Relation between the Gradient of the ATP/ADP Ratio and the Membrane Potential across the Mitochondrial Membrane

Abstract

The relation between the intramitochondrial and extramitochondrial ratio ATP/ADP, the transmembrane potential and pH gradient is investigated in the present communication. For this purpose mitochondria are equilibrated with added [¹⁴C]ATP in the presence of substrate and oligomycin for eliminating phosphate transfer by ATPase. The membrane potential was measured by the distribution of⁸⁶Rb⁺ in the presence of valinomycin, the ΔpH by the distribution of [¹⁴C]acetate. In the energized state by varying ΔΨ between 60 and 160 mV, the internal (ATP/ADP)_i is decreased 30-fold, the external (ATP/ADP)_e remains largely constant. As a result, the Δlog (ATP/ADP)_e/(ATP/ADP)_i=Δlogφ is increased linearly with ΔΨ according to the following relation: Δlogφ= 0.85 ΔΨ– 0.35. The ΔpH was changed between 0.1 and 0.8 by increasing the P_iconcentration causing only a minor decrease of Δlogφ. An increase of Δlogφ would be expected if the ATP-ADP exchange has a significant electroneutral portion. Also in the uncoupled and respiration-inhibited state the same function between Δlogφ and Δφ is found as in the energized states. It is concluded that under these conditions the ATP-ADP exchange is largely electrical.