Radicals as Reaction Product

In the oxidation of ascorbate by Oj catalysed by ascorbate oxidase, the formation of the monodehydroascorbate free radical was demonstrated by EPR spectroscopy in a flow cell. A steady state was usually reached within 50 ms. The production of the free radical was also followed by the reduction of Fe(in)-cytochrome c. Thus the oxidation of ascorbate occurs in a one-electron step The formation of the monodehydroascorbate free radical was also measured directly by spectrophotometry at 360 nm, where the free radical shows an absorption maximum 

The formation of the monodehydroascorbate free radical by dopamine P-mono-oxygenase was similarly demonstrated by the acceleration of the reduction of Fe(III)-cytochrome c and by direct spectrophotometry at 360 nm The action of the laccase from the mushroom Agaricus hisporum on catechol yielded also the semiquinone as the result of a one-electron transfer 

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 oxidation of dopamine by ceruloplasmin yielded a free radical with a very short lifetime, characterized by an EPR signal at g = 2.006 Dimethyl-p-phenylenediamine was oxidized faster by the enzyme to the free radical Wurster s red, which was further oxidized by the enzyme in another one-electron step. The free radical seemed, moreover, to be stabilized by ceruloplasmin 

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C02 radicals. Reaction of C02 radicals with S208 ions, reaction (R7), yields again SOt" radicals as reaction products, and its main effect is that of depleting formate and peroxodisulfate from the reaction system. 

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