Effect of cations on the temperature sensitivity of Ca2+ transport in rat-liver mitochondria and safranine uptake by liposomes.

The activation energy of mitochondrial Ca2+ transport has been studied in various conditions by Arrhenius plots in the temperature range 6-20 degrees C. In the presence of Mg2+ the activation energy is decreased to 18 kJ/mole from that of 40 kJ/mole found in a sucrose medium. In the presence of the polyamine spermine the activation energy is practically 0 kJ/mole. A lanthanide Eu3+, which is a potent inhibitor of Ca2+ transport, has no significant effect on the activation energy. In a KCl medium the activation energy is increased to 70 kJ/mole. When both K+ and Mg+ are present the activation energy is nonlinear between 11 and 18 degrees C. In the presence of K+ and spermine it is about 0 kJ/mole between 6 and 13 degrees C and at higher temperatures 68 kJ/mole. Neither Mg2+ nor spermine affect the slope of the Arrhenius plot for state 4 respiration. Spermine decreases slightly the activation energy of Ca2+-stimulated respiration. Spermine also decreases the activation energy of valinomycin- or gramicidin-induced safranine uptake by liposomes from 68 to almost 0 kJ/mole between 17 and 30 degrees C. The results indicate that Ca2+ binding to the polar head groups of the phospholipids at the membrane surface is the rate-limiting step of mitochondrial Ca2+ transport, because agents that inhibit Ca2+ binding to these sites (Mg2+, spermine, K+) have the most marked effect, whereas Eu3+, which, because of the small concentration used, ought to interact mainly with the mitochondrial Ca2+ transport system, has no significant effect on the temperature sensitivity of mitochondrial Ca2+ transport.