Energy-linked Adenosine Diphosphate Accumulation by Corn Mitochondria

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Abstract
The requirement for phosphate and Mg2+ in energy-linked [3H]ADP accumulation by corn mitochondria was studied. Arsenate will fully substitute for phosphate; sulfate partially substitutes; acetate, bicarbonate and pyrophosphate are ineffective. Phosphate is also taken up by the mitochondria, but the ADP/Pi ratio varies widely with experimental treatments. ADP does not exchange with endogenous labeled phosphate, although Pi/32Pi exchange occurs. Mg2+ is also accumulated during ADP uptake. Mg2+ can be substituted with varying efficiency by other divalent cations, but not monovalent cations. Effective cations typically increase phosphate uptake, particularly Ca2+. Ca2+-activated ADP accumulation is insensitive to carboxyatractyloside over a wide range of Ca2+ concentrations. When Ca2+ is substituted for Mg2+ it is not necessary to block ATP formation to secure high levels of ADP accumulation, since Ca2+ will divert energy from ATP formation into ion uptake. The transport mechanism may carry out a concerted transport of ADP and phosphate with bound divalent cation. The phosphate transporter may be involved, or alternatively, a special mechanism for trivalent anion transport may exist which acts cooperatively with the phosphate transporter.