ON THE RELATIONSHIP BETWEEN MATRIX FREE MG2-MITOCHONDRIA( CONCENTRATION AND TOTAL MG2+ IN HEART)

The matrix free magnesium ion concentration, [Mg2+](m), estimated usin g 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 respira tion-dependent uptake and efflux of Mg2+ and during osmotic swelling. In the absence of external P-i these mitochondria contain about 32 nmo l/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 abil ity of matrix anions to provide Mg-ligands. P-i interacts strongly wit h Mg2+ to decrease [Mg2+](m) and, in the absence of external Mg2+, pro motes 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 i n both total Mg2+ and [Mg2+](m). Mersalyl increases thr permeability o f 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 le ak pathways, rather than via a specific Mg2+ transporter. Mitochondria swollen by respiration dependent uptake of potassium phosphate show d ecreased [Mg2+](m), whereas those swollen to the same extent in potass ium acetate do not. This suggests that [Mg2+](m) is well-buffered duri ng osmotic volume changes unless there is also a change in ligand avai lability.