Energy-linked Adenosine Diphosphate Accumulation by Corn Mitochondria: II. Phosphate and Divalent Cation Requirement

The requirement for phosphate and Mg(2+) in energy-linked [(3)H] ADP accumulation by corn mitochondria has been 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/(32)Pi exchange occurs.Mg(2+) is also accumulated during ADP uptake. Mg(2+) can be substituted with varying efficiency by other divalent cations, but not monovalent cations. Effective cations typically increase phosphate uptake, particularly Ca(2+) . Ca(2+) -activated ADP accumulation is insensitive to carboxyatractyloside over a wide range of Ca(2+) concentrations. When Ca(2+) is substituted for Mg(2+) it is not necessary to block ATP formation to secure high levels of ADP accumulation, since Ca(2+) will divert energy from ATP formation into ion uptake.It is suggested that 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.