Are mitochondria a spontaneous and permanent source of reactive oxygen species?

Abstract

The bioenergetic properties of mitochondria in combination with the high turnover rate of dioxygen qualify these organelles for the formation of reactive oxygen species (ROS). The assumption that mitochondria are the major intracellular source of ROS was essentially based on in vitro experiments with isolated mitochondria. The transfer of these data to the living cell may, however, be incorrect. Artefacts due to the preparation procedure or inadequate detection methods of ROS may lead to false positive results. Inhomogeneous results were found to be due to an interaction of the detection system with components of the respiratory chain which could be avoided by a recently developed non-invasive method. One of the most critical electron transfer steps in the respiratory chain is the electron bifurcation from ubiquinol to the cytochrome bc(1) complex. This electron bifurcation requires the free mobility of the head domain of the Rieske iron-sulfur protein. Inhibition of electron bifurcation by antimycin A causes leakage of single electrons to oxygen which results in the release of ROS. Hindrance of electron bifurcation was also observed following alterations of the physical state of membrane phospholipids in which the cytochrome bc(1) complex is inserted. Irrespective of whether the fluidity of the membrane was elevated or decreased, electron flow rates to the Rieske iron-sulfur protein were drastically reduced. Concomitantly superoxide radicals were released from these mitochondria, strongly suggesting the involvement of the ubiquinol/cytochrome bc(1) redox couple in this process.

Hydrogen Peroxide

“…If the electron flow were reverse (from complex II to I), then rotenone would be expected to decrease the production of O 2 ⅐Ϫ ; however, this was not the observation. Both rotenone and antimycin are known to increase O 2 ⅐Ϫ generation during forward electron flow, [23][24][25] but myxothiazol reportedly has the opposite effect, 24,25 although admittedly this point is controversial. 26 -28 Thus our results suggest that increases in the rate of forward electron flow in the mitochondria, which would occur with heightened myocardial oxygen metabolism, lead to the production of O 2 ⅐Ϫ , which then is converted to the coronary vasodilator, H 2 O 2 .…”

Endothelial Mitochondria

“…157,155 Mitochondria have been proposed as the source of the superoxide because its effects could be abrogated by inhibition of mitochondrial electron transport at either complex I or complex III, and not by inhibitors of xanthine oxidoreductase, NOS, or NADPH oxidase. 157,158 However, the story may not be so straightforward, because inhibition of complex III with antimycin is more commonly used to increase mitochondrial ROS production, 159 and diphenyleneiodonium, which inhibited superoxide production in these studies, 157 inhibits NADPH oxidase. 160 If the endothelium is indeed an important target of reperfusion injury, then this raises the question of whether ischemic pre-or postconditioning functions primarily at the level of the myocytes or the endothelium.…”

Visible light alters yeast metabolic rhythms by inhibiting respiration

“…YAP1 encodes a basic leucine zipper transcription factor that localizes to the yeast nucleus in the presence of H 2 O 2 and up-regulates transcription of oxidative stress genes such as thioredoxin reductase (TTR1), cytosolic thioredoxin (TRX2), and cytochrome-c peroxidase (CCP1) (32). If visible light impairs electron transport, high-energy electrons react prematurely with O 2 and form superoxide radicals (O 2 · − ), H 2 O 2 , and hydroxyl radicals ( · OH) (32,33). Therefore, yeast with an impaired ROS defense might be more susceptible to the deleterious effects of visible light.…”