Role of calcium and superoxide dismutase in sensitizing mitochondria to peroxynitrite-induced permeability transition.

The mitochondrial permeability transition pore (PTP) is a membrane protein complex assembled and opened in response to Ca(2+) and oxidants such as peroxynitrite (ONOO(-)). Opening the PTP is mechanistically linked to the release of cytochrome c, which participates in downstream apoptotic signaling. However, the molecular basis of the synergistic interactions between oxidants and Ca(2+) in promoting the PTP are poorly understood and are addressed in the present study. In isolated rat liver mitochondria, it was found that the timing of the exposure of the isolated rat liver mitochondria to Ca(2+) was a critical factor in determining the impact of ONOO(-) on PTP. Specifically, addition of Ca(2+) alone, or ONOO(-) and then Ca(2+), elicited similar low levels of PTP opening, whereas ONOO(-) alone was ineffective. In contrast, addition of Ca(2+) and then ONOO(-) induced extensive PTP opening and cytochrome c release. Interestingly, Cu/Zn-superoxide dismutase enhanced pore opening through a mechanism independent of its catalytic activity. These data are consistent with a model in which Ca(2+) reveals a molecular target that is now reactive with ONOO(-). As a test of this hypothesis, tyrosine nitration was determined in mitochondria exposed to ONOO(-) alone or to Ca(2+) and then ONOO(-), and mitochondrial membrane proteins were analyzed using proteomics. These studies suggest protein targets revealed by Ca(2+) include dehydrogenases and CoA - containing enzymes. These data are discussed in the context of the role of mitochondria, Ca(2+), and ONOO(-) in apoptotic signaling.