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

The matrix free magnesium ion concentration, [Mg2+](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 Mg2+ per mg protein. [Mg2+](m) was compared with total Mg2+ during respiration-dependent uptake and efflux of Mg2+ 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. [Mg2+](m) depends on both the size of the total Mg2+ pool and the ability of matrix anions to provide Mg-ligands. P-i interacts strongly with Mg2+ to decrease [Mg2+](m) and, in the absence of external Mg2+, promotes respiration-dependent Mg2+ efflux and a decrease in [Mg2+](m) to very low levels. The uptake of P-i by respiring mitochondria converts Delta pH to membrane potential (Delta Psi) and provides additional Mg-binding sites. This permits large accumulations of Mg2+ and P-i with little change in [Mg2+](m). Nigericin also converts Delta pH to Delta Psi in respiring mitochondria and induces a large and rapid increase in both total Mg2+ and [Mg2+](m). Mersalyl increases thr permeability of the mitochondrial membrane to cations and this also induces a marked increase in both total Mg2+ and [Mg2+](m). These results suggest that mitochondria take up Mg2+ by electrophoretic flux through membrane leak pathways, rather than via a specific Mg2+ transporter. Mitochondria swollen by respiration dependent uptake of potassium phosphate show decreased [Mg2+](m), whereas those swollen to the same extent in potassium acetate do not. This suggests that [Mg2+](m) is well-buffered during osmotic volume changes unless there is also a change in ligand availability.