Oxidation of hydroquinones by the versatile ligninolytic peroxidase from Pleurotus eryngii -: H2O2 generation and the influence of Mn2+

Formation of H(2)O(2) during the oxidation of three lignin-derived hydroquinones by the ligninolytic versatile peroxidase (VP), produced by the white-rot fungus Pleurotus eryngii, was investigated. VP can oxidize a wide variety of phenols, including hydroquinones, either directly in a manner similar to horseradish peroxidase (HRP), or indirectly through Mn(3+) formed from Mn(2+) oxidation, in a manner similar to manganese peroxidase (MnP). From several possible buffers (all pH 5), tartrate buffer was selected to study the oxidation of hydroquinones as it did not support the Mn(2+)-mediated activity of VP in the absence of exogenous H(2)O(2) (unlike glyoxylate and oxalate buffers). In the absence of Mn(2+), efficient hydroquinone oxidation by VP was dependent on exogenous H(2)O(2). Under these conditions, semiquinone radicals produced by VP autoxidized to a certain extent producing superoxide anion radical (O(2)(.-)) that spontaneously dismutated to H(2)O(2) and O(2). The use of this peroxide by VP produced quinone in an amount greater than equimolar to the initial H(2)O(2) (a quinone/H(2)O(2) molar ratio of I was only observed under anaerobic conditions). In the presence of Mn(2+), exogenous H(2)O(2) was not required for complete oxidation of hydroquinone by VP. Reaction blanks lacking YT revealed H(2)O(2) production due to a slow conversion of hydroquinone into semiquinone radicals (probably via autooxidation catalysed by trace amounts of free metal ions), followed by O(2)(.-) production through semiquinone autooxidation and O(2)(.-) reduction by Mn(2+). This peroxide was used by VP to oxidize hydroquinone that was mainly carried out through Mn(2+) oxidation. By comparing the activity of VP to that of MnP and HRP, it was found that the ability of VP and MnP to oxidize Mn(2+) greatly increased hydroquinone oxidation efficiency.