The biomass-processing enzymes laccase and xylanase can each be used to moderately reduce the amount of chlorine dioxide required for bleached pulp production. Although combined enzyme treatment is anticipated to be appealing to biomass-converting industries seeking to implement green processes, simultaneous incubation of these enzymes in the presence of a laccase mediator such as 1-hydroxybenzotriazole (HBT) results in rapid loss of xylanase activity. To characterize this inactivation, Thermomyces lanuginosus xylanase was treated with Trametes versicolor laccase in the presence of HBT. The time-dependent loss of xylanase activity showed first order kinetics, and a plot of inactivation rate constant versus HBT concentration exhibited saturation kinetics. Further, the xylanase substrate xylan protected against the inactivation. These results are consistent with an active-site directed modification, and the change in the xylanase UV absorbance and fluorescence emission spectra upon inactivation suggests tryptophan oxidation. To further characterize the nature of the modification, inactivated xylanase was digested with chymotrypsin and the resulting peptides were analyzed via mass spectrometry. The laccase-generated HBT oxidant was demonstrated to oxidize tryptophan residues to oxindolealanine residues, including Trp17 and known xylan-binding Trp18. Mechanistic detail-of the oxidative inactivation will provide the basis for future studies focused on construction of redox-resistant xylanases. (C) 2007 Elsevier Inc. All rights reserved.