Characterization of Energy-Dependent Ca2+ Transport in Maize Mitochondria

Experimental conditions which optimize both substrate- and ATP-dependent Ca(2+) transport in corn (Zea mays) mitochondria have been determined. It has been found that a substrate (pyruvate + succinate) dependent, Pi independent, binding of Ca(2+) occurs. This reaction is very rapid and complete in less than 30 seconds. For massive accumulation of calcium, Pi is essential. Phosphate is accumulated along with the calcium and the ratio of Ca:Pi accumulated is about 1.6:1 indicating the precipitation of hydroxyapatite inside the mitochondria.The activation energies and Michaelis constants for both the substrate- and ATP-driven reactions have been determined. It has also been shown that the substrate-driven system is more efficient in Ca(2+) accumulation than the ATP-driven system. This is partially due to the fact that Mg(2+) is essential for the ATP-driven system but not for the substrate-driven system and that Mg(2+) acts as a strong competitor of Ca(2+) transport. The effect of other inorganic ions on Ca(2+) transport energized by both substrate and ATP were examined.The results lend support to the hypothesis that high energy intermediates of oxidative phosphorylation participate directly in Ca(2+) binding and transport in plant mitochondria.