Complex I-mediated reactive oxygen species generation: modulation by cytochrome c and NAD(P)+ oxidation-reduction state

Dissociation of cytochrome c from the inner mitochondrial membrane during cardiac ischemia

Mitochondria isolated from ischemic cardiac tissue exhibit diminished rates of respiration and ATP synthesis. The present study was undertaken to determine whether cytochrome c release was responsible for ischemia-induced loss in mitochondrial function. Rat hearts were perfused in Langendorff fashion for 60 min (control) or for 30 min followed by 30 min of no flow ischemia. Mitochondria isolated from ischemic hearts in a buffer containing KCl exhibited depressed rates of maximum respiration and a lower cytochrome c content relative to control mitochondria. The addition of cytochrome c restored maximum rates of respiration, indicating that the release of cytochrome c is responsible for observed declines in function. However, mitochondria isolated in a mannitol/sucrose buffer exhibited no ischemia-induced loss in cytochrome c content, indicating that ischemia does not on its own cause the release of cytochrome c. Nevertheless, state 3 respiratory rates remained depressed, and cytochrome c release was enhanced when mitochondria from ischemic relative to perfused tissue were subsequently placed in a high ionic strength buffer, hypotonic solution, or detergent. Thus, events that occur during ischemia favor detachment of cytochrome c from the inner membrane increasing the pool of cytochrome c available for release. These results provide insight into the sequence of events that leads to release of cytochrome c and loss of mitochondrial respiratory activity during cardiac ischemia/reperfusion.

Regulation of brain mitochondrial H2O2 production by membrane potential and NAD(P)H redox state

Mitochondrial production of reactive oxygen species (ROS) at Complex I of the electron transport chain is implicated in the etiology of neural cell death in acute and chronic neurodegenerative disorders. However, little is known regarding the regulation of mitochondrial ROS production by NADH-linked respiratory substrates under physiologically realistic conditions in the absence of respiratory chain inhibitors. This study used Amplex Red fluorescence measurements of H2O2 to test the hypothesis that ROS production by isolated brain mitochondria is regulated by membrane potential (DeltaPsi) and NAD(P)H redox state. DeltaPsi was monitored by following the medium concentration of the lipophilic cation tetraphenylphosphonium with a selective electrode. NAD(P)H autofluorescence was used to monitor NAD(P)H redox state. While the rate of H2O2 production was closely related to DeltaPsi and the level of NAD(P)H reduction at high values of DeltaPsi, 30% of the maximal rate of H2O2 formation was still observed in the presence of uncoupler (p-trifluoromethoxycarbonylcyanide phenylhydrazone) concentrations that provided for maximum depolarization of DeltaPsi and oxidation of NAD(P)H. Our findings indicate that ROS production by mitochondria oxidizing physiological NADH-dependent substrates is regulated by DeltaPsi and by the NAD(P)H redox state over ranges consistent with those that exist at different levels of cellular energy demand.

Mitochondrial functional state in clonal pancreatic beta-cells exposed to free fatty acids

Excessive free fatty acid (FFA) exposure represents a potentially important diabetogenic condition that can impair insulin secretion from pancreatic beta-cells. Because mitochondrial oxidative phosphorylation is a main link between glucose metabolism and insulin secretion, in the present work we investigated the effects of the FFA oleate (OE) on mitochondrial function in the clonal pancreatic beta-cell line, MIN6. Both the long term (72 h) and short term (immediately after application) impact of OE exposure on beta-cells was investigated. After 72 h of exposure to OE (0.4 mm, 0.5% bovine serum albumin) cells were washed and permeabilized, and mitochondrial function (respiration, phosphorylation, membrane potential formation, production of reactive oxygen species) was measured in the absence or presence of OE. MIN6 cells exposed to OE for 72 h showed impaired glucose-stimulated insulin secretion and decreased cellular ATP. Mitochondria in OE-exposed cells retained normal functional characteristics in FFA-free medium; however, they were significantly more sensitive to the acute uncoupling effect of OE treatment. The mitochondria of OE-exposed cells displayed increased depolarization caused by acute OE treatment, which is attributable to the elevation in the FFA-transporting function of uncoupling protein 2 and the dicarboxylate carrier. These cells also had an increased production of reactive oxygen species in complex I of the mitochondrial respiratory chain that could be activated by FFA. A high level of reduction of respiratory complex I augmented acute FFA-induced uncoupling in a way compatible with activation of mitochondrial uncoupling protein by intramitochondrial superoxide. A stronger augmentation was observed in OE-exposed cells. Together, these events may underlie FFA-induced depression of the ATP/ADP ratio in beta-cells, which accounts for the defective glucose-stimulated insulin secretion associated with lipotoxicity.

Mitochondrial membrane permeabilization and superoxide production during apoptosis. A single-cell analysis

The temporal relationship between mitochondrial membrane permeabilization and reactive oxygen species production during apoptosis remains unknown. We analyzed the rate of superoxide production of human breast carcinoma cells expressing a cytochrome c-green fluorescent protein (cyt-c-GFP) fusion protein at the single-cell level during apoptosis. In cells treated with the proapoptotic agents staurosporine (3 microm) or tumor necrosis factor-alpha (100 ng/ml), the release of cyt-c-GFP was individually set for each cell, and the majority of the fusion protein was released in less than 10 min. Prior to the release of the fusion protein, cells demonstrated a constant rate of superoxide production determined with the probe hydroethidine. After the release was completed, the superoxide concentration increased rapidly to a level more than 3-fold above baseline. Treatment with the broad spectrum caspase inhibitor z-Val-Ala-Asp(O-methyl)-fluoromethylketone (z-VAD-fmk; 200 microm) did not alter the kinetics of the cyt-c-GFP release but significantly reduced superoxide concentration after the release of cyt-c-GFP. Interestingly, treatment with z-VAD-fmk also reduced the increase in superoxide concentration in response to menadione in the absence of mitochondrial cyt-c-GFP release. Mitochondrial depolarization with the protonophore carbonyl cyanide p-trifluoromethoxy-phenylhydrazone per se did not trigger cyt-c-GFP release or an increase in superoxide production. Our data suggest that mitochondria increase their superoxide production during apoptosis directly after the quantitative release of soluble intermembrane proteins and demonstrate novel antioxidative effects of the commonly used caspase inhibitor z-VAD-fmk.