Oxidative stress and adenine nucleotide control of mitochondrial permeability transition

Mitochondria can initiate apoptosis by releasing cytochrome c after undergoing a calcium-dependent permeability transition (MPT). Although the MPT is enhanced by oxidative stress and prevented by adenine nucleotides such as adenosine 5'-diphosphate (ADP), the hypothesis has not been tested that oxidants regulate the effects of exogenous adenine nucleotides on the MPT and cytochrome c release. We found that cytochrome c release from intact rat liver mitochondria depended strictly on pore opening and not on membrane potential, and that MPT-enhancing oxidative stress also augmented cytochrome c release. At low oxidative stress, micromolar (ADP) and low adenosine 5'-triphosphate (ATP)/ADP ratio inhibited the MPT and cytochrome c release, whereas ATP or high ATP/ADP had only a slight effect. In freshly isolated mitochondria, the time to half-maximal MPT was related to the log of the ATP/ADP ratio. This function was shifted to shorter times by oxidative stress which decreased ADP protection and caused ATP to accelerate the calcium-dependent MPT. By comparison, mitochondria treated with reducing agents and those isolated from septic rats were protected from the MPT by both nucleotides. These results indicate that oxidation-sensitive site(s) in the membrane regulate the effects of adenine nucleotides on the MPT. The oxidant-based differences in the effects of ADP and ATP on the pore support the novel hypothesis that failure of the cell to consume ATP and provide adequate ADP at the adenine nucleotide transporter during oxidative stress predisposes to cytochrome c release and initiation of apoptosis.