Laccase-catalyzed decolorization of synthetic dyes

Commercial dyes are not uniformly susceptible to microbial attack in conven tional aerobic treatment because of their unique and stable chemical struct ures. Three synthetic dyes with typical chromophores (anthraquinone, azo an d indigo) were decolorized by a white-rot fungus Trametes versicolor. The r esponsible enzyme for dye decomposition was laccase, an extracellular oxida se released by the fungus under the conditions of slow growth or in its sta tionary phase. The mechanism of laccase-catalyzed dye decomposition, howeve r, was different depending on dye structures. Anthraquinone dye was an enzy me substrate that was directly oxidized by laccase while decolorization of azo and indigo dyes involved some small molecule (< 8 kDa) metabolites. It was demonstrated that azo and indigo dyes were not the substrates of laccas e and the small molecule metabolites mediated the interaction between the d yes and the enzyme. The decolorization rate of the nonsubstrate dyes was ac tually limited by the concentration of mediating compounds rather than lacc ase activity in the solutions. Some synthetic compounds such as 2,2'-azino- bis(3-ethylthiazoline-6-sulfonate) or ABTS and anthraquinone dye could also mediate the decolorization of azo and indigo dyes. The mediating function of ABTS and anthraquinone dye was quantitatively compared in the decomposit ion of two nonsubstrate dyes. This fact implies that the laccase-substrate dyes in an industrial effluent can promote the decolorization of those nons ubstrate dyes. Effluent decolorization, therefore, may not be limited by th e small molecule metabolites which are not produced in large amount by fung us in most industrial effluents. A laccase-catalyzed and mediator-involved dye degradation mechanism is proposed For further kinetic studies. (C) 1999 Elsevier Science Ltd. All rights reserved.