Phenoloxidase-mediated interactions of phenols and anilines with humic materials

Phenoloxidases present in terrestrial systems may contribute to the formati on of humus through random coupling of a variety of aromatic compounds, inc luding xenobiotic chemicals. Because of their structural similarity to natu ral substrates originating mainly from lignin decomposition, xenobiotic phe nols and anilines can be readily incorporated into the soil organic matter, a phenomenon referred to as binding. The underlying mechanism of binding i nvolves oxidation of the xenobiotic substrates to free radicals or quinone products that subsequently couple directly to humus or to naturally occurri ng phenols that also are subject to oxidation. The oxidation can be mediate d by soil phenoloxidases as well as by abiotic catalysts. The ability of th e enzymes to mediate the oxidation was demonstrated in a number of model st udies, in which selected pollutants were incubated with humic monomers or n atural humic acids in the presence of different phenoloxidases (laccase, pe roxidase, tyrosinase). Analysis of the formed complexes by mass spectrometr y and C-13 nuclear magnetic resonance (NMR) spectroscopy left no doubt abou t the formation of covalent bonds between the pollutants and humic material s. Some bonds were formed at the chlorinated sites, leading to partial deha logenation of the aromatic contaminants. Experimental data indicated that b ound phenols and anilines were unlikely to adversely affect the environment ; their release from humic complexes by soil microorganisms was very limite d and once released, they were subjected to mineralization. For those reaso ns, phenoloxidases, which proved capable of mediating the underlying reacti on, are currently considered as a tool for enhancing immobilization phenome na in soil.