Phenol oxidase systems

Humic substances are abundant in DOM as well as POM, but the role of phenol oxidases in DOM metabolism has been little studied (Munster and De Haan, 1998). Foreman (1999) found that phenol oxidase activity in water from five contrasting Michigan streams was correlated with phenol concentration and that bacterial production in systems with significant humic DOC input was stimulated by the addition of phenols, suggesting that phenols may be an important growth substrate for some bacterial guilds. 

Coche-Guerente, L., Labbe, P., Mengeaud, V. (2001). Amplification of amperometric biosensor responses by electrochemical substrate recycling. 3. Theoretical and experimental study of the phenol-polyphenol oxidase system immobilized in Laponite hydrogels and layer-by-layer self-assembled structures. Anal. Chem. 73 3206-18. 

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 . 

In immunologically stressed mosquitoes some or all of the ookinetes reaching the basal lamina may be subjected to melanization via the phenol oxidase cascade (Lehane et al., 2004b). Development underneath the basal lamina is thought to shield the developing oocyst from the mosquito immune system because the basal lamina is self however when unprotected the immune system can attack the developing oocyst. Indeed, melanized oocysts were first reported in non-natural mosquito vectors by Ross as black spores (Ross, 1923). 

The polycyclic aromatic hydrocarbon carcinogens, which are very ubiquitous, are metabolized by the microsomal mixed-function oxidase system of target tissues to a variety of metabolites such as phenols, quinones, epoxides, dihydrodiols and dihydrodiol-epoxides ( ). The mqjor pathway of activation of benzo(a)pyrene (BP) leads to the formation of dihydrodiol-epoxide of BP which interacts predominantly with the 2-amino of guanine of DNA. The dihydrodiol-epoxide of BP appears to be the major ultimate electrophilic, mutagenic, and carcinogenic metabolite of BP ( ). Nevertheless, other metabolites such as certain phenols, epoxides and quinones may contribute to the overall carcinogenic activity of BP. In addition, a free radical mechanism may also be partly involved in its carcinogenic activity. 

Among the enzyme systems performing redox reactions oxidations copper containing oxidases perform the irreversible oxidation of phenols. Two main types of phenol oxidases are known, the catechol oxidases and the laccases of which the latter is supposed to be... 

FIGURE 5.21 Seasonal variation in phenol oxidase activity in detrital material and soils (0-10 cm deep, 10-30 cm deep) in different trophic systems (Wright and Reddy, 2001a). 

The microsomal mixed-function oxidase system containing multiple forms of cytochrome P-450 is the catalytic site for the initial oxidation of BaP. The primary products are the 2,3-, 4,5-, 7,8-, and 9,10-epoxide. The 4,5-epoxide has been isolated, while the formation of the other epoxides was demonstrated indirectly (757, 754, 236, 415, 428, 482, 505, 506). Primary oxidation at the 6 position results in formation of the unstable 6-hydroxy-BaP which is further oxidized by way of the 6-oxo radical to the 1,6-, 3,6-, and 6,12-quinones 290, 291, 350, 351). The major phenolic metabolite of BaP is 3-hydroxy-BaP lesser amounts of 1-, 7-, and 9-hydroxy-BaP are also formed. These phenols are produced at least partially by nonenzymatic rearrangement of the epoxides (the NIH shift 85, 93, 164, 236, 272, 416, 422, 424, 428, 482, 505, 506). 

Marchantin H is a natural compound isolated from Marchantia diptera (Wu 1990). Marchantins are naturally phenolic structures isolated from different species of liverwort (Tori et al. 1985, Asakawa et al. 1987). Marchantin H could scavenge the stable free radical l,l-diphenyl-2-picrylhydrazyl and per-oxyl radical derived from 2,2 -azobis(2-amidino-propane) dihydrochloride in aqueous phase, but not the peroxyl radical derived from 2,2 -azobis (2,4-dimethylvaleronitrile) in hexane (Hsiao et al. 1996). It was reactive toward superoxide anion generated by the xanthine/xanthine oxidase system. Marchantin H inhibited copper-catalysed oxidation of human low-density lipoprotein, as measured by... 

Potato Phenol Oxidase. The studies of Kubowitz on the copper oxidase of potatoes were part of the efforts of Warburg s institute to find enzyme systems that could catalyze oxygen consumption coupled with substrate oxidation. The enzyme was purified on the basis of an assay involving transfer of electrons from pyridine nucleotides to o-quinone and from the resulting catechol to oxygen (I). The reduction of catalytic quantities of quinone was probably catalyzed by a flavoprotein present in the Zwischenferment preparation used to reduce TPN. In the presence of... 

A much better case can be made for the possible function of GSH in the hydrogen transport systems of higher plants. Ascorbic-acid oxidase appears to be characteristically a plant enzyme. Phenol oxidases and peroxidase systems are also found in many higher plants. Such enzymes can cause an oxidation of ascorbate indirectly by virtue of the nonenzymatic oxidation of ascorbic acid by compounds of quinoid structure formed from the phenolic substrate (39). Such systems are not universally distributed in plants, but where they are present in sufficient amount it is not unreasonable to suppose that some substrates may be oxidized by way of a respiratory chain consisting of substrate, TPN, GSH, ascorbic acid, and a terminal oxidase. 

A widely distributed group of enzymes known as the tyrosinases or polyphenol oxidases catalyzes the oxidation of phenolic substances by oxygen. Where these have been isolated, they have been shown to be copper proteins. These enzymes are particularly abundant in plant tissues where they may function as terminal oxidases in place of the cytochrome system the relative importance of the phenol oxidases in plant cell respiration, however, has not yet been determined.The oxidation of ascorbic acid in plant tissues is also due to the presence of a copper enzyme. 

A better model for the investigation of this hypothesis is the gene system in D. melanogaster. Gene sequences which are located on the second and third chromosomes are especially convenient for analysis. Genes of alcohol dehydrogenase (Adh-50.1), phenol oxidase (Tyr-52.4), and kinurenin hydroxylase (cn-57.5) are lo-... 

---