Phenoloxidases

Hurst (19) discusses the similarity in action of the pyrethrins and of DDT as indicated by a dispersant action on the lipids of insect cuticle and internal tissue. He has developed an elaborate theory of contact insecticidal action but provides no experimental data. Hurst believes that the susceptibility to insecticides depends partially on the cuticular permeability, but more fundamentally on the effects on internal tissue receptors which control oxidative metabolism or oxidative enzyme systems. The access of pyrethrins to insects, for example, is facilitated by adsorption and storage in the lipophilic layers of the epicuticle. The epicuticle is to be regarded as a lipoprotein mosaic consisting of alternating patches of lipid and protein receptors which are sites of oxidase activity. Such a condition exists in both the hydrophilic type of cuticle found in larvae of Calliphora and Phormia and in the waxy cuticle of Tenebrio larvae. Hurst explains pyrethrinization as a preliminary narcosis or knockdown phase in which oxidase action is blocked by adsorption of the insecticide on the lipoprotein tissue components, followed by death when further dispersant action of the insecticide results in an irreversible increase in the phenoloxidase activity as a result of the displacement of protective lipids. This increase in phenoloxidase activity is accompanied by the accumulation of toxic quinoid metabolites in the blood and tissues—for example, O-quinones which would block substrate access to normal enzyme systems. The varying degrees of susceptibility shown by different insect species to an insecticide may be explainable not only in terms of differences in cuticle make-up but also as internal factors associated with the stability of oxidase systems. 

Sugumaran, M., Comparative biochemistry of eumelanogenesis and the protective roles of phenoloxidase and melanin in insects. Pigment Cell Res., 15, 2, 2002. Riley, P.A., Melanin, Int. J. Biochem. Cell. Biol, 29, 1235, 1997. 

Duran, N. and Esposito, E., Potential applications of oxidative enzymes and phenoloxidase-like compounds in wastewater and soil treatment a review, Appl. Catal. B Environmental, 28, 83-99, 2000. 

Fungal laccases (benzenediokoxygen oxidoreductase, EC 1.10.3.2) belong to the multicopper blue phenoloxidases. They comprise glycosylated proteins expressed in multiple forms and variable molecular weight, ranging from 59 to 110 kDa. Laccase is expressed as multiple constitutive and induced isoenzymes [30, 64]. The enzyme contains four copper atoms (Cu), in different states of oxidation (I, II, III) [65], which play an important role in the catalytic mechanism. Laccase oxidizes different compounds while reducing O2 to H20, a total reduction of four electrons. 

WRF are key regulators of the global C-cycle. Some WRF produce all three LME, while others produce only one or two of them [10]. The main LME are oxidor-eductases, that is two types of peroxidases, LiP and MnP, and a phenoloxidase Laccase. Catalytic cycles of peroxidases and laccases are given in Figs. 1 and 2, respectively. LME are produced by WRF during their secondary metabolism. 

A class of enzymes that sometimes causes problems is the poly-phenoloxidases, which are present in the outer layers of the kernel. These enzymes can cause dark spots to appear in pastry offcuts that are being recycled, e.g. to make pie cases. The enzymes act on any minute bran particles that are present. 

PO = phenoloxidase, AMPs = antimicrobial peptides, NK cells = natural killer cells. Adapted from Flajnik and Du... 

Decker, H. and Rimke, T., Tarantula haemocyanin shows phenoloxidase activity, J. Biol. Chem., 273, 25889, 1998. 

It is possible to prepare a lignin-like polymer, known as dehydrogenated polymerisate (DHP) in the laboratory by treating con-iferyl alcohol in vitro under aerobic conditions with a phenoloxidase... 

Various chloro- and alkyl-substituted anilines, which represent the aromatic base of a large number of organic pollutants, were shown to react with phenolic humic constituents in the presence of a phenoloxidase isolated from R. praticola, while no reaction occurred when only the anilines were incubated with the fungal lactase. 

Kharazipour etal. (1998) used a peroxidase enzyme in combination with H2O2 to activate the surface of TMP fibres for self-bonding. Fibres were activated in a wet system, then dewatered and fluffed out before pressing at 190 °C for 5 minutes. The best IBS recorded for boards of 5 mm thickness made from the activated fibres was 0.55 MPa. The IBS was found to be dependent upon the pH of the treatment solution, the time of treatment and the board density. The authors noted that phenoloxidase gave comparable results to laccase, which was unexpected, since it was thought that the phenoloxidase would lead only to depolymerization of the lignin. 

Laccase is perhaps the metallo-enzyme most widely used for this aim. Laccases are a family of multicopper ( blue copper ) oxidases widely distributed in nature Many laccases have fungal origin, while others are produced in plants. They contain four Cu(II) ions, and catalyse the one-electron oxidation of four molecules of a reducing substrate with the concomitant four-electron reduction of oxygen to water . In view of their low redox potential, which is in the range of 0.5-0.8 V vs. NHE depending on the fungal source laccases typically oxidize phenols (phenoloxidase activity) or anilines. 

Milled wood lignin was mixed with the crude enzyme solution of Tram-ties versicolor extracellular phenoloxidases produced on spent sulfite liquor in a ratio of approximately 2 1. This comprised the main part of the two-component bio-adhesive. Industrial particles were bonded with 15% bioadhesive under conventional pressing conditions to have 19 mm particle boards (40 x 50 cm) of the properties described in Table IV. The bonding reaction (crosslinking) took place in aqueous solution at room temperature. If conventional pressing technology is applied, the temperature should be elevated in order to maintain water evaporation within a reasonable press time. 

A new putative member of the blue multi-copper oxidases has been isolated using the Escherichia coli yacK gene. Six copper ions per polypeptide chain were determined and assigned to two type 1 copper centers and further one type 2 and one type 3 copper. Phenoloxidase and ferroxidase properties were ascertained98 A new copper containing nitrite reductase was purified from a halophilic archaeon and the ligands to type 1 and type 2 coppers in the sequence were... 

J.C. Rhodes, I. Polacheck, and K.J. Kwon-Chung, Phenoloxidase activity and virulence in isogenic strains of Cryptococcus neoformans, Infect. Immun., 36, 1175, 1982. 

In order to eluddate the mechanism of the further transformations of the primary lignin decomposition products including the cleavage reactions, we synthesized some of the important primary products labelled with Cu and introduced these into the cultures of fungi or enzymes. Thus, it could be shown, for example, that the breakdown of the side chain of ferulic acid occurs at the double bond vanillic acid is found. During polymerization in the presence of phenoloxidases, in the case of carboxyl-labelled ferulic acid, about 60% of the activity is split off as Cli02. The polymers labelled in the 2 and 3 position in the side chain or in the methoxyl group contain the whole applied activity. 

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