KINETICS OF MG-2+ FLUX INTO RAT-LIVER MITOCHONDRIA

Unidirectional fluxes of Mg2+ across the limiting membranes of rat liver mitochondria were measured in the presence of the respiratory substrate succinate by 28Mg. Rates of influx and efflux of Mg2+ decreased when respiration was inhibited. A linear dependence of the reciprocal of the Mg2+ influx rate on the reciprocal of the external Mg2+ concentration was observed. The apparent KM for Mg2+ averaged about 0.7 mM. N-Ethylmaleimide [malNEt], an inhibitor of transmembrane phosphate-hydroxyl exchanges, enhanced the observed pH dependence of Mg2+ influx. In the presence of MalNEt, the apparent Vmax of Mg2+ influx was greater at pH 8 than at pH 7, and there was a linear dependence of the Mg2+ influx rate on the external OH- concentration. The K+ analog Tl+ inhibited Mg2+ influx, while La3+, an inhibitor of mitochondrial Ca2+ transport, has no effect on Mg2+ influx. Mg2+ competitively inhibited the flux of K+ into rat liver mitochondria. The mechanism(s) mediating mitochondrial Mg2+ and K+ fluxes appeared to be similar in their energy dependence, pH dependence, sensitivity to Tl+, and insensitivity to La3+.