It has recently been discovered that lignin peroxidase isozyme H2 (LiPH2) has the ability to oxidize Mn2+ (Khindaria et al., 1995). Furthermore, at pH 4.5, the physiological pH of Phanerochaete chrysosporium, LiPH2 oxidizes Mn2+ at a much faster rate (25 times) than veratryl alcohol (VA). The ability of Mn2+ to act as a redox mediator for indirect oxidations catalyzed by LiPH2 was therefore investigated. In the presence of physiologically relevant levels of oxalate and Mn2+, the rate of LiPH2- catalyzed oxidation of all substrates studied was dramatically increased. Up to 10-fold stimulations were observed compared to the rates of oxidation of substrate in either the presence or absence of VA. We propose that the stimulation is due to the ability of LiPH2 to oxidize Mn2+, producing the strong oxidant Mn3+, at a high rate. The rates of oxidation of the substrates showed a hyperbolic dependence on Mn2+ in the presence of oxalate, a chelator which was required for maximal activity. The oxalate dependence of the oxidation rates correlated well with the concentration of the 1:1 complex of Mn2+-oxalate. The relative concentrations of the substrates and H2O2 and the rate constants for their reactions with Mn3+ determined which chemical was oxidized by the enzymatically produced Mn3+. The importance of the ability of Mn2+-oxalate to stimulate the oxidation of chemicals by LiPH2 is discussed.
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