Correlation of Glutathione Peroxidase to Paraquat/Oxidative Stress Resistance in Conyza Determined by Direct Fluorometric Assay

Glutathione peroxidase is involved in scavenging free radicals in many biological systems. Its presence in plants has been implicated by identification of genes with nucleotide sequences similar to those of mammals, as well as by indirect, coupled enzyme assays. We directly quantified glutathione peroxidase activity in crude plant extracts using an organic hydroperoxide substrate and measuring GSSG, the direct reaction product, by adapting a sensitive fluorometric method. The constitutive levels of glutathione peroxidase in a paraquatresistant biotype of Conyza bonariensis (resistant to many oxidative stresses) were almost double those of the sensitive biotype, providing correlative evidence that glutathione peroxidase is part of the system preventing oxidative damage. Glutathione peroxidase activity was separated from both glutathione transferase and glutathione reductase activities by specific antibody immunoprecipitation and by substrate-affinity chromatography. Conyza glutathione peroxidase cross-reacted with citrus polyclonal anti-phospholipid hydroperoxide glutathione peroxidase. The data suggest that this glutathione peroxidase is a component of the systems dealing with oxidative damage protection in plants and is biochemically different from glutathione peroxidases that also have glutathione transferase activity.