The role of oxalate in lignin peroxidase-catalyzed reduction: protection from compound III accumulation.

Reduction may be an important step in the degradation of some highly oxidized environmental pollutants by Phanerochaete chrysosporium. Lignin peroxidases (LiP) from P. chrysosporium are able to catalyze reductive reactions using veratryl alcohol (VA) as a mediator and either oxalate or EDTA as electron donors. Reduction of oxygen to superoxide, monitored by oxygen consumption, was used as a measure of the reductive activity of LiP. In the presence of EDTA, the rate of O2 reduction catalyzed by LiP decreased with time and increasing concentrations of H2O2. When oxalate replaced EDTA, LiP-catalyzed O2 reduction did not decrease with time, and increasing concentrations of H2O2 increased the duration and extent of O2 reduction. LiP was converted to the compound III state in the presence of EDTA, H2O2, and veratryl alcohol. When oxalate replaced EDTA, compound II was observed. The importance of the veratryl alcohol cation radical (VA.+) in the conversion of LiP compound III to active enzyme has been previously examined (D.P. Barr and S.D. Aust, 1994, Arch. Biochem. Biophys. 311, 378-382). We propose that rapid reduction of VA.+ by EDTA results in accumulation of LiP compound III and the loss of activity resulting in a decrease in LiP-catalyzed reduction reactions. Oxalate is less effective in reducing the VA.+, therefore, some VA.+ remains to convert compound III to active enzyme and maintain LiP-catalyzed reduction reactions. Thus oxalate, a normal secondary metabolite of P. chrysosporium, is a suitable candidate for mediating reduction reactions by LiP in vivo.