Enzymatically generated o-quinones

The PPO-B-immunosensors are potentiostated at —0.2 V vs. SCE in order to reduce the enzymatically generated o-quinone. 

A.2.1 Reactions of Anthocyanins with Enzymatically Generated o-Quinones. .443... 

There is an irreversible enzymatic inactivation reaction, which occurs during the oxidation of the cyclizable and noncyclizable diphenols to oquinones. This inactivation process has been interpreted as being the result of a direct attack of an o-quinone on a nucleophilic residue (His) near the active enzyme center or of an attack of a copper-bound hydroxyl radical generated by the Cu(I)-peroxide complex. However, the latter hypothesis seems to be more probable, because inactivation also occurs in the presence of reducing agents that remove the o-quinones generated. 

Precursors. Precursors for this reaction are anthocyanins and t -quinones of caf-feoyltartaric (caftaric acid) and p-coumaroyltartaric (cutaric acid) acids generated by enzymatic oxidation by grape polyphenoloxidase (PPO) (Singleton et al. 1985). PPO first adds an OH group to monophenols and then oxidizes the resulting o-diphenol to o-quinone. 

Tyrosinase or polyphenol oxidase (EC 1.14.18.1) is a bifunctional, copper-containing enzyme widely distributed on the phylogenetic tree. This enzyme uses molecular oxygen to catalyze the oxidation of monophenols to their corresponding o-diphenols (cresolase activity) as well as their subsequent oxidation to o-quinones (catecholase activity). The o-quinones thus generated polymerize to form melanin, through a series of subsequent enzymatic and nonenzymatic reactions [1-3]. 

N-Acetyldopamine A -[2-(3,4-dihydroxyphenyl)ethyl]-acetamide and N-P-alanyldopamine (3-amino-A -[2-(3,4-dihydroxyphenyl)ethyl]propanamide) occur in insects where they serve to form the sclerotized cuticle required for the exoskeleton. They are transformed by enzymatic oxidation to the corresponding o-quinones and p-quinonemethides which cross-link structural proteins with each other and with chitin. The biosynthesis of N-acetyldopamine starts from dopamine and acetyl-CoA by way of arylamine-A acyltransferase (EC 2.3.1.5). In contrast to the biosynthesis of (/f)- noradrenaline, the p-quinonemethides, generated non-selectively by enzymatic oxidation from N-ace-tyldopamine or )V-)3-alanyldopamine, undergo addition of water to furnish the racemic side-chain hydrox-ylated derivatives N-acetylnoradrenaline ()V-[2-(3,4-dihydroxyphenyl)-2-hydroxyethyl]acetamide) or, respectively N- alanylnoradrenaline (3-amino-)V-[2-(3,4-dihydroxyphenyl)-2-hydroxyethyl]propan-amide). 

Capsaicin and capsaicinoids undergo Phase I metabolic conversion involving both oxidative and non-oxidative paths. The liver is the major site of this enzymatic activity. Lee and Kumar (1980) demonstrated the conversion of catechol metabolites via hydroxylation of vanil-lyl ring. In rats, dihydrocapsaicin is metabolized to products that are excreted in the urine as glu-curonides (Kawada and Iwai, 1985). The generation of a quinone derivative occurs via O-demethylation at the aromatic ring with concomitant oxidation of the semiquinone and quinone derivatives or via demethylation of the phenoxy radical intermediate of capsaicin. Additionally, the alkyl side chain of capsaicin is also susceptible to oxidative deamination (Wehmeyer et al., 1990). There is evidence that capsaicinoids can undergo aliphatic oxidation (cu-oxidation) (Surh et al, 1995 Reilly et al, 2003) which is a possible detoxification pathway. Non-oxidative pathways are also involved in the bioconversion of capsaicin, e.g. hydrolysis of the acid-amide bond to yield vanillylamine and fatty acyl moieties (Kawada et al, 1984 Kawada and Iwai, 1985 Oi et al, 1992). 

L-ascorbic acid (AA) and its isomer D-erythorbic acid (EA) (also called D-isoascorbic acid) have been used as inhibitors of enzymatic browning in fruit and vegetable products for at least 50 years, (15-17). These compounds prevent quinone accumulation and subsequent pigment formation by reducing the 0-quinones generated from the phenolic substrates of PPO back to O-dihydroxyphenolic compounds (17-18). AA also can act as a PPO inhibitor (19-20). AA and EA are used interchangeably although there are indications that AA is more effective in some systems (21-22). 

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