Analysis of the pathways of nitric oxide utilization in mitochondria

The regulatory role that mitochondria play in cell dysfunction and cell-death pathways involves the concept of a complex and multisite regulation of cellular respiration and energy production signaled by cellular and intercellular messengers. Hence, the role of nitric oxide, as a physiological regulator acting directly on the mitochondrial respiratory chain acquires further relevance. This article provides a survey of the major regulatory roles of nitric oxide on mitochondrial functions as an expression of two major metabolic pathways for nitric oxide consumption: a reductive pathway, involving mitochondrial ubiquinol and yielding nitroxyl anion and an oxidative pathway involving superoxide anion and yielding peroxynitrite. The modulation of the decay pathways for nitrogen-and oxygen-centered radicals is further analyzed as a function of the redox transitions of mitochondrial ubiquinol. The interplay among these redox processes and its implications for mitochondrial function is discussed in terms of the mitochondrial steady-state levels (and gradients) of nitric oxide and superoxide anion.     

Effect of nitric oxide donors on oxygen-dependent cytotoxic responses mediated by neutrophils

We analyzed the effect of nitric oxide (NO) on oxygen-dependent cytotoxic responses mediated by neutrophils against unopsonized erythrocytes using three NO donors: S-nitrosoglutathione (GSNO), S-nitroso-N-acetylpenicillamine (SNAP), and sodium nitroprusside (SNP). Neutrophils were treated with these compounds for 1-2 min at 37 degrees C and cytotoxicity was then triggered in the presence of NO donors by precipitating immune complexes, aggregated IgG, the chemotactic peptide FMLP, or opsonized zymosan. GSNO induced, in all cases, a marked increase in cytotoxic responses, while SNAP moderately increased cytotoxicity triggered by immune complexes, aggregated IgG, or Z, opsonized zymosen, without modifying those responses induced by FMLP. By contrast, SNP dramatically suppressed cytotoxicity triggered by all of the stimuli assessed. The enhancing effects mediated by GSNO and SNAP did not depend on the stimulation of guanylyl cyclase and were prevented by the NO scavengers hemoglobin and PTIO (2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl 3-oxide). The inhibitory activity of SNP, on the other hand, was not prevented by NO scavengers, suggesting that it cannot be ascribed to the release of NO. In another set of experiments, neutrophils were pretreated with GSNO or SNAP for different times. Then cells were washed to remove NO donors from the culture medium, and cytotoxicity was triggered by different stimuli. It was found that neutrophils must be pretreated with NO donors for at least 4 h to increase cytotoxic responses, and pretreatment for longer periods (i.e., 8 or 18 h) further increased cytotoxicity. Not only cytotoxic responses, but also the production of O2- and H2O2, and the release of myeloperoxidase were increased under these conditions.     

Nitric oxide, complex I, and the modulation of mitochondrial reactive species in biology and disease

Mitochondria are the specialized organelles for energy metabolism but also participate in the production of O(2) active species, cell cycle regulation, apoptosis and thermogenesis. Classically, regulation of mitochondrial energy functions was based on the ADP/ATP ratio, which dynamically stimulates the transition between resting and maximal O(2) uptake. However, in the last years, NO was identified as a physiologic regulator of electron transfer and ATP synthesis by inhibiting cytochrome oxidase. Additionally, NO stimulates the mitochondrial production of O(2) active species, primarily O(2)(-) and H(2)O(2), and, depending on NO matrix concentration, of ONOO(-), which is responsible for the nitrosylation and nitration of mitochondrial components. By this means, alteration in mitochondrial complexes restricts energy output, further increases O(2) active species and changes cell signaling for proliferation and apoptosis through redox effects on specific pathways. These mechanisms are prototypically operating in prevalent generalized diseases like sepsis with multiorgan failure or limited neurodegenerative disorders like Parkinson's disease. Complex I appears to be highly susceptible to ONOO(-) effects and nitration, which defines an acquired group of mitochondrial disorders, in addition to the genetically induced syndromes. Increase of mitochondrial NO may follow over-expression of nNOS, induction and translocation of iNOS, and activation and/or increased content of the newly described mtNOS. Likewise, mtNOS is important in the modulation of O(2) uptake and cell signaling, and in mitochondrial pathology, including the effects of aging, dystrophin deficiency, hypoxia, inflammation and cancer.     

Analysis of the pathways of nitric oxide utilization in mitochondria

The regulatory role that mitochondria play in cell dysfunction and cell-death pathways involves the concept of a complex and multisite regulation of cellular respiration and energy production signaled by cellular and intercellular messengers. Hence, the role of nitric oxide, as a physiological regulator acting directly on the mitochondrial respiratory chain acquires further relevance. This article provides a survey of the major regulatory roles of nitric oxide on mitochondrial functions as an expression of two major metabolic pathways for nitric oxide consumption: a reductive pathway, involving mitochondrial ubiquinol and yielding nitroxyl anion and an oxidative pathway involving superoxide anion and yielding peroxynitrite. The modulation of the decay pathways for nitrogen-and oxygen-centered radicals is further analyzed as a function of the redox transitions of mitochondrial ubiquinol. The interplay among these redox processes and its implications for mitochondrial function is discussed in terms of the mitochondrial steady-state levels (and gradients) of nitric oxide and superoxide anion.