Kinetics of a Stepwise Photoxidation of Ascorbic Acid by a Manganese‐flavin‐catalase System*

Summary 1. Helianthus chloroplasts to which flavin, MnC12, catalase and ascorbic acid have been added catalyze the photoxidation in red light of ascorbic acid in a three‐step reaction with the absorption of not one but two equivalents of oxygen. 2. In white or blue light the chloroplasts can be omitted and the entire sequence of photochemical reactions occurs unchanged with flavin as the light absorbing dyestuff. 3. New to this reaction are the continuation beyond the oxidation to dehydroascorbic acid and the peculiar kinetics which treat successive intermediate products not as competing but as successive substrates of various photochemical steps. 4. The sequence of reactions depends specifically on the combination of flavin, manganous ion and iron porphyrin. To photoxidize dehydroascorbic acid to a 2 carbon and 4 carbon oxidative product none of the three components can be omitted from the reaction mixture. 5. Kinetics and specificity of the reaction are discussed on the basis of a complex which imitates the action of natural flavin‐iron porphyrin oxidase.