The effect of membrane potential on the protonophoric action of oleic acid.

Free fatty acids, such as oleic acid, are able to act as uncouplers of oxidative phosphorylation in mitochondria [l]. They can also transport cations across membranes and may have a physiological role in iron transport in the gut [2]. We have previously shown that oleic acid can act as a K+/H+ ion exchange carrier in liposomes [3]. It was proposed that oleic acid is able to transport H+ and K+ as the protonated species and K+ soap, respectively. The action of oleic acid is therefore similar to that of the ionophore, nigericin. Herein, we present the result of an experiment designed to detect i f oleic acid action is affected by a membrane potential. The effect of the ionophores valinomycin and oleic acid on the rate of internal alkalinization of liposomes is shown in figure 1. The first trace shows the effect of a K+ pulse on liposomes in the absence of ionophores. The second trace shows the effect of the ionophore valinomycin. It can be seen the effect of valinomycin on the rate of internal alkalinization is negligible. It would appear that the K+ permeability is not rate limiting under these conditions. The third trace shows the effect of a low concentration of oleic acid where an increase in the rate of internal alkalinization can be seen. The final trace shows the effect of oleic acid in the presence of valinomycin. Here the rate of internal alkalinization is much greater than the rates of the second and third traces combined. Thus, oleic acid and valinomycin act synergistically on the rate of internal alkalinization following a K+ pulse. In the absence of a membrane potential, oleic acid is able to act as an electroneutral K+/H+ exchange carrier across membranes. Following the external addition of K+ in the presence of valinomycin, a membrane potential is induced and oleic acid appears to act as a more potent protonophore. We propose, that in the presence of a membrane potential there is an increase in the rate of the transmembrane movement of the anionic species of oleic acid. The presence of the membrane potential drives the movement of the anionic species across the membrane, hence the movement is electrophoretic. This mode of action may be significant with respect to the uncoupling of mitochondria by free fatty acids, where a steady state membrane potential exists. pH 0.1 1