KINETIC STUDIES OF THE REVERSE REACTION CATALYSED BY ADENOSINE TRIPHOSPHATE-CREATINE PHOSPHOTRANSFERASE. THE INHIBITION BY MAGNESIUM IONS AND ADENOSINE DIPHOSPHATE.

1. Kinetic investigations of the reaction catalysed by ATP-creatine phosphotransferase have been carried out. 2. No firm conclusions could be reached about the reaction of Mg(2+) at the nucleotide-binding site of the enzyme. The value of the kinetic constant for this reaction depends on the value used for the apparent stability constant of the metal ion-nucleotide complex and, to a smaller extent, on the method of plotting the results. 3. At higher concentrations Mg(2+) is a non-competitive inhibitor of the enzyme with respect to both MgADP(-) and phosphocreatine. 4. ADP(3-) is a competitive inhibitor of the enzyme with respect to MgADP(-) and a non-competitive inhibitor with respect to phosphocreatine. 5. The concentration of phosphocreatine has little, if any, effect on the kinetic constants for the nucleotide reactants.