Oxygen Activation in Isolated Chloroplasts. Mechanism of Ferredoxin-Dependent Ethylene Formation from Methionine

Abstract

1 Low-potential electron acceptors of photosystem I of chloroplast lamellae produce superoxide anions (O₂^-) and hydrogen peroxide by autoxidation, but have no effect on ethylene formation from methionine; equimolar amounts of ferredoxin are less active in photosynthetic O₂^- and H₂O₂ production but strongly stimulate ethylene production from methionine. 2 Ten to fifty units of superoxide dismutase inhibit fifty to two hundred units of superoxide dismutase stimulate ethylene formation from methionine by chloroplast lamellae in the presence of ferredoxin. This stimulation is stronger at pH 7.0 than at pH 7.8. Catalase inhibits ethylene formation from methionine. 3 Pulse-radiolytic production of nitrite (NO₂^-) from hydroxylamine, initiated by hydroxyl radicals (^.OH) or O₂^-, shows no difference in the presence or absence of ferredoxin, nor do the decay kinetics of O₂^-. 4 From the above observations and from model reactions (xanthine/xanthine oxidase; iron salts in the presence of H₂O₂), it is concluded that reduced ferredoxin in the presence of H₂O₂ forms a Fenton-type oxidizing species for methionine, generating ethylene in the presence of pyridoxal phosphate. 5 Inhibitory effects of both superoxide dismutase and catalase in oxygen-dependent reactions need not necessarily indicate the participation of the ‘Haber-Weiss’ reaction.