Transformation of the fungicide cyprodinil by a laccase of Trametes villosa in the presence of phenolic mediators and humic acid

Xenobiotic chemicals can be transformed or covalently bound to humic materials by oxidoreductive enzymes present in terrestrial systems. Chemicals that are not substrates for oxidoreductive enzymes may undergo transformation in the presence of certain reactive compounds, which are often referred to as mediators. In this study, cyprodinil, a broad-spectrum fungicide, did not show any transformation when incubated alone with a laccase from Trametes villosa. It was transformed to a significant extent, however, when a mediator was present. All of the 13 tested mediators belonged to the group of naturally occurring phenols. With some exceptions (2,6-dimethoxyphenol, syringic acid, and ferulic acid), phenols substituted with one or two methoxy groups were very effective mediators. In experiments with 14C-labeled cyprodinil, the radioactive label was largely associated with brown transformation products that precipitated out of the aqueous solution. As determined by mass spectrometry, the products were mixed oligomers resulting from cross-coupling between cyprodinil and a mediator. The addition of large amounts of humic acid (HA) (400 mg/L) to the reaction mixtures involving the most effective mediators reduced cyprodinil transformation (42.6–68.6%) by 12–48%, probably due to an inhibitory effect. The inhibition decreased with decreasing concentration of HA. The addition of HA (400 mg/L) to the reaction mixtures involving the least effective mediators or no mediators (control) enhanced cyprodinil transformation (0.3–17.6%) by 2.9–17.1%, probably as a result of binding to HA.