On the relationship between matrix free Mg2+ concentration and total Mg2+ in heart mitochondria

Abstract The matrix free magnesium ion concentration, [Mg 2+ ] m , estimated using the fluorescent probe furaptra, averaged 0.67 mM in 15 preparations of beef heart mitochondria containing an average of 21 nmol total Mg 2+ per mg protein. [Mg 2+ ] m was compared with total Mg 2+ during respiration-dependent uptake and efflux of Mg 2+ and during osmotic swelling. In the absence of external P i these mitochondria contain about 32 nmol/mg non-diffusible Mg-binding sites with an apparent K d of 0.34 mM. [Mg 2+ ] m depends on both the size of the total Mg 2+ pool and the ability of matrix anions to provide Mg-ligands. P i interacts strongly with Mg 2+ to decrease [Mg 2+ ] m and, in the absence of external Mg 2+ , promotes respiration-dependent Mg 2+ efflux and a decrease in [Mg 2+ ] m to very low levels. The uptake of P i by respiring mitochondria converts ΔpH to membrane potential (Δ Ψ ) and provides additional Mg-binding sites. This permits large accumulations of Mg 2+ and P i with little change in [Mg 2+ ] m . Nigericin also converts ΔpH to Δ Ψ in respiring mitochondria and induces a large and rapid increase in both total Mg 2+ and [Mg 2+ ] m . Mersalyl increases the permeability of the mitochondrial membrane to cations and this also induces a marked increase in both total Mg 2+ and [Mg 2+ ] m . These results suggest that mitochondria take up Mg 2+ by electrophoretic flux through membrane leak pathways, rather than via a specific Mg 2+ transporter. Mitochondria swollen by respiration dependent uptake of potassium phosphate show decreased [Mg 2+ ] m , whereas those swollen to the same extent in potassium acetate do not. This suggests that [Mg 2+ ] m is well-buffered during osmotic volume changes unless there is also a change in ligand availability.