title: A calcium ion-dependent atp pyrophosphohydrolase in Physarum polycephalum.
abstract: 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.
mesh_terms: D000255:Adenosine Triphosphate; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology*; D002850:Chromatography, Gel; D002852:Chromatography, Ion Exchange; D004591:Electrophoresis, Polyacrylamide Gel; D004789:Enzyme Activation; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008970:Molecular Weight; D009235:Myxomycetes / Q000201:enzymology*; D010804:Physarum / Q000201:enzymology*; D011487:Protein Conformation; D011755:Pyrophosphatases / Q000037:antagonists & inhibitors / Q000302:isolation & purification / Q000378:metabolism*; D012995:Solubility; D013329:Structure-Activity Relationship; D013696:Temperature
chemical_list: D002110:Caffeine; D000255:Adenosine Triphosphate; D011755:Pyrophosphatases; D008274:Magnesium; D002118:Calcium