Nitric oxide catalyzed

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. 

In the chamber process, nitric oxide catalyzes the oxidation of sulfur dioxide to trioxide ... 

A homogeneous catalyst exists in the same phase as the reacting molecules. There are many examples in both the gas and liquid phases. One such example is the unusual catalytic behavior of nitric oxide toward ozone. In the troposphere, the part of the atmosphere closest to earth, nitric oxide catalyzes ozone production. However, in the upper atmosphere it catalyzes the decomposition of ozone. Both of these effects are unfortunate environmentally. 

A hot platinum wire glows when held over a concentrated ammonia solution. The oxidation of ammonia to produce nitric oxide, catalyzed by platinum, is highly exothermic. 

Arginine + 02 -> Citrulline + Nitric Oxide (catalyzed by Nitric Oxide Synthase). 

Dimmeler, S., and BrOne, B. (1992). Characterization of a nitric-oxide-catalyzed ADP-ribosylation of glyceraldehyde-3-phosphate dehydrogenase. Eur. J. Biochem. 210, 305-310. 

Pure nitroglycerin is a stable Hquid at temperate conditions. It decomposes above 60°C to form nitric oxides which in turn catalyze further decomposition. Moisture increases the rate of decomposition under these conditions. Double- and multibase propellants containing nitroglycerin have substantially shorter stabiHty Hves at 65 and 80°C than do single-base propellants. The decomposition of nitroglycerin proceeds as... 

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. 

Receptors linked to guanylyl cyclase and which catalyze the formation of guanosine triphosphate (GMP) to guanosine-3A -cychc monophosphate (cychc GMP) include those for atrial natriuretic factor (ANF) and endothehal-derived relaxing factor (EDRF), mediating vasodilatation, and nitric oxide [10102 3-9], NO, or a clearly related derivative. 

NO synthases (NOS, L-arginine, NADPH oxygen oxi-doreductases, nitric oxide forming EC 1.14.13.39) represent a family of enzymes that catalyze the formation of nitric oxide (NO) from the amino acid L-arginine. In mammals, three isoforms of NOS have been identified. They are termed neuronal NOS (nNOS, NOS I, NOS1), inducible NOS (iNOS, NOS H, NOS2), and endothelial... 

The catalytic activity of ln/H-ZSM-5 for the selective reduction of nitric oxide (NO) with methane was improved by the addition of Pt and Ir which catalyzed NO oxidation, even in the presence of water vapor. It was also found that the precious metal, particularly Ir loaded in/H-ZSM-5 gave a low reaction order with respect to NO, and then showed a high catalytic activity for the reduction of NO at low concentrations, if compared with ln/H-ZSM-5. The latter effect of the precious metal is attributed to the enhancement of the chemisorption of NO and also to the increase in the amount of NO2 adsorbed on in sites. 

The first interest in the electroreduction of N02 or NO catalyzed by metal complexes is to model the activity of nitrite reductase enzymes.327 There is also an extensive growth in studies related to the development of metal complex-based electrochemical sensors for NO determination in biological and environmental samples 328 329 Nitrate disproportionates to nitric oxide and nitrate in aqueous solution. 

Catalytic oxidation with dioxygen. The well-known aerial oxidation of the nitric oxide (NO ) produced in the stoichiometric oxidations in equation (90) to nitrogen dioxide (NO ) forms the basis for the NO -catalyzed oxidation of various donors with dioxygen251-253 (equation 96). 

In addition to nitric oxide, superoxide, and peroxynitrite, NO synthases are able to generate secondary free radicals because similar to cytochrome P-450 reductase, the reductase domain can transfer an electron from the heme to a xenobiotic. Thus it has been found [158,159] that neuronal NO synthase NOS I catalyzed the formation of CH3CH(OH) radical from ethanol. It was suggested that the perferryl complex of NOS I is responsible for the formation of such secondary radicals. Miller [160] also demonstrated that 1,3-dinitrobenzene mediated the formation of superoxide by nNOS. It was proposed that the enhancement of superoxide production in the presence of 1,3-dinitrobenzene converted nNOS into peroxynitrite-produced synthase and may be a mechanism of neurotoxicity of certain nitro compounds. 

It follows from the above that MPO may catalyze the formation of chlorinated products in media containing chloride ions. Recently, Hazen et al. [172] have shown that the same enzyme catalyzes lipid peroxidation and protein nitration in media containing physiologically relevant levels of nitrite ions. It was found that the interaction of activated monocytes with LDL in the presence of nitrite ions resulted in the nitration of apolipoprotein B-100 tyrosine residues and the generation of lipid peroxidation products 9-hydroxy-10,12-octadecadienoate and 9-hydroxy-10,12-octadecadienoic acid. In this case there might be two mechanisms of MPO catalytic activity. At low rates of nitric oxide flux, the process was inhibited by catalase and MPO inhibitors but not SOD, suggesting the MPO initiation. 

Belkner et al. [32] demonstrated that 15-LOX oxidized preferably LDL cholesterol esters. Even in the presence of free linoleic acid, cholesteryl linoleate continued to be a major LOX substrate. It was also found that the depletion of LDL from a-tocopherol has not prevented the LDL oxidation. This is of a special interest in connection with the role of a-tocopherol in LDL oxidation. As the majority of cholesteryl esters is normally buried in the core of a lipoprotein particle and cannot be directly oxidized by LOX, it has been suggested that LDL oxidation might be initiated by a-tocopheryl radical formed during the oxidation of a-tocopherol [33,34]. Correspondingly, it was concluded that the oxidation of LDL by soybean and recombinant human 15-LOXs may occur by two pathways (a) LDL-free fatty acids are oxidized enzymatically with the formation of a-tocopheryl radical, and (b) the a-tocopheryl-mediated oxidation of cholesteryl esters occurs via a nonenzymatic way. Pro and con proofs related to the prooxidant role of a-tocopherol were considered in Chapter 25 in connection with the study of nonenzymatic lipid oxidation and in Chapter 29 dedicated to antioxidants. It should be stressed that comparison of the possible effects of a-tocopherol and nitric oxide on LDL oxidation does not support importance of a-tocopherol prooxidant activity. It should be mentioned that the above data describing the activity of cholesteryl esters in LDL oxidation are in contradiction with some earlier results. Thus in 1988, Sparrow et al. [35] suggested that the 15-LOX-catalyzed oxidation of LDL is accelerated in the presence of phospholipase A2, i.e., the hydrolysis of cholesterol esters is an important step in LDL oxidation. 

At present, there is growing interest in the study of the effects of nitrogen oxides on LOX-catalyzed processes. It has been shown that nitric oxide is able to inhibit LOX-catalyzed lipid... 

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