Temperature Dependence of the Mitochondrial Inner Membrane Anion Channel

Abstract

The mitochondrial inner membrane anion channel (IMAC) carries a wide variety of anions and is postulated to be involved in mitochondrial volume homeostasis in conjunction with the K+/H+ antiporter, thus allowing the respiratory chain proton pumps to drive salt efflux. How it is regulated is uncertain; however, it is inhibited by matrix Mg2+ and matrix protons. Previously determined values for the ICs0 IC50 suggested that the channel would be closed under physiological conditions. In a previous study (Liu, G., Hinch, B., Davatol-Hag, H., Lu, Y., Powers, M., and Beavis, A. D. (1996) J. BioL Chem. 271, 19717-19723), it was demonstrated that the channel is highly temperature-dependent, and that a large component of this sensitivity resulted from an effect on the pIC50 for protons. We have now investigated the effect of temperature on the inhibition by Mg2+ and have found that it too is temperature-dependent. When the temperature is raised from 20 °C to 45 °C, the IC50 increases from 22 to 350 μm at pH 7.4 and from 80 to 1.5 mM at pH 8.4, respectively. The Arrhenius plot for the IC50 is linear with a slope = -80 kJ/mol. The IC50 is also strongly pH-dependent, and at 37 °C increases from 90 μM at pH 7.4 to 1230 μM at pH 8.4. In view of the extremely rapid fluxes that IMAC is capable of conducting at 37 °C, we conclude that inhibition by matrix Mg2+ and protons is necessary to limit its activity under physiological conditions. We conclude that the primary role of Mg2+ is to ensure IMAC is poised to allow regulation by small changes in pH in the physiological range. This control is mediated by a direct effect of H + on the activity, in addition to an indirect effect mediated by a change in the Mg2+ IC50. The question that remains is not whether IMAC can be active at physiological concentrations of Mg2+ and H+, but what other factors might increase its sensitivity to changes in mitochondrial volume.