Contracted State as an Energy Source for Ca Binding and Ca + Inorganic Phosphate Accumulation by Corn Mitochondria 1

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An investigation has been made of the possibility of utilizing the potential energy of the contracted state of corn mitochondria to drive Ca + inorganic phosphate accumulation. Contraction was obtained with succinate or NADH oxidation. In the succinate experiments the mitochondria were contracted in buffered KCl layered over sucrose in centrifuge tubes and centrifuged down through distinct wash, reactive and isotope exchange layers. In the NADH experiments, ion accumulation was initiated upon exhaustion of the substrate. The results show that mitochondria in the contracted state will actively bind some 45Ca, but no real accumulation occurs until inorganic phosphate is available. Substrate powered contraction in the presence of inorganic phosphate also provides a potential for accumulation upon subsequent reaction of the mitochondria with Ca. It is deducted that contraction is due to X∼I formation, to which Ca will bind. Subsequent reaction with inorganic phosphate produces CaX∼P, which is the transport moiety. When X∼P is formed first, Ca also reacts to produce CaX∼P. Hence it is immaterial which ion reacts first with the contracted state. Contraction is believed to result from the action of a mechanoenzyme, presumably I∼. The stability of CaX∼I must be low for the mitochondria swell very rapidly upon exhaustion of NADH or blocking of succinate oxidation by cyanide.

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