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.
Akerman
K E
KE
Department of Medical Chemistry, University of Helsinki, Siltavuorenpenger 10, SF-00170 Helsinki 17, Finland.
D000818:Animals; D001693:Biological Transport, Active / Q000502:physiology; D002118:Calcium / Q000378:metabolism*; D002412:Cations / Q000378:metabolism*; D008081:Liposomes / Q000378:metabolism*; D008099:Liver / Q000378:metabolism; D008928:Mitochondria / Q000378:metabolism*; D010619:Phenazines / Q000378:metabolism*; D010743:Phospholipids / Q000378:metabolism; D051381:Rats; D013053:Spectrophotometry; D013096:Spermine / Q000378:metabolism; D013696:Temperature
D002412:Cations; D008081:Liposomes; D010619:Phenazines; D010743:Phospholipids; D013096:Spermine; D002118:Calcium; C009195:safranine T