Peroxidase-catalysed oxidation reaction

Oscillatory reactions are a typical class of phenomena, which display unusual features. After the discovery of Belousov-Zhabotinskii (B-Z) reaction, there has been a tremendous flurry of activity [1] and a large number of such reactions have been discovered during recent years. Biochemical reactions [2-10] such as glycolytic oscillations and peroxidase catalysed oxidation of nicotinamide adenosine deoxyhydrogenase (NADH) have also generated considerable interest. The interest in such reactions is stiU sustained in view of their importance in understanding cardiac and neuronal oscillations. In the case of many oscillatory chemical reactions [1], detailed reaction mechanisms have been postulated and verified with the help of numerical computation. This has also been particularly so for B-Z reaction where Field-Koros-Noyes (FKN) mechanism [11] has been invoked. 

Misleading conclusions can be drawn if only the reduction potentials of, for example, the GS, HVGSH and PhO, H /PhOH couples are considered and the law of mass action is ignored. Rate constants for repair of phenoxyl radicals by GSH, producing thiyl radicals, of < 10 dm moP s have been estimated in the cases of 1-naphthol [67] or acetaminophen [68]. The latter phenol was oxidized by thiyl radicals (cysteine) with 5 ca. 7x10 dm moP s at pH 7, an estimate of the reduction potential of the acetaminophen phenoxyl radical, (PhO, H /PhOH) = 0.71 V confirming that reaction (5), as an equilibrium, is well over to the left [69]. In spite of this, thiyl radicals are undoubtedly formed during peroxidase catalysed oxidation of acetaminophen in the presence of thiols [64,70]. Similar results were obtained with 4-ethoxyaniline (p-pheneti-dine) [71]. 

The use of mediators to i uce the applied potential in amperometric electrodes, and hence interference, sometimes requires multienzymatic immobilization. Glucose oxidase and peroxidase are cdmmobilized for the determination of glucose without interference firom ascorbic and uric acids. The peroxidase catalyses the reaction of the hydrogen peroxide arising from the oxidation of glucose by the mediator hexacyanoferrate [38]. 

Oxidoreduciases. Enzymes catalysing redox reactions. The substrate which is oxidized is regarded as the hydrogen donor. This group includes the trivially named enzymes, dehydrogenases, oxidases, reductases, peroxidases, hydrogenases and hydroxylases. 

Horseradish peroxidase catalysed kinetic resolution of racemic secondary hydroperoxides has been described by Adam et al. [79]. The reaction yields (i )-hy-droperoxides up to ee>99% and (S)-alcohols up to ee>97%. Optically active hydroperoxides as potential stereoselective oxidants can be obtained by this process. 

Figure 1. A reaction scheme for peroxidase catalysed amino-fluorene oxidation.

Certain fluorescent compounds, such as the coumarin derivative, scopoletin, can act as hydrogen donors in the oxidative reaction catalysed by horseradish peroxidase (Udenfriend, 1969), an enzyme that exhibits substrate specificity for hydrogen peroxide... 

For foreign compounds the majority of oxidation reactions are catalysed by mono oxygenase enzymes found in the SER and known as microsomal enzymes. Other enzymes involved in the oxidation of xenobiotics are found in other organelles such as the mitochondria and the cytosol. Thus amine oxidases located in the mitochondria, xanthine oxidase, alcohol dehydrogenase in the cytosol, the prostaglandin synthetase system and various other peroxidases may all be involved in the oxidation of foreign compounds. 

Peroxidase catalyses the rapid oxidation of AA CYamazakl, 1962) and both AA as well as phenols serve as donors in peroxidase mediated reactions. The observation that AA accumulates in a cell having high peroxidase activity indicates that the enzymic activity is being regulated by some other factors like flavonoid pigments present in the cell. It was reported that the flavones of the leaves and roots of wheat sprout and anthocyanins of the coleoptiles accelerated the peroxidase oxidation of AA (Kolesnikov and Zore, 1964). Moreover, morin also accelerated the peroxidase oxidation of AA (Kursanov et al., 1950). A detailed study on the effect of flavonoids and oxidation of AA in plants has been conducted by Stenlld and Samorodova-Bianci (1969). These authors have compared the effect of flavonoid on different enzymic and non-enzymlc systems, viz., I by copper ions ... 

It is important to note that the oxidation produces the superoxide free radical. Since it is toxic, the radical produced in reaction (a) must be removed. This is done in a reaction catalysed by superoxide dismutase, which produces hydrogen peroxide. However, this also must be removed (see Appendix 9.6 for discussion of free radicals). Removal of hydrogen peroxide is achieved in a reaction with reduced glutathione, catalysed by glutathionine peroxidase. 

In an enzymic reaction catalysed by glutathione peroxidase, GSH reacts with peroxides and becomes oxidized to form a dimer (GSSG) linked by a disulfide bridge. 

The haem peroxidases are a superfamily of enzymes which oxidise a broad range of structurally diverse substrates by using hydroperoxides as oxidants. For example, chloroperoxidase catalyses the regioselective and stereoselective haloge-nation of glycals, the enantioselective epoxidation of distributed alkenes and the stereoselective sulfoxidation of prochiral thioethers by racemic arylethyl hydroperoxides [62]. The latter reaction ends in (i )-sulfoxides, (S)-hydroperoxides and the corresponding (R)-alcohol, all In optically active forms. 

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