Oxaloacetate inhibition of succinate oxidation in tightly coupled liver mitochondria with ferricyanide as an electron acceptor

Inhibitors of succinate: quinone reductase/Complex II regulate production of mitochondrial reactive oxygen species and protect normal cells from ischemic damage but induce specific cancer cell death

Succinate:quinone reductase (SQR) of Complex II occupies a unique central point in the mitochondrial respiratory system as a major source of electrons driving reactive oxygen species (ROS) production. It is an ideal pharmaceutical target for modulating ROS levels in normal cells to prevent oxidative stress-induced damage or alternatively,increase ROS in cancer cells, inducing cell death.The value of drugs like diazoxide to prevent ROS production,protecting normal cells, whereas vitamin E analogues promote ROS in cancer cells to kill them is highlighted. As pharmaceuticals these agents may prevent degenerative disease and their modes of action are presently being fully explored. The evidence that SDH/Complex II is tightly coupled to the NADH/NAD+ ratio in all cells,impacted by the available supplies of Krebs cycle intermediates as essential NAD-linked substrates, and the NAD+-dependent regulation of SDH/Complex II are reviewed, as are links to the NAD+-dependent dehydrogenases, Complex I and the E3 dihiydrolipoamide dehydrogenase to produce ROS. This review collates and discusses diverse sources of information relating to ROS production in different biological systems, focussing on evidence for SQR as the main source of ROS production in mitochondria, particularly its relevance to protection from oxidative stress and to the mitochondrial-targeted anti cancer drugs (mitocans) as novel cancer therapies [corrected].

Does bacteriorhodopsin energize the membranes of animal mitochondria under light?

A suspension of freeze-thawed mitochondria mixed with purple membranes from Halobacteria was illuminated with visible light. It was found that the light exposure prevented inhibition of succinate oxidation. The illumination also led to a decrease in inhibition of the rate of ferricyanide reduction by rat liver mitochondria in the presence of succinate. Both phenomena are explained by the fact that oxalacetate inhibition of succinate dehydrogenase is prevented by light-induced energization of mitochondrial membranes due to the contact with purple membranes.

Bépridil a new effector of oxidative phosphorylations

Bépridil action on oxidative phosphorylations depends on the nature of the oxidized substrate. Thus it seemed to act as an uncoupler on the FAD-linked substrates as we have shown with succinate. But this property could not be applied to the NAD-linked substrates. Thus, in the presence of Bépridil, ADP would have opposite effects on the electron flow across the first site of oxidative phosphorylations; from one side the direct electron flow was decreased while from the other side, the reverse one was increased. Confirming this difference in its action, Bépridil did not affect the oxidative phosphorylation property of the cytochrome oxidase as could be deduced from the invariability of TMPD + ascorbate oxidation whether Bépridil was added or not. Moreover the effect of Bépridil on the P/O ratio was a dramatic demonstration of its selectivity. As a matter of fact this ratio was increased to a value near 5 for the NAD-linked substrate while it was decreased to near 0.4 with the FAD-linked substrate.

Correlation of the effects of citric acid cycle metabolites on succinate oxidation by rat liver mitochondria and submitochondrial particles

1. Succinate dehydrogenase is inhibited by citrate and beta-hydroxy-butyrate in a complex manner, both in mitochondria and submitochondrial particles. Kinetics of inhibition in the particles points to a competitive component in the mechanism involved. 2. Pyruvate, alpha-ketoglutarate, malate, and glutamate stimulate oxidation of succinate by mitochondria. 3. Stimulation by alpha-ketoglutarate and glutamate is not influenced by the presence of rotenone. 4. Stimulation by pyruvate is higher in the absence of rotenone and increases significantly in the presence of K+ and valinomycin. Pyruvate supplies in mitochondria reducing equivalents for malate dehydrogenase operating in the reverse direction-reduction of oxaloacetate to malate. 5. Stimulation by malate is higher in the presence of rotenone.