Role in Nature

Due to the abundance of laccase and laccase-like enzymes, there are numerous and diverse natural functions for these oxidoreductases. Although laccase is able to polymerize lignin precursors, and its presence has been identified in xylem tissue of higher plants, there is still discussion about their involvement in lignification (Dean and Eriksson 1994; Thurston 1994; Mayer and Staples 2002). Peroxidases are regarded as the main biocatalysts in that process, but laccases operate in the absence of toxic peroxide and could play a role in the early stages of lignification in living cells (Sterjiades et al. 1992).

Evidence of laccase activity in the cuticles of larval and adult insects suggests their involvement in sclerotization (Dittmer et al. 2004; Suderman et al. 2006).

Physiological functions of laccase-like activities in bacteria include melanin production, spore coat resistance, morphogenesis, and detoxification of copper (Sharma et al. 2007). Laccase-like genes have been identified in important human pathogens such as Escherichia coli, Bordetella pertusis, Pseudomonas aeruginosa, Campylobacter jejuni, Yersinia pestis, and Mycobacterium leprae (Alexandre and Zhulin 2000). In all of these pathogens, the potential mechanism of virulence is suspected to be the production of melanin and laccase activity. M. leprae has an ability, unique among mycobacteria, to oxidize diphenols to o-quinones, and so the oxidation of L-DOPA has become a diagnostic feature for M. leprae (Prabhakaran and Harris 1985).

Fungal laccases probably play diverse roles in spore pigmentation and morphogenesis (Leatham and Stahmann 1981), fungal plant-pathogen/host interactions and stress defense (Mayer and Staples 2002), degradation of lignin (Thurston 1994; Leonowicz et al. 2001; Baldrian 2006), and turnover of humic matter (Claus and Filip 1998; Filip et al. 1998).

Similar to bacteria, laccase has been identified as a virulence factor in several human-pathogenic fungi such as Cryptococcus neoformans, Aspergillus fumigatus, and Filobasidiella neoformans due to the synthesis of melanin or the involvement of laccase in polysaccharide capsule formation (Mayer and Staples 2002).

Bollag et al. (1988) showed that the addition of laccase reversed the inhibitory effects of a number of phenolic compounds upon the growth of Rhizoctonia prati-cola inocula. They attributed the detoxification of the original phenolic compound to an ability of the laccase to transform it or cross-couple it with another phenol. This allows phytopathogenic fungi such as B. cinerea to detoxify phytoalexins and tannins, thereby increasing fungal virulence (Mayer and Staples 2002). It has been also demonstrated that interactions of different microorganisms, including soil fungi and bacteria, can be accompanied by strong laccase induction (Freitag and Morrell 1992; Savoie et al. 1998; Savoie 2001; Velazquez-Cedeno et al. 2004). This has been shown for laccase-producing basidiomycetes (Iakovlev and Stenlid 2000; Baldrian 2004), and also for the plant-pathogenic soil fungus Rhizoctonia solani when exposed to Pseudomonas strains producing antifungal compounds (Crowe and Olsson 2001). Laccase can probably also contribute to the degradation of phenolic antibiotics that inhibit fungal growth, like 2,4-diacetylphloroglucinol. The role of laccases in defense against heavy metals has been attributed to the production of melanins (Galhaup and Haltrich 2001; Baldrian et al. 2000; Baldrian 2003).

White-rot fungi secrete laccases and other oxidative enzymes in order to degrade complex natural polymers such as lignin (O'Malley et al. 1993; Dean and Eriksson

1994; Leonowicz et al. 2001). Laccase activity also plays an important role during composting processes, and it was isolated from both compost-specific fungi and the compost itself (Chefetz et al. 1998a,b; Chamuris et al. 2000).

Soil humic substances are considered to be the most stable part of decomposing organic matter in nature, and there is evidence that they are in a steady-state equilibrium of formation and degradation. Laccases have been shown to participate in the transformation of humic substances (Dehorter and Blondeau 1992; Chefetz et al. 1998a; Filip et al. 1998; Fakoussa and Frost 1999; Kluczek-Turpeinen et al. 2003, 2005). Laccase activity was also positively correlated with the degradation and synthesis of humic matter in experiments with Cladosporium cladosporiodis (Claus and Filip 1998). In vitro studies have demonstrated a 50% decolorization of humic acids by a laccase preparation from T. versicolor in the presence of a redox mediator (Claus and Filip 1998).

Ectomycorrhizal (EM) symbiotic fungi play a central role in the nutrition of trees by mobilizing and transporting nutrients to the roots (Smith and Read 1997). Phosphorus- and nitrogen-delivering compounds are entrapped in the complex organic macromolecules of litter and humic matter of forest soils (Ponge 2003). Acid phosphatases, proteases, and laccases are important exoenzymes that help to release matrix-bound nutrients and make them accessible to plant roots (Courty et al. 2006).

Laccase gene sequences have been identified in several EM fungi (Chen et al. 2003) and the enzymes have been purified from Cantharellus cibarius, Lactarius piperatus, Russula delica, Thelephora terrestris, and Armillaria mellea (Baldrian 2006). Other researchers have pointed out that tyrosinase appears to be the major phenoloxidase of EM because the oxidation of the laccase-specific substrate syrin-galdazine has scarcely been reported (Burke and Cairney 2002).

The seasonal dynamics of the laccase and acid phosphatase activities of EM were monitored in an oak forest. Among the most frequent and abundant EM morphotypes, those of Lactarius quietus and Cortinarius anomalus showed a peak in laccase activity in spring, while those of Xerocomus chrysenteron displayed their highest laccase activities in summer and fall (Courty et al. 2006).

Several authors have investigated the production of enzymes by fungi introduced into soils, and a number of protocols for laccase extraction have been proposed to optimize the extraction yield (Lang et al. 1997, 1998; Criquet et al. 1999; Baldrian et al. 2000). Laccase activities in soil extracts have been repeatedly demonstrated (Suflita and Bollag 1980; McClaughtery and Linkins 1990). An enzyme purified from a soil sample exhibited a high similarity to a laccase from Polyporus versicolor (Mayaudon and Sarkar 1975). A thermostable humic acid-laccase complex was isolated by Ruggiero and Radogna (1984).

Relatively high activities of laccase - compared to agricultural or meadow soils -can be detected in forest litter and soils (Rosenbrock et al. 1995; Criquet et al. 2000; Carreiro et al. 2000; Ghosh et al. 2003). The laccase activities reflect the temporal course of organic substance degradation (Fioretto et al. 2000), and their isoenzyme patterns vary during the succession (Nardo et al. 2004). Laccase activities in soil correlate with fungal biomass, which in turn is influenced by factors like temperature (Criquet et al. 2000) or nitrogen fertilization (Carreiro et al. 2000; Gallo et al. 2004).

Laccase activity in water-saturated environments (peatlands) is low due to poor oxygen availability, but increases dramatically when the oxygen concentration increases (Pind et al. 1994; Williams et al. 2000). The burst of laccase activity can lead to the depletion of phenolic compounds that inhibit organic matter degradation by oxidative and hydrolytic enzymes (Freeman et al. 2004), and it can be assumed that oxygen-regulated laccase activity plays an important role in carbon cycling in such environments (Baldrian 2006).

Body Detox Made Easy

Body Detox Made Easy

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