Oxidation, phenolic compound, enzymatic

PO performs vitally important functions in the plant cell and is mainly associated with the oxidation of phenolic compounds and with the formation and strengthening of the cell wall (Passardi et al., 2004). PO is involved in the oxidative transformation of molecules in growth-regulating or signalling activities and - as a result - can also perform regulatory functions in the cell. Plant POs are represented by genetically different proteins with the same enzymatic activity (Welinder et al., 2002). 

There are several chemical compounds found in the waste waters of a wide variety of industries that must be removed because of the danger they represent to human health. Among the major classes of contaminants, several aromatic molecules, including phenols and aromatic amines, have been reported. Enzymatic treatment has been proposed by many researchers as an alternative to conventional methods. In this respect, PX has the ability to coprecipitate certain difficult-to-remove contaminants by inducing the formation of mixed polymers that behave similarly to the polymeric products of easily removable contaminants. Thus, several types of PX, including HRP C, LiP, and a number of other PXs from different sources, have been used for treatment of aqueous aromatic contaminants and decolorization of dyes. Thus, LiP was shown to mineralize a variety of recalcitrant aromatic compounds and to oxidize a number of polycyclic aromatic and phenolic compounds. Furthermore, MnP and a microbial PX from Coprinus macrorhizus have also been observed to catalyze the oxidation of several monoaromatic phenols and aromatic dyes (Hamid and Khalil-ur-Rehman 2009). 

Enzymatic transformations of alkaloids by peroxidases most probably occur by single-step oxidations catalyzed by the HRP-I and HRP-II forms of the enzyme. The catalysis of one-electron oxidations of compounds containing aromatic hydrocarbon, hydrazine, phenol, hydroxamic acid, and amine functional groups has been recently reviewed (45, 58, 82). A brief summary of those HRP reactions that involve functional groups most commonly occurring in alkaloids is presented below. 

Moreover, under certain conditions these phenolic compounds could also act as pro-oxidants. In the presence of redox-active metal ions such as Cu or Fe, phenolic compounds react with O2 to generate phenoxyl radicals. Under normal growth conditions phenoxyl radicals can be rapidly deactivated by polymerization or enzymatic reduction. However, if the phenoxyl radical concentrations are too high and/or the lifetime is increased, they could initiate DNA damage or lipid peroxidation and exhibit cytotoxicities. Curcumin, demethoxycurcumin, and bisdemethoxycurcumin have been reported to induce... 

The production of humic substances by microorganisms is an extracellular process, because the enzymes are secreted into the external solution that contains the phenolic compounds derived from lignin and tannic acid degradation and microbial and plant metabolites. These phenolic compounds can then be enzymatically oxidized to quinones, which can undergo further polymerization or polycondensation reactions with other biomolecules (e.g., amino acids) to form humic polymers (Stevenson, 1994 Bollag et al., 1998 Burton, 2003). 

Different drawbacks arise from the application of peroxidases (1) the enzymatic inactivation, usually caused by the presence of high concentrations of hydrogen peroxide, which oxidizes the porphyrin ring [15] moreover, when phenolic compounds are being treated, the phenoxy radicals produced as intermediates... 

These reactions involve, on one hand, the nucleophilic A-rings of flavonoids such as flavanols, and on the other hand, electrophilic quinones arising from enzymatic or chemical oxidation of phenolic compounds. 

A crnde extract of sweet potato Ipomoea-batatas (L.) Lam.) was nsed as a source of phenol oxidases (polyphenoloxidase, tyrosinase, catecholoxidase, EC 1.14.18.1). The extract was directly placed in the carrier of a FIA system with UVD, to promote oxidation of phenolic compounds to o-quinones that condense to form melanin-like pigments with a strong absorption at 410 nm. The determination of phenols in industrial wastewaters showed good agreement with conventional methods (correlation coefficient 0.9954) LOD was 10 p,M, with RSD <2.7% (w = 6). Under optimal storage conditions the enzymatic activity did not vary for at least five months . 

A large amount of phenols is released in wastewater and can be lost to waste streams. A rapid increase in the distribution and abundance of plastic debris in the ocean around the world was reported, and the adverse influence of plastic s phenol residues has been of great interest Polluted water disinfection, enzymatic oxidation of chlorinated phenols, decomposition of alkylphenol polyethoxylates and combustion of phenols can lead to the formation of highly toxic compounds. High adsorption of phenols on sludge and sediments requires that their distribution in these systems also be followed. All of these facts have promoted extensive research on phenolic compounds and their fate in the environment. 

In living cells, various oxidoreductases play an important role in maintaining the metabolism of living systems. Most of oxidoreductases contain low valent metals as their catalytic center. In vitro enzymatic oxidoreductions have afforded functional organic materials. Some oxidoreductases such as peroxidase, laccase, and polyphenol oxidase have received much attention as catalysts for oxidative polymerizations of phenol derivatives to produce novel polyaromatics [1-10], This chapter deals with enzymatic oxidative polymerization of phenolic compounds. 

Procyanidins are quite reactive and are therefore considered as some of the most unstable natural phenolic compounds [19-20]. They are subject to enzymatic oxidation by polyphenol oxidases as well as to spontaneous oxidation [21], Coupled oxidation reactions involving o-quinones of phenolic acids have been reported [22-24], Procyanidins are thermally labile [25] and can easily undergo molecular rearrangements in acidic or basic media [26]. In model solutions interflavanoid bonds of procyanidins were found to be unstable, but also new carbon-carbon bonds were formed... 

An alternative or complimentary theory for the mode of action of simple phenolic compounds is that they are converted to much more toxic quinones. Pillinger et al. [69] found that various phenolic decomposition products of barley straw were most toxic under conditions favorable for oxidation of the compounds to quinones, and that quinones were up to one thousand-fold more toxic to algae than the parent compounds. The most likely route to conversion to a quinone is enzymatic. Peroxidases and polyphenol oxidases can perform such a reaction, depending on the substrate. However, polyphenol oxidase cannot be detected in most green algae [139] and has not been reported in cyanobacteria. 

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