Further studies on the inactivation by sodium azide of lignin peroxidase from Phanerochaete chrysosporium

Abstract

Azide ion is a mechanism-based inactivator of horseradish peroxidase [Ortiz de Montellano et al. (1988) Biochemistry 27, 5470-5476] and the peroxidase from the coprophilic fungus Coprinus macrorhizus [DePillis and Ortiz de Montellano (1989) Biochemistry 28, 7947-7952]. These peroxidases mediate the one-electron oxidation of azide ion-forming azidyl radical. Inactivation of these enzymes is caused by covalent modification of the heme prosthetic groups by azidyl radical. Lignin peroxidases from the wood- rotting fungus Phanerochaete chrysosporium are also inactivated when they catalyze oxidation of azide ion [Tuisel et al. (1991) Arch. Biochem. Biophys. 288, 456-462; DePillis et al. (1990) Arch. Biochem. Biophys. 280, 217223]. Following inactivation of horseradish peroxidase and the peroxidase from C. macrorhizus substantial amounts of azidyl-heme adducts have been found. Only trace amounts of such adducts have been found following azide-mediated inactivation of lignin peroxidase. Nevertheless, we have shown that during oxidation of azide by lignin peroxidase H8 destruction of heme occurred and a substantial fraction of the enzyme is irreversibly inactivated. However, the rest of the enzyme forms a relatively stable ferrous-nitric oxide (NO) complex. Although this complex appears to be an inactivated form of the enzyme, we have shown that, when present as the ferrous-NO complex, the enzyme is actually protected from inactivation. The lignin peroxidase ferrous-NO complex reverts slowly (t(1/2) = 6.3 x 103 s) to the ferric form. Reversion is accelerated if the complex is chromatographed on a PD-10 (Sephadex G-25) column or if veratryl alcohol is added. If azide and hydrogen peroxide (a required cosubstrate) are present (or added), the enzyme undergoes another cycle of catalysis and further inactivation. A detailed reaction mechanism is proposed that is consistent with our experimental observations, the chemistry of azide, and our current understanding of peroxidases.