Nitric oxide cycle

Figure 2 shows the nitric oxide cycle resulting in the emission of NO and pollutants arising from it at atmospheric temperatures8. 

Effect of Nitric Oxide on Ozone Depletion. Nitrous oxide is injected into the atmosphere from natural sources on earth about 10% is converted to nitric oxide (N20 + 0( D) — 2 NO), which in turn can catalyze the destmction of ozone (11,32,75). The two main cycles are 1 and 2. Rate constant data are given in Reference 11. 

B. Nitric Oxide, Nitrogen Dioxide, and Ozone Cycles... 

Nitric Oxide Synthase 266 4.5 Iron Redox Cycle Modifiers 272... 

Moreover in the retina, iron is a cofactor of a number of other enzymes, including nitric oxide synthase, (i-carotene monooxygenase, and RPE65-isomerohydrolase converting all-tranx-retinol to 11 -m -retinol in the visual cycle. 

Burlet, S. Cespuglio, R. (1997). Voltametric detection of nitric oxide (NO) in the rat brain its variations throughout the sleep-wake cycle. Neurosci. Lett. 226, 131-5. 

Nitric oxide has been implicated in sleep regulation. NO concentrations undergo state-dependent modulation during the sleep-wake cycle in the cortex (Burlet Cespuglio, 1997) and thalamus (Williams et al., 1997). Intraperitoneal,... 

Monti, J. M. Jantos, H. (2004). Microinjection of the nitric oxide synthase inhibitor L-NAME into the lateral basal forebrain alters the sleep/wake cycle of the rat. Prog. Neuropsychopharmacol. Biol Psychiatry 28 (2), 239-47. 

Importantly, the purple color is completely restored upon recooling the solution. Thus, the thermal electron-transfer equilibrium depicted in equation (35) is completely reversible over multiple cooling/warming cycles. On the other hand, the isolation of the pure cation-radical salt in quantitative yield is readily achieved by in vacuo removal of the gaseous nitric oxide and precipitation of the MA+ BF4 salt with diethyl ether. This methodology has been employed for the isolation of a variety of organic cation radicals from aromatic, olefinic and heteroatom-centered donors.174 However, competitive donor/acceptor complexation complicates the isolation process in some cases.175... 

In this chapter the generation of free radicals, mainly superoxide and nitric oxide, catalyzed by prooxidant enzymes will be considered. Enzymes are apparently able to produce some other free radicals (for example, HO and N02), although their formation is not always rigorously proved or verified. The reactions of such enzymes as lipoxygenase and cyclooxygenase also proceed by free radical mechanism, but the free radicals formed are consumed in their catalytic cycles and probably not to be released outside. Therefore, these enzymes are considered separately in Chapter 26 dedicated to enzymatic lipid peroxidation. 

Simultaneous generation of nitric oxide and superoxide by NO synthases results in the formation of peroxynitrite. As the reaction between these free radicals proceeds with a diffusion-controlled rate (Chapter 21), it is surprising that it is possible to detect experimentally both superoxide and NO during NO synthase catalysis. However, Pou et al. [147] pointed out that the reason is the fact that superoxide and nitric oxide are generated consecutively at the same heme iron site. Therefore, after superoxide production NO synthase must cycle twice before NO production. Correspondingly, there is enough time for superoxide to diffuse from the enzyme and react with other biomolecules. 

In denitrification, part of the biological nitrogen cycle, nitrate in the soil is converted via four enzymatic reactions stepwise to nitrite, nitric oxide and nitrous oxide to finally yield gaseous nitrogen. 

Reaction 2-6 is sufficiently fast to be important in the atmosphere. For a carbon monoxide concentration of 5 ppm, the average lifetime of a hydroxyl radical is about 0.01 s (see Reaction 2-6 other reactions may decrease the lifetime even further). Reaction 2-7 is a three-body recombination and is known to be fast at atmospheric pressures. The rate constant for Reaction 2-8 is not well established, although several experimental studies support its occurrence. On the basis of the most recently reported value for the rate constant of Reaction 2-8, which is an indirect determination, the average lifetime of a hydroperoxy radical is about 2 s for a nitric oxide concentration of 0.05 ppm. Reaction 2-8 is the pivotal reaction for this cycle, and it deserves more direct experimental study. 

Reactions 2-6 through 2-8 form a catalytic cycle, in that the hydroxyl radical that is used in Reaction 2-6 is r enerated in Reaction 2-8. The net results of this cycle are the oxidations of nitric oxide to nitrogen dioxide and carbon monoxide to carbon dioxide by the oxygen present in the air. 

There were two important innovations in the development of these oxidative cycles the use of carbon monoxide which had previously been considered a relatively inert molecule in the atmosphere to regenerate the hydroperoxy radical via Reactions 2-6 and 2-7 and the use of peroxy radicals HO, and RO, to oxidize nitric oxide to nitrogen dioxide. 

Nitrite reductases (NiRs)—enzymes found in several strains of denitrifying bacteria— catalyze the one-electron reduction of nitrite anion to nitric oxide (Equation 1). - In addition to the importance of this process in the global nitrogen cycle (Figure 1), further incentive for the study of the denitrification process is provided by its environmental impact, ranging from the production of NO as a pollutant and NjO as a potent greenhouse gas, to lake eutrophication due to farm runoff that contains high concentrations of nitrates and nitrites. 

The role of biomass in the natural carbon cycle is not well understood, and in the light of predictions of a future atmospheric energy balance crisis caused by carbon dioxide accumulation, in turn the result of an exponential increase in the consumption of carbon fuel, the apparent lack of concern by scientists and policy makers is most troubling. Yet there is no other single issue before us in energy supply which will require action long before the worst effects of excess production will be apparent. The only satisfactory model is the action taken by the R D community with respect to the SST in nitric oxide potential and chloro-halocarbon emissions, when it was realised that the stratospheric ozone layer was vulnerable to interference. Almost all other responses to pollution" have been after definitive effects have become apparent. 

The reaction of nitric oxide with superoxide dismutase is a simple reversible equilibrium, whereas the catalytic cycle with superoxide involves a two step sequence. Consequently, superoxide dismutase may be reduced by superoxide and then react with nitric oxide to form nitroxyl anion. Nitroxyl anion may react with molecular oxygen to form peroxynitrite anion (ONOO"). 

Welsh, N., Eizirik, D. L., Bendtzen, K., and Sandler, S. (1991a). Interleukin-1/3-induced nitric oxide production in isolated rat pancreatic islets requires gene transcription and may lead to inhibition of Krebs cycle enzyme aconitase. Endocrinology (Baltimore) 129, 3167-3173. 

Zafiriou, O. C., Hanley, Q. S., and Snyder, G. (1989). Nitric oxide and nitrous oxide production and cycling during dissimilatory nitrite reduction by Pseudomonas perfec txmuirina. J. Biol. Chem. 264, 5694-5699. 

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