Free radicals nitrogen oxide

As reviewed in Chapter 3 of this book, nitric oxide is the product of the enzymatic oxidation of one of the guanidino nitrogen groups of L-arginine to the free radical nitric oxide and L-citrulline (Marietta etal., 1988). At present there appears to be at least three distinct isoforms of nitric oxide synthase that are primarily differentiated at the level of gene expression (Schmidt, 1992 Nathan,... 

An additional group of oxygen-containing radicals, termed RNOS, contain nitrogen as well as oxygen. These are derived principally from the free radical nitric oxide (NO), which is produced endogenously by the enzyme nitric oxide synthase. Nitric oxide combines with O2 or superoxide to produce additional RNOS. 

Consider the free radical nitrogen dioxide, NO2, a major contributor to urban smog that is formed when the NO in auto exhaust is oxidized. NO2 has several resonance forms. Two differ in terms of which O atom is doubly bonded, as in the case of ozone. Two others have the lone electron residing on the N or on an O, so the resonance hybrid has the lone electron delocalized over these two atoms ... 

In vitro studies have indicated that increasing the vitamin E content of LDL particles increases their resistance to oxidation and decreases their uptake by macrophages. Vitamin E supplementation has also been reported to suppress macrophage uptake of oxidized LDL in human arterial lesions and decrease urinary Fi-isoprostane (a footprint of free radical-mediated oxidation of arachidonic acid) concentrations. Reactive nitrogen species are also implicated in aortic oxidation of LDL and therefore potentially in atherosclerosis. 

Saturated fatty acids are very stable but unsaturated acids are susceptible to oxidation the more double bonds the greater the susceptibility. Unsaturated fatty acids, therefore, have to be handled under an atmosphere of inert gas (e.g. nitrogen) and kept away from (photo) oxidants or substances giving rise to free radicals. Anti-oxidant compounds have frequently to be used in the biochemical laboratory just as organisms and cells have to utilize similar compounds to prevent potentially harmful attack of acyl chains in vivo (section 8.11). 

Nitrogen monoxide ( nitnc oxide ) is another stable free radical Although known for hundreds of years NO has only recently been discovered to be an extremely important biochemical messenger and moderator of so many biological processes that it might be better to ask Which ones is it not involved m ... 

NO formation occurs by a complex reaction network of over 100 free-radical reactions, and is highly dependent on the form of nitrogen in the waste. Nitro-compounds form NO2 first, and then NO, approaching equiHbrium from the oxidized side. Amines form cyano intermediates on their way to NO, approaching equiHbrium from the reduced side. Using air as the oxidant, NO also forms from N2 and O2. This last is known as thermal NO. ... 

The reactive species that iaitiate free-radical oxidatioa are preseat ia trace amouats. Exteasive studies (11) of the autoxidatioa mechanism have clearly estabUshed that the most reactive materials are thiols and disulfides, heterocycHc nitrogen compounds, diolefins, furans, and certain aromatic-olefin compounds. Because free-radical formation is accelerated by metal ions of copper, cobalt, and even iron (12), the presence of metals further compHcates the control of oxidation. It is difficult to avoid some metals, particularly iron, ia fuel systems. 

The principal components of atmospheric chemical processes are hydrocarbons, oxides of nitrogen, oxides of sulfur, oxygenated hydrocarbons, ozone, and free radical intermediates. Solar radiation plays a crucial role in the generation of free radicals, whereas water vapor and temperature can influence particular chemical pathways. Table 12-4 lists a few of the components of each of these classes. Although more extensive tabulations may be found in "Atmospheric Chemical Compounds" (8), those listed in... 

Any substance capable of reacting with free radicals to form products that do not reinitiate the oxidation reaction could be considered to function as free-radical traps. The quinones are known to scavenge alkyl free radicals. Many polynuclear hydrocarbons show activity as inhibitors of oxidation and are thought to function by trapping free radicals [25]. Addition of R to quinone or to a polynuclear compound on either the oxygen or nitrogen atoms produces adduct radicals that can undergo subsequent dimerization, disproportionation, or reaction with a second R to form stable products. 

Titov claims that the free radical mechanism applies for nitration of aliphatic hydrocarbons, of aromatic side chains, of olefins, and of aromatic ring carbons, if irf the latter case the nitrating agent is ca 60—70% nitric acid that is free of nitrous acid, or even more dil acid if oxides of nitrogen are present... 

The concentration of ozone in the stratosphere is lower than predicted from reactions 1-4. This is due to the presence of trace amounts of some reactive species known as free radicals. These species have an odd number of electrons and they can speed up reaction 4 by means of catalytic chain reactions. Nitrogen oxides, NO and NO2, which are naturally present in the stratosphere at levels of a few parts per billion (ppb), are the most important catalysts in this respect. The reactions, first suggested by Paul Crutzen (2) and by Harold Johnston (3) in the early 1970 s, are as follows ... 

The stratosphere contains, however, only small amounts--a few tenths of a ppb-of chlorine free radicals of natural origin. They are produced by the decomposition of methyl chloride, CH3Q. The nitrogen oxides (NO and NO2) are more abundant and are produced in the stratosphere by the decomposition of nitrous oxide, N2O. Both CH3CI and N2O are of biological origin these compounds, released at the Earth s surface, are sufficiently stable to reach the stratosphere in significant amounts. 

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