Titration of mitochondrial buffer by accumulated anions

Membrane-potential-dependent changes in the stoichiometry of charge translocation by the mitochondrial electron transport chain

The charge/oxygen (q+/O) stoichiometry of mitochondria respiring on succinate was measured under conditions of high membrane potential (delta psi). The technique used was a variation of the steady-state method of Al-Shawi and Brand [(1981) Biochem. J. 200, 539-546]. We show that q+/O was about 2.7 at high values of delta psi (170 mV). As delta psi was lowered from 170 mV to 85 mV with the respiratory inhibitor malonate the q+/O stoichiometry increased to 6.0. A number of artefacts which could have led to an underestimation of the q+/O stoichiometry were eliminated. These included effects of any rapid change in mitochondrial volume, internal pH, activity of the endogenous K+/H+ exchanger or in H+ conductance due to changes in delta psi after the addition of inhibitor. The experiments presented here are the first direct demonstration that the stoichiometry of proton pumping by the mitochondrial respiratory chain changes as delta psi is varied.

Effects on mitochondrial K flux of pH, K concentration, and N-ethyl maleimide

Based on published evidence that cation transport in mitochondria is not significantly dependent on a membrane potential, it is suggested that the process of mitochondrial cation transport may be nonelectrogenic. These experiments focused on the possibility that K+ flux into rat liver mitochondria may be directly coupled, via an energy-linked carrier mechanism, to OH- influx or H+ efflux. The dependence of the unidirectional K+ influx on the external K+ concentration indicates involvement of a saturable mechanism. Increasing the external pH from 7.0 to 8.0 increases the apparent V max of the K+ influx without significantly altering the apparent Km for K+. The pH dependence is greater in the presence of N-ethyl maleimide, a known inhibitor of the mitochondrial Pi/OH- exchange mechanism. N-Ethyl maleimide decreases the apparent V max at pH 7.0 and increases it at pH 8.0. Evidence indicates that both N-ethyl maleimide and a high external Pi concentration may stimulate the K+ influx at alkaline external pH (8.0) by preventing net exchanges between endogenous Pi and external OH-. An apparent first-order dependence of the K+ influx on the external OH- concentration is observed in the presence of N-ethyl maleimide. These results are consistent with a possible role of external OH- as a cosubstrate of the K+ transport mechanism.