In situ inactivation of the oxidase activity of xylem peroxidases by H2O2 in the H2O2-producing xylem of Zinnia elegans

Staining of lignifying Zinnia elegans stems with 3,3',5,5'-tetramethylbenzidine (TMB) in the presence and in the absence of catalase reveals the presence of xylem oxidase activities in the H2O2-producing lignifying xylem cells. This staining of lignifying xylem cells with TMB is the result of two independent mechanisms:one is the catalase-sensitive (H2O2-dependent) peroxidase-mediated oxidation of TMB, and the other the catalase-insensitive (H2O2-independent) oxidation of TMB, probably due to the oxidase activity of xylem peroxidases The response of this TMB-oxidase activity of xylem peroxidases to different exogenous H2O2 concentrations was studied, and the results showed that H2O2 at high concentrations (100-1,000 mM) clearly acted as an inactivator of this xylem TMB-oxidase activity, although-some inhibitory effect could still be appreciated at 40 mM H2O2. This xylem TMB-oxidase activity resided in a strongly basic cell wall-bound peroxidase (pI about 10.5). Given such a scenario, it may be concluded that this TMB-oxidase activity of peroxidase is located in tissues capable of sustaining H2O2 production, and that the in situ oxidase activity shown by this enzyme is inactivated by high H2O2 concentrations.