Printed in Great Britain Concentration of MgATP2- and Other Ions in Solution CALCULATION OF THE TRUE CONCENTRATIONS OF SPECIES PRESENT IN MIXTURES OF ASSOCIATING IONS

1. A simple method is described for calculating the free concentrations of all species in a mixture of several ionic components that associate at equilibrium to any extent and with any stoicheiometry. 2. It can readily be adapted to take account of species such as protons for which the free rather than the total concentrations are controlled. 3. It was applied to mixtures of adenine nucleotides, Mg2+ and other ions relevant to the study of glucokinase (EC 2.7.1.2), but the qualitative conclusions are not peculiar to this system. 4. ATP exists in a high and nearly constant proportion (about 80%) as MgATP2- in solu-tions in which the total MgCl2 concentration exceeds the total ATP concentration by 1-10mM. 5. By contrast, the proportion ofATP present as MgATP2- varies greatly if the total MgCl2 and total ATP concentrations are varied in constant proportion. A major problem in the study of enzymes that require nucleoside phosphates is the multifarious composition of mixtures of such phosphates with metal ions. For example, an equimolar mixture of ATP and MgCI2 at pH7 contains appreciable pro-portions of MgATP2-, ATP4-, HATP3-, Mg2+ and Cl-, as well as traces of MgHATP-, Mg2ATP and MgCl+. Until fairly recently it was common practice to oversimplify this problem, by assuming, for example, that if the concentrations of metal ion and nucleotide were varied in constant ratio the nucleotide would exist largely as a 1:1 complex with the metal ion. The need to place stricter controls on the way in which cation and nucleotide concentra-tions are varied has led to several methods for calcu-lating the concentrations of the various species i