Transformation of polycyclic aromatic hydrocarbons by laccase is strongly enhanced by phenolic compounds present in soil

Efficient transformation of several polycyclic aromatic hydrocarbons (PAHs) was obtained using a fungal laccase in the presence of phenolic compounds related to those formed in nature during the turnover of lignin and humus. The effect of these natural mediators, namely vanillin, acetovanillone, acetosyringone, syringaldehyde, 2,4,6-trimethylphenol, p-coumaric acid, ferulic acid, and sinapic acid, was compared with that of synthetic mediators such as 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonate) (ABTS) and 1-hydroxybenzotriazole (HBT). Anthracene was significantly degraded by laccase in the absence of mediators, whereas benzoa]pyrene and pyrene were weakly transformed (less than 15% after 24 h). Vanillin, acetovanillone, 2,4,6-trimethylphenol, and, above all, p-coumaric acid strongly promoted the removal of PAHs by laccase. 9,10-Anthraquinone was the main product detected from anthracene oxidation by all the laccase-mediator systems. The yield of anthraquinone formed was directly correlated with the amount of p-coumaric acid used. This compound resulted in a better laccase mediator than ABTS and close similarity to HBT, attaining 95% removal of anthracene and benzoa]pyrene and around 50% of pyrene within 24 h. Benzoa]pyrene 1,6-, 3,6-, and 6,12-quinones were produced during benzoa]pyrene oxidation with laccase and p-coumaric acid, HBT, or ABTS as mediators, although use of the latter mediator gave further oxidation products that were not produced by the two other systems.