Auto-oxidation

Benzaldehyde is easily oxidised by atmospheric oxygon giving, ultimately, benzoic acid. This auto-oxidation is considerably influenced by catalysts tiiose are considered to react with the unstable peroxide complexes which are the initial products of the oxidation. Catalysts which inhibit or retard auto-oxidation are termed anti-oxidants, and those that accelerate auto-oxidation are called pro-oxidants. Anti-oxidants find important applications in preserving many organic compounds, e.g., acrolein. For benzaldehyde, hydroquinone or catechol (considerably loss than U-1 per cent, is sufficient) are excellent anti-oxidants. 

Verbenone results from the auto-oxidation of turpentine oil, d-... 

Solubility and stability of coelenterazine. Coelenterazine is very poorly soluble in neutral aqueous buffer solutions, and the solutions are unstable in air. It can be easily dissolved in water in the presence of alkali, but the resulting solution is extremely unstable under aerobic conditions. Coelenterazine is soluble in methanol, and the solution is relatively stable. The stability is enhanced by the addition of a trace of HCl. A methanolic solution of coelenterazine can be stored for several days at — 20°C, and a methanolic solution containing 1-2 mM HCl can be stored for several months at — 70°C under aerobic conditions without significant oxidation. In many other organic solvents, coelenterazine is less stable, and spontaneously auto-oxidized at significant rates. In dimethylformamide and DMSO, it is rapidly decomposed accompanied by the emission of chemiluminescence. e-Coelenterazines are generally less stable than coelenterazines. 

The absorption spectra of Watasenia luciferin (coelenterazine disulfate) and Watasenia oxyluciferin (coelenteramide disulfate) are shown in Fig. 6.3.2. Watasenia luciferin in neutral aqueous solutions is auto-oxidized in air more rapidly than coelenterazine, and the compound emits a strong chemiluminescence in the presence of H2O2 ( 10mM) plus Fe2+ ( 0.2mM). Watasenia oxyluciferin is strongly fluorescent in aqueous solutions (Amax 400 nm), 500 times stronger than the fluorescence of coelenteramide in aqueous media (Goto et al., 1974). 

Harvey (1952) demonstrated the luciferin-luciferase reaction with O. phosphorea collected at Nanaimo, British Columbia, Canada, and with O. enopla from Bermuda. McElroy (1960) partially purified the luciferin, and found that the luminescence spectrum of the luciferin-luciferase reaction of O. enopla is identical to the fluorescence spectrum of the luciferin (A.max 510 nm), and also that the luciferin is auto-oxidized by molecular oxygen without light emission. Further investigation on the bioluminescence of Odontosyllis has been made by Shimomura etal. (1963d, 1964) and Trainor (1979). Although the phenomenon is well known, the chemical structure of the luciferin and the mechanism of the luminescence reaction have not been elucidated. 

Fig. 7.2.4 Absorption spectra of the pink substance obtained from Odontosyllis luciferin by auto-oxidation in ethanol/water (5 6) containing 8% NaCl A, in water (A-max 520 nm) B, in 30 mM HCl C, in 0.1 M NaOH D, after heating C at 90°C for 5 min, then cooled. The concentrations of the substance are normalized. From Shimomura et al., 1963d, with permission from John Wiley Sc Sons Ltd.

Additionally, sulfite may undergo simultaneous auto-oxidation and reduction reactions at higher pressures (see equation 2). 

At elevated temperatures, methylene carbons cleave from aromatic rings to form radicals (Fig. 7.44). Further fragmentation decomposes xylenol to cresols and methane (Fig. 7.44a). Alternatively, auto-oxidation occurs (Fig. 1.44b ). Aldehydes and ketones are intermediates before decarboxylation or decarbonylation takes place to generate cresols and carbon dioxide. These oxidative reactions are possible even in inert atmospheres due to the presence of hydroxyl radicals and water.5... 

Stokes K, Urbanski P, and Upton J. The in vivo auto-oxidation of polyether pol3furethane hy metal ions. 

Peroxidation is also catalyzed in vivo by heme compounds and by lipoxygenases found in platelets and leukocytes. Other products of auto-oxidation or enzymic oxidation of physiologic significance include oxysterols (formed from cholesterol) and isoprostanes (prostanoids). 

CHj—), which do not form a conjugated ring system. Thus, these compounds are colorless (as are all porphyrinogens). However, the porphyrinogens are readily auto-oxidized to their respective colored porphyrins. These oxidations are catalyzed by light and by the porphyrins that are formed. 

Superoxide is formed (reaction 1) in the red blood cell by the auto-oxidation of hemoglobin to methemo-globin (approximately 3% of hemoglobin in human red blood cells has been calculated to auto-oxidize per day) in other tissues, it is formed by the action of enzymes such as cytochrome P450 reductase and xanthine oxidase. When stimulated by contact with bacteria, neutrophils exhibit a respiratory burst (see below) and produce superoxide in a reaction catalyzed by NADPH oxidase (reaction 2). Superoxide spontaneously dismu-tates to form H2O2 and O2 however, the rate of this same reaction is speeded up tremendously by the action of the enzyme superoxide dismutase (reaction 3). Hydrogen peroxide is subject to a number of fates. The enzyme catalase, present in many types of cells, converts... 

Serra, PA, Esposita, G, Enrico, P, Mura, M, Migheli, R, Delogu, MR, Miele, M, Desole, MS, Grella, G and Miele, E (2000) Manganese increases L-dopa auto-oxidation in the striatum of the freely moving rat potential implications to L-dopa long-term therapy of Parkinson s disease. Brit. J. Pharmacol. 130 937-945. 

The second pathway is the eccentric cleavage that occurs at double bonds other than the central 15,15 -double bond of the P-carotene molecule to produce different products called P-apocarotenals with various chain lengths. Because only trace amounts of apocarotenals were detected in vivo from tissues of animals fed P-carotene and these compounds can be formed non-enzymatically from P-carotene auto-oxidation, the existence of this pathway was controversial until recently. The identification of P-carotene 9, 10 -oxygenase (BC02), which acts specifically at the 9, 10 double bond of P-carotene to produce P-apo-lO -carotenal and P-ionone, provided clear evidence of the eccentric cleavage pathway in vivo. Lycopene was also reported as a substrate for BC02 activity. 

During ischaemia, the activity of cellular antioxidant systems may be reduced (Ferrari et al. 1985 GaUnanes etal. 1992). In addition, a number of cellular pathways that produce free radicals are primed during ischaemia such as the xanthine/xanthine oxidase system (McCord, 1987), catecholamine auto-oxidation (Jackson et al., 1986) and the arachadonic acid pathway (Halliwell and Gutteridge, 1989). Thus, during early reperfusion there is a burst of free radical production (see Fig. 4.1) that may overwhelm the antioxidant systems of the cells. 

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