Purification, Properties, and Distribution of Ascorbate Peroxidase in Legume Root Nodules

Abstract

All aerobic biological systems, including N2-fixing root nodules, are subject to O2 toxicity that results from the formation of reactive inter- mediates such as H2O2 and free radicals of O2. H2O2 may be removed from root nodules in a series of enzymic reactions involving ascorbate peroxidase, dehydroascorbate reductase, and glutathione reductase. We confirm here the presence of these enzymes in root nodules from nine species of legumes and from Alnus rubra. Ascorbate peroxidase from soybean nodules was purified to near homogeneity. This enzyme was fcbund to be a hemeprotein with a molecular weight of 30,000 as deter- ruined by sodium dodecyi sulfate gel electrophoresis. KCN, NaN3, CO, and C2H2 were potent inhibitors of activity. Nonphysiological reductants such as guaiacol, o-dianisidine, and pyrogallol functioned as substrates for the enzyme. No activity was detected with NAD(P)H, reduced glu- tathione, or urate. Ascorbate peroxidation did not follow Michaelis- Menten kinetics. The substrate concentration which resulted in a reaction rate of 0.5 Vmax was 70 micromolar for ascorbate and 3 micromolar for H2O2. The high affinity of ascorbate peroxidase for H2O2 indicates that this enzyme, rather than cataiase, is responsible for most H2O2 removal outside of peroxisomes in root nodules.