M8OI toxicity is associated with an inhibition of ubiquinone reduction by complex I in the mitochondrial electron transport chain

Abstract

Methylimidazolium ionic liquids (MILs) are solvents used in an increasing variety of industrial applications. Recent studies identified the 8C MIL (M8OI) contaminating the environment, detected exposure in humans and proposed M8OI to be a potential trigger for the autoimmune liver disease primary biliary cholangitis (PBC). To gain a better understanding of any PBC trigger mechanism(s), the interaction of M8OI with mitochondria has been examined. M8OI inhibited oxygen consumption in intact cells and induced cell death (IC_50%–10 μM). Results from permeabilized cells indicated M8OI inhibits the mitochondrial electron transport chain at complex I, not complexes II, III or IV. Accordingly, succinate supported mitochondrial oxygen consumption and reduced cell death in the presence of M8OI. M8OI inhibited NADH oxidation by both mitochondrial membranes and purified complex I with IC_50% values of 470 μM and 340 μM respectively. Based on direct determinations of M8OI in non-mitochondrial and mitochondrial compartments, toxic M8OI concentrations were estimated to result in mitochondrial concentrations commensurate with complex I inhibition. Mitochondrial accumulation followed by complex I inhibition is therefore a possible molecular initiating event for M8OI-dependent cell death. NADH oxidation by purified complex I in combination with a flavin-site electron acceptor was not inhibited by M8OI, indicating no interaction of M8OI at the NADH-binding active site. Modelling supported M8OI binding to the ubiquinone-binding site. By inhibiting turnover, M8OI also gave rise to increases in complex-I-linked reactive oxygen species. However, inhibitors of oxidative stress did not affect M8OI-mediated cell death. The metabolic consequences of M8OI-mediated complex I inhibition, not increased reactive oxygen species production, are therefore the likely cause of apoptotic cell death. Understanding the effects on complex I and the pathways activated and leading to cell death may be informative regarding mitochondrial stress, cell death and diseases such as PBC.