Characterization of Energy-Dependent Ca2+ Transport in Maize Mitochondria

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Abstract
Experimental conditions which optimize both substrate- and ATP-dependent Ca2+ transport in corn (Zea mays) mitochondria have been determined. It has been found that a substrate (pyruvate + succinate) dependent, Pi independent, binding of Ca2+occurs. This reaction is very rapid and complete in less than 30 sec. For massive accumulation of Ca, Pi is essential. Phosphate is accumulated along with the Ca and the ratio of Ca:Pl accumulated is about 1.6:1 indicating the precipitation of hydroxyapatlte inside the mitochondria. The activation energies and Mlchaells constants for both the substrate- and ATP-driven reactions have been determined. It also has been shown that the substrate-driven system is more efficient in Ca2+ accumulation than the ATP-driven system. This is partially due to the fact that Mg2+ Is essential for the ATP-driven system but not for the substrate-driven system and that Mg acts as a strong competitor of Ca''2''1" transport. The effect of other Inorganic ions on Ca2+ 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 Ca2+ binding and transport In plant mitochondria.

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