Peroxidase catalysed conversion

Free radicals are by-products of prostaglandin metabolism and may even regulate the activity of the arachidonate pathway. Arachidonic acid, released from lipids as a result of activation of phospholipases by tissue injury or by hormones, may be metabolized by the prostaglandin or leu-kotriene pathways. The peroxidase-catalysed conversion of prostaglandin G2 to prostaglandin H2 (unstable prostanoids) and the mechanism of hydroperoxy fatty acid to the hydroxy fatty acid conversion both yield oxygen radicals, which can be detected by e.s.r. (Rice-Evans et al., 1991). 

Klibanov AM, Tu T, Scott KP (1983) Peroxidase-catalysed removal of phenols from coal-conversion waste waters. Science 221 259-261... 

With tyrosinase, on the contrary, a two-electron oxidation occurs, as no EPR signal was detected in the catechol oxidation at pH 5.3 Melanins are polymerization products of tyrosine, whereby tyrosinase catalyses the first steps the formation of dopa (3,4-dihydroxyphenylalanine) and of dopaquinone, leading to an indolequi-none polymer The peroxidase mechanism for the conversion of tyrosine into dopa in melanogenesis was not substantiated In natural and synthetic melanins free radicals of a semiquinone type were detected by EPR 4-10 x 10 spins g of a hydrated suspension (the material was modified on drying and the number of free spins increased). The fairly symmetrical EPR signal had a g-value of 2.004 and a line-width of 4-10 G The melanins seem to be natural radical scavengers. 

The seminal ideas relating to the oxidative coupling of phenols have found very widespread application, and yet the actual mechanism of coupling in vivo remains obscure. In new work, it has been found that extracts of Papaver somniferum which had peroxidase activity were unable to catalyse the conversion, by phenol oxidative coupling, of reticuline into salutaridine. 

Catalase, which catalyses the conversion of hydrogen peroxide to oxygen and water (Eqn. 29), and peroxidases, which catalyse the conversion of alkyl peroxides to alcohol and water with concomitant dehydrogenation of a substrate (Eqn. 30), are Fe(III) haemoproteins. 

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