Catecholase effect

This mechanism has been supported by data from experiments with isotopically labeled oxygen (O ). The majority of phenol oxidases can carry out the oxidation of hydroquinone derivatives (lower line of the diagram above) by themselves without coupling. This is termed the catecholase effect because catechols are oxidized. Here H2O (and not H2O2) is formed. 

Tyrosinase obtained relatively pure was tested for its effect in rats, dogs (53), and man (50), and was found to exert a specific depression of blood pressure only in hypertensive animals. Tyrosinase contains catecholase and cresolase, and has the property of... 

The dinucleating ligand (58) has been prepared by standard methods via the bis-hydroxymethyl-ation of 4R-substituted phenols. It has been observed that modification of the R substituent induces a drastic effect on the catecholase activity of the dicopper(II) complexes containing (58) in the deprotonated form.85 A similar coordination pattern to those found in metal derivatives of (58) is realized in bis(triazacyclononane) systems bridged by a m-xylyl fragment (59)86 and in (60).87 /)-Xylyl ligands exhibit different coordination chemistry and reactivity 85... 

In the catecholase cycle, both the oxy and met sites can react with catechol derivatives, oxidizing them to the corresponding quinones (paths (f) and (g) in Scheme 2) Electron-donating substituents on the catechol ring have been shown to enhance the reactivity. However, steric and polarity effects of the substituents may also be important for the observed difference in reactivity among the substrates. " ... 

Further validation of the mechanism proposed for the catecholase activity of the dicopper complexes [Cu2(L66)]" , [Cu2(L55)], and [Cu2(EBA)]" " (Scheme 17) has been obtained investigating the inhibitory effect of kojic acid [5-hydroxy-2-(hydroxymethyl)-y-pyrone] (154). This fungal metabolite is one of the most efficient inhibitors of mushroom tyrosinase and other polyphenol oxidases (160,161). When the catalytic oxidation of DTBCH2 was studied in the presence of kojic acid, strong competitive inhibition was observed in the steps exhibiting substrate concentration dependence,... 

In the light of the foregoing discussion we must note a certain discordance about the activity of Mn++. Here this cation is effective as a catecholase in contrast with the findings of Brackman and Havinga (1955). However the latter have studied the reactions in methanol as a solvent and in the presence of morpholine. 

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