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 Mg 2؉ and matrix protons. Although, in energized mitochondria, the anion flux through IMAC is expected to be in the outward direction, IMAC is most easily assayed in de-energized mitochondria by monitoring the rate of passive mitochondrial swelling that occurs after the addition of the potassium ionophore valinomycin to mitochondria suspended in potassium salts of the test anion (1). By using this assay, it has been shown that potential physiological regulators of IMAC include matrix Mg 2ϩ and matrix protons (5, 6). A number of nonphysiological inhibitors have also been identified. These include amphiphilic amines such as propranolol, the irreversible inhibitor N,NЈ-dicyclohexylcarbodiimide, and tributyltin, which is probably the most potent inhibitor of IMAC identified to date (see Ref. 1 for review of properties).More recently, we demonstrated that IMAC is extremely sensitive to temperature, in a manner that suggests the open probability is temperature-dependent (7). For example, using malonate as the substrate anion, the flux increased about 1000-fold when the temperature was raised from 5 to 40°C. A large part of this stimulation seems to result from a decrease in the pIC 50 of one of the inhibitory protonation sites. The most significant aspect of these findings is that they suggest the activity of IMAC might be significantly higher at physiological pH values and temperatures than what has been predicted from previous studies that were carried out at 25°C. The actual activity under physiological conditions cannot be predicted, however, because the effects of temperature were determined in Mg 2ϩ -depleted mitochondria, and the effect of temperature on the inhibition by matrix Mg 2ϩ is unknown. On the basis of published values for the IC 50 for Mg 2ϩ determined at 25°C (6), Jung and Brierley (8) have pointed out that activation of IMAC is unlikely at physiological concentrations of Mg 2ϩ and, for similar reasons, O'Rourke (3) has stated that the physiological role of IMAC is unclear. Thus, the goal of the present work was to determine the effect of temperature on the inhibition of IMAC by Mg 2ϩ and thus shed light on its physiological significance. * This work was supported in part by the American Heart Association, Ohio Affiliate, Columbus, Ohio. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.§ To whom correspondence should be addressed. Tel.: 419-383-4125; Fax: 419-383-2871; E-mail: abeavis@mco.edu.1 The abbreviations used are: IMAC, inner membrane anion channel; MOPS, 3-(N-morpholino)propanesu...

Development of Quantum Chemical Method to Calculate Half Maximal Inhibitory Concentration (IC50)

“…The rate of increase with the temperature is very slow. Similar kind of dependence of IC 50 on the temperature was observed by the experiment 16…”

“…For a particular Δ G 0 s , IC 50 for Δ G 0 I =−28 kcal/mol is much higher for the corresponding IC 50 value of Δ G 0 I =−30 kcal/mol. Experimentally this type of dependence of IC 50 on the temperature was observed before 16. The sharp increase of IC 50 with the temperature implies that the enzyme‐inhibitor complex is more sensitive to the thermal fluctuation than the enzyme‐substrate complex.…”

Cellular magnesium homeostasis

“…Additional mitochondrial function modulated by changes in Mg 2+ within the organelle are anion channels present in the mitochondrial membrane [287] as well as the opening of the permeability transition pore [288]. The mitochondrial inner membrane anion channel (IMAC) transports various anions, and is involved in regulating the organelle volume in conjunction with the K + /H + antiporter.…”

“…The mitochondrial inner membrane anion channel (IMAC) transports various anions, and is involved in regulating the organelle volume in conjunction with the K + /H + antiporter. Although its fine regulation is not fully elucidated as yet, experimental evidence suggests that matrix Mg 2+ and protons maintain the channel in its closed state [287]. Kinetic studies by Beavis and collaborators support a main role of Mg 2+ in maintaining the channel in a conformation that would allow fine modulation by small changes in pH and proton distribution under physiological conditions [287].…”

“…Although its fine regulation is not fully elucidated as yet, experimental evidence suggests that matrix Mg 2+ and protons maintain the channel in its closed state [287]. Kinetic studies by Beavis and collaborators support a main role of Mg 2+ in maintaining the channel in a conformation that would allow fine modulation by small changes in pH and proton distribution under physiological conditions [287]. The end results will be the maintenance of an optimal proton gradient and Δψ across the mitochondrial membrane, essential to retain proper organelle function and intra-mitochondrial Mg 2+ content [115].…”