INTERACTIONS OF ASCORBATE AND CHELATED IRON IN A METHYLVIOLOGEN‐MEDIATED MEHLER REACTION

— –In the light, isolated spinach thylakoids consumed O 2 in the presence of methylviologen, and ascorbate was found to interact with this reaction in various ways. Chelating‐resin was used to remove metal impurities from the assay medium. Ascorbate diminished the H 2 0 2 pool in resin‐untreated solutions, while in resin‐treated solutions ascorbate had no effect on H 2 O 2 concentrations. A Fenton catalyst (Fe‐EDTA) increased O 2 uptake in the presence of ascorbate and decreased the amount of O 2 recovered by catalase. Ascorbate tripled the rate of the methylviologen‐mediated Mehler reaction, and the O 2 consumed was liberated to 50% of its original concentration by catalase. Superoxide dismutase reversed the effects of ascorbate on the Mehler reaction rates. These results indicate that ascorbate can stimulate Mehler reactions indirectly by promoting a Fenton‐type reaction as well as stimulating Mehler reactions directly by reducing 2O 2 ‐ to 2H 2 O 2 . The promotion of a Fenton‐type reaction by ascorbate appears to be the cause of H 2 O 2 depletion in resin‐untreated solutions.