A calcium ion-dependent atp pyrophosphohydrolase in Physarum polycephalum

An activity of ATP Pyrophosphohydrolase (EC 3.6.1.8) was found in the soluble fraction of the plasmodium of Physarum polycephalum. The products of the enzyme reaction were inorganic pyrophosphate and 5'-AMP in equimolar quantities. The enzyme had a pronounced requirement for Ca2+ with high specificity. Mg-2+ was not an essential cofactor but stimulated the enzyme activity about 2.5-fold of the control. The enzyme hydrolyzed ITP, GTP and beta,gamma-methylene ATP at a limited rate. Among inhibitors tested, 3 mM caffeine reduced the activity to about 75% of the control. The enzyme had a broad pH optimum around pH=7.0 and the Km for ATP was 2.0 mM. An Arrhenius plot showed a break at about 18 degrees C and the calculated activation energies were 6.7 and 11.4 kcal/mol above and below the transition temperature, respectively. Disc electrophoresis in dodecyl sulfate and gel filtration on Sephadex -g-200 gave apparent molecular weights of 56 000 and 240 000, respectively, suggesting that the native enzyme was built up from 4 polypeptide chains. The possible role of the enzyme in vivo was discussed.