Cytochrome bc1 regulates the mitochondrial permeability transition by two distinct pathways

The mitochondrial permeability transition (MPT) pore is a calcium-sensitive channel in the mitochondrial inner membrane that plays a crucial role in cell death. Here we show that cytochrome bc(1) regulates the MPT in isolated rat liver mitochondria and in CEM and HL60 cells by two independent pathways. Glutathione depletion activated the MPT via increased production of reactive oxygen species (ROS) generated by cytochrome bc1. The ROS producing mechanism in cytochrome bc1 involves movement of the Rieske iron-sulfur protein subunit of the enzyme complex, because inhibition of cytochrome bc1 by pharmacologically blocking iron-sulfur protein movement completely abolished ROS production, MPT activation, and cell death. The classical inhibitor of the MPT, cyclosporine A, had no protective effect against MPT activation. In contrast, the calcium-activated, cyclosporine A-regulated MPT in rat liver mitochondria was also blocked with inhibitors of cytochrome bc1. These results indicate that electron flux through cytochrome bc1 regulates two distinct pathways to the MPT, one unregulated and involving mitochondrial ROS and the other regulated and activated by calcium.