Mg‐Atp Binding: Its Modification by Spermine, the Relevance to Cytosolic Mg²⁺ Buffering, Changes in the Intracellular Ionized Mg²⁺ Concentration and the Estimation of Mg²⁺ by ³¹P‐NMR

Abstract

It is now generally accepted that the intracellular ionized magnesium concentration ([Mg 2+ ] i ) in muscle cells is around 1 mmol l −1 ; in heart muscle this means that from the total some 90‐95% is bound (see McGuigan et al. 1991 a ). Although binding will include sequestration by intracellular organelles, a large part of the binding is by ATP in the cytosol and an equilibrium exists in the cytosol between free ATP, ionized magnesium and Mg‐ATP. The extend of this equilibrium depends on the equilibrium constant of the reaction, which is a function of pH, temperature and ionic strength. This equilibrium constant is also important in the estimation of [Mg 2+ ] i using 31 P‐NMR. In this method the difference between the α and β peaks of ATP is measured and from this shift and the equilibrium constant between Mg 2+ and ATP in the cytosol, the [Mg 2+ ] i can be calculated (Vink, 1993).