 30 per cent of the total protein and 80 per cent of the total ATP-diphosphohydrolase activity was obtained from submitochondrial particles. ATP-diphosphohydrolase activity was characterized in this fraction. The enzyme had a pH optimum of 8 and catalysed the hydrolysis of triphospho- and diphosphonucleosides other than ATP or ADP. Pyrophosphate was also hydrolysed, but AMP or other monoester phosphates were not. The activity of ATP-diphosphohydrolase was dependent on Mg2+, Ca2+ or Mn2+ and the enzyme substrate was the cation-nucleotide complex. An excess of free cation produced inhibition. ATP-diphosphohydrolase activity was stimulated at micromolar concentrations of calcium or magnesium in the presence of La-PPi. Negative cooperativity kinetics was observed with all substrates tested. The V-max ranged from 150 to 300 nmol of Pi released/mg/min. The [S](0.5) for nucleotides was 1-10 mM and 182 mM for PPi. The enzyme was inhibited by orthovanadate, but not by L-phenylalanine, oligomycin, sodium azide, P-1,P-5-di(adenosine-5')pentaphosphate or sodium fluoride. The experimental evidence showing absence of inhibition bq sodium azide and sodium fluoride, hydrolysis of pyrophosphate but not of monoester phosphates, and negative cooperativity suggested that this enzyme was a novel ATP-diphosphohydrolase. (C) 1999 W. B. Saunders Company Ltd.