Effect of thiocyanate on the peroxidase and pseudocatalase activities of Leishmania major ascorbate peroxidase

Abstract

We report here that the Leishmania major ascorbate peroxidase (LmAPX), having similarity with plant ascorbate peroxidase, catalyzes the oxidation of suboptimal concentration of ascorbate to monodehydroascorbate (MDA) at physiological pH in the presence of added H₂O₂ with concurrent evolution of O₂. This pseudocatalatic degradation of H₂O₂ to O₂ is solely dependent on ascorbate and is blocked by a spin trap, α-phenyl-n-tert-butyl nitrone (PBN), indicating the involvement of free radical species in the reaction process. LmAPX thus appears to catalyze ascorbate oxidation by its peroxidase activity, first generating MDA and H₂O with subsequent regeneration of ascorbate by the reduction of MDA with H₂O₂ evolving O₂ through the intermediate formation of O₂^-. Interestingly, both peroxidase and ascorbate-dependent pseudocatalatic activity of LmAPX are reversibly inhibited by SCN^- in a concentration dependent manner. Spectral studies indicate that ascorbate cannot reduce LmAPX compound II to the native enzyme in presence of SCN^-. Further kinetic studies indicate that SCN^- itself is not oxidized by LmAPX but inhibits both ascorbate and guaiacol oxidation, which suggests that SCN^- blocks initial peroxidase activity with ascorbate rather than subsequent nonenzymatic pseudocatalatic degradation of H₂O₂ to O₂. Binding studies by optical difference spectroscopy indicate that SCN^- binds LmAPX (Kd = 100 ± 10 mM) near the heme edge. Thus, unlike mammalian peroxidases, SCN^- acts as an inhibitor for Leishmania peroxidase to block ascorbate oxidation and subsequent pseudocatalase activity.