Effect of membrane potential on equilibrium poise between cytochrome a and cytochrome c in rat liver mitochondria.

Abstract

The object of this work was to test the suggestion that the equilibrium poise between cytochromea and cytochromec in mitochondria might be influenced by the membrane potential. 1. The midpoint potentials of cytochromes (c+c1) and cytochromea (CO present) were found to be 250 mV and 245 mV, respectively, by equilibrating rat liver mitochondria with mixtures of ferrocyanide and ferricyanide anaerobically in presence of antimycin A and measuring the redox state of the cytochromes spectrophotometrically. In absence of CO, cytochrome oxidase gave an anomalous redox titration curve with a “midpoint” at about 275 mV. 2. When the mitochondria were equilibrated with ferricyanide/ferrocyanide, the redox poise of cytochromea (CO present) and of cytochromes (a+a3) but not of cytochromes (c+c1) was dependent on the sign and magnitude of the membrane potential developed by treating the mitochondria as follows: by adding ATP, by chaging the composition of the suspension medium so as to vary the Donnan or Nernst potential, by adding valinomycin in a medium of low K+ ion content, or by adding a pulse of acid or alkali when the membrane was made permeable to protons with FCCP. 3. The findings agree with the suggestion that the respiratory chain is arranged across the cristae membrane with cytochromesc1 andc in contact with the outer phase and cytochromesa anda3 plugged through, so that the equilibrium distribution of electrons between thec anda cytochromes is influenced by the electric field across the membrane.