Polyphenols polyphenol quinones

FIa.VOnoIOxida.tlon, The fermentation process is initiated by the oxidation of catechins (1) to reactive catechin quinones (13), a process catalyzed by the enzyme polyphenol oxidase (PPO) (56). Whereas the gaHocatechins, epigaHocatechin, and epigaHocatechin gaHate, are preferred, polyphenol oxidase can use any catechin (Table 2) as a substrate. This reaction is energy-dependent and is the basis of the series of reactions between flavanoids that form the complex polyphenoHc constituents found in black and oolong teas. 

The initial oxidation of the flavanol components of fresh leaf to quinone structures through the mediation of tea polyphenol oxidase is the essential driving force in the production of black tea. While each of the catechins is oxidizable by this route, epigallocatechin and its galloyl ester are preferentially oxidized.68 Subsequent reactions of the flavonoid substances are largely nonenzymic. 

Polyphenoloxidase (PPO, EC 1.14.18.1) is one of the most studied oxidative enzymes because it is involved in the biosynthesis of melanins in animals and in the browning of plants. The enzyme seems to be almost universally distributed in animals, plants, fungi, and bacteria (Sanchez-Ferrer and others 1995) and catalyzes two different reactions in which molecular oxygen is involved the o-hydroxylation of monophenols to o-diphenols (monophenolase activity) and the subsequent oxidation of 0-diphenols to o-quinones (diphenolase activity). Several studies have reported that this enzyme is involved in the degradation of natural phenols with complex structures, such as anthocyanins in strawberries and flavanols present in tea leaves. Several polyphenols... 

Laccase is one of the main oxidizing enzymes responsible for polyphenol degradation. It is a copper-containing polyphenoloxidase (p-diphenoloxidase, EC 1.10.3.2) that catalyzes the oxidation of several compounds such as polyphenols, methoxy-substituted phenols, diamines, and other compounds, but that does not oxidize tyrosine (Thurston, 1994). In a classical laccase reaction, a phenol undergoes a one-electron oxidation to form a free radical. In this typical reaction the active oxygen species can be transformed in a second oxidation step into a quinone that, as the free radical product, can undergo polymerization. 

Polyphenol oxidase occurs within certain mammalian tissues as well as both lower (46,47) and higher (48-55) plants. In mammalian systems, the enzyme as tyrosinase (56) plays a significant role in melanin synthesis. The PPO complex of higher plants consists of a cresolase, a cate-cholase and a laccase. These copper metalloproteins catalyze the one and two electron oxidations of phenols to quinones at the expense of 02. Polyphenol oxidase also occurs in certain fungi where it is involved in the metabolism of certain tree-synthesized phenolic compounds that have been implicated in disease resistance, wound healing, and anti-nutrative modification of plant proteins to discourage herbivory (53,55). This protocol presents the Triton X-114-mediated solubilization of Vida faba chloroplast polyphenol oxidase as performed by Hutcheson and Buchanan (57). 

The various chemical processes influencing the colour formation in tea have been vigorously investigated. The self-association of black tea polyphenol theaflavin and its com-plexation with caffeine [175], and the role of epicatechin quinone in the synthesis and degradation of theaflavin [176] have been studied in detail. 

It is interesting to underline that there is another (plant) enzyme which possesses a coordinatively similar dicopper environment catechol oxidase.11 As already mentioned in Chapter 6, Section 3, such an ubiquitous enzyme catalyses the two-electron oxidation by molecular dioxygen of catechols to the corresponding quinones (the so-generated quinones in turn polymerize to form brown polyphenolic catechol melanins, which protect damaged plants from pathogens or insects). 

FREE RADICALS, QUINONES, PHENOLS POLYPHENOLS, AROMATICS, ETC. 

Although many polyphenolic compounds can quench DPPH215 218 and scavenge radicals in competition with spin traps,225 232,278 consideration must also be given to the fate of the resultant phenoxyl radicals, which can include their direct reaction with important biomolecules, further oxidation to cytotoxic quinones and, in the case of semiquinones, their reduction of oxygen to superoxide. Thus,... 

In addition to being hepatotoxic, bromobenzene is also nephrotoxic because of the production of reactive polyphenolic GSH conjugates, covalent binding to protein, and the production of ROS. 2-Bromophenol and 2-bromohydroquinone are both nephrotoxic metabolites of bromobenzene. Quinones are both oxidants and electrophiles, undergoing both one and two electron reduction and reaction with sulfydryl groups such as GSH and... 

Fig. 26.2. Amperometric immunosensors set-up using a biotinylated copolymer poly(pyrrole-biotin, pyrrole-lactitob-ionamide) coated platinum or glassy carbon electrodes and three enzymatic markers (GOX-B, PPO-B, HRP-Ab) for the detection of cholera antitoxin. (A) HRP-immunosensor, (B) GOX-B-immunosensor, (C) PPO-B-immunosensor. Mred/Mox = hydroquinone/quinone Gox = biotinylated glucose oxidase PPO — biotinylated polyphenol oxidase HRP-Ab = peroxidase-labeled IgG anti-rabbit antibody.

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