Divalent cation stimulation of substrate oxidation by corn mitochondria

The oxidation of reduced nicotinamide adenine dinucleotide, malate-pyruvate, and succinate by corn mitochondria in buffered 0.2 m KCl was determined as a function of divalent cations. Ni²⁺, Mg²⁺, Co²⁺, Ca²⁺, Mn²⁺, Sr²⁺, and Ba²⁺ stimulated reduced nicotinamide adenine dinucleotide oxidation in the absence of inorganic phosphate, with Ca²⁺ and Sr²⁺ having the greatest effect. Malate-pyruvate and succinate oxidation was stimulated by Ca²⁺, Ba²⁺, and Sr²⁺, but only in the presence of inorganic phosphate. Ca²⁺, Sr²⁺, and Ba²⁺ produced a simulated state 4 to state 3 transition with all three substrates, but only with malate-pyruvate and succinate was there a return to state 4. The order of divalent cation effectiveness suggests that the rate of water substitution from the cation inner coordination hydration sphere may be a rate-limiting step in certain mitochondrial reactions involving electron transport and phosphorylation.