The Role of Ascorbate Peroxidase and Monodehydroascorbate Reductase in H2O2 Scavenging in Plants

Abstract

Ascorbate is a hydrophilic antioxidant found in most eukaryotes. The first isolation of ascorbate for structural analysis was made by Svirbely and Szent-Györgyi (1933) from the Hungarian pepper, which indicates the high level of ascorbate found in plant tissues as compared with those in other organisms. Since then, many nutritional studies of ascorbate in vegetables have been made, but the physiological function of ascorbate in plant tissues remained obscure for a long time. Recent work, triggered by the proposal that ascorbate is the reductant of hydrogen peroxide (H₂O₂) in chloroplasts (Foyer and Halliwell 1977) and by the finding of ascorbate peroxidase (APX) in plants (Groden and Beck 1979; Kelly and Latzko 1979) and Euglena (Shigeoka et al. 1980), suggested that ascorbate plays a central role in scavenging of H₂O₂ in plant tissues, especially in leaf cells.

Plant peroxidase is one of the classic enzymes, whose reaction mechanism has been most intensively analyzed (Everse et al. 1991). This type of plant peroxidase is referred to as guaiacol peroxidase (GuPX) to distinguish it from APX, and it has diverse physiological functions, such as lignin biosynthesis, degradation of indole-3-acetate, ethylene biosynthesis, cross-linking of hydroxyproline-rich glycoprotein (Brownleader et al. 1995), wound healing, and defense against pathogens. GuPX is classified as the metabolic peroxidase whose oxidation products have physiological functions. In contrast to GuPX, APX is a H₂O₂-scavenging peroxidase, and its primary function is rapid removal of H₂O₂ at the site of its generation (Asada 1992). The primary oxidation products of APX-catalyzed reactions, the monodehydroascorbate...