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

Habermann, Helen M.; Gaffron, Hans

doi:10.1111/j.1751-1097.1962.tb08089.x

Cited by 16 publications

(5 citation statements)

References 26 publications

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“…The best explanation at this point seems an oxidation of ascorbate brought about by either singlet oxygen, super-oxide or hydroxyl radicals formed in the light. Habermann and Gaffron (1962) have shown with an 0;-generating model system that such an oxidation is initially accompanied by proton uptake. From the data presented here it is impossible to decide whether the superoxide formation we have observed in Chllipid vesicles is brought about by a direct electron transfer from excited Chl to oxygen or by a secondary reaction like (Thomas, 1965).…”

Section: Discussionmentioning

confidence: 99%

Photoreactivity of Isolated Photosystem I Particles Upon Combination With Artificial Lipid Membranes or Triton X‐100 Micelles*

Ginkel

1979

Photochem & Photobiology

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Abstract— PS‐I particles isolated according to Shiozawa et al. (1974) show increased rates of O2‐‐ and H +‐uptake with ascorbate as electron donor upon combination with an artificial vesicular lipid membrane. The amount of increase varies depending on the reconstitution procedure used. Combination of PS‐I particles with Triton X‐100 micelles increases these photochemical activities even more. The observed proton uptake in PS‐I lipid vesicles is not caused by the well‐known proton gradient found in thylakoid membranes, since lipid vesicles containing extracted leaf Chl show the same activities and uncouplers have no effect. Because these phenomena are also caused by solubilized Chl, it is concluded that there is no obvious correlation with PS‐I activity. Proton uptake most probably is caused by oxidation of ascorbate by either singlet oxygen, superoxide or OH‐radicals formed in the light. Experimental results are obtained which indicate that Chl in lipid catalyzes formation of superoxide and singlet oxygen. However, it is not clear whether superoxide formation is caused by direct electron transport from excited Chl to oxygen or by a secondary reaction. Diphenylcarbazone disproportionation has been reported as a specific photosystem I reaction. However, PS‐I lipid vesicles and Chl‐lipid‐Triton X‐100 mixtures oxidize DPCN at comparable rates, showing that the reaction is not specific for PS‐I. Cations stimulate DPCN disproportionation in Chl‐lipid‐Triton X‐100 mixtures but do not affect the rate of P700 photooxidation at all. Therefore it is suggested that Gross and Greniers (1978) conclusion that cation regulation of PSI electron flow (studied by DPCN disproportionation of PS‐I particles in Triton X‐100 micelles) provides a fine tuning mechanism for energy transfer, has to be reevaluated.

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Section: Discussionmentioning

confidence: 99%

Photoreactivity of Isolated Photosystem I Particles Upon Combination With Artificial Lipid Membranes or Triton X‐100 Micelles*

Ginkel

1979

Photochem & Photobiology

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“…Manganese. Manganous ions are essential for the complete photoxidation of ascorbic acid in the artificial flavin-sensitized system [8]. In the active chloroplast system, manganese is not an essential component of the reaction mixture.…”

Section: Results a N D Discussionmentioning

confidence: 99%

“…In the aerobic photoxidation of manganese by chloroplasts, Bachofen [3] has found two pH optima reflecting conditions favoring H,O, formation (at neutrality) and Mn3+ formation (at pH [8][9]. The pH response of the chloroplast-mediated photoxidation of DKGA suggests a relationship to Bachofen's system.…”

Section: (F) P H Ofreaction Mixturementioning

confidence: 99%

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Kinetics of Chloroplast‐mediated Photoxidation of Diketogulonate*

Habermann

Handel

McKELLAR

1968

Photochem & Photobiology

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Illuminated chloroplasts can mediate a photoxidation of diketogulonic acid (DKGA) with rates of oxygen uptake equivalent to rates of Hill reactions with ferricyanide or quinone. The photoxidation of DKGA is sensitive to dichlorophenyl dimethylurea (DCMU) and exhibits the drop in quantum yield at long wavelengths characteristic of photosystem 11. Still, the reaction is only partially inactivated by heating chloroplasts at 50" for 10 min (which destroys oxygen evolution). The photoxidation is inhibited by copper and detergents; and is stimulated by added flavin (or methyl viologen) and manganous ions.A model system containing Mn3+ (as manganipyrophosphate) and DKGA, mimics the chloroplast system. Pre-illuminated chloroplast suspensions can be substituted for Mn3+ in the model dark reaction.I t seems that a light-dependent oxidation of MnZ+ to Mn3+ by photosystem I1 is the essential contribution of the chloroplasts. Electrons from Mn2+ move through the electron transport system to femcyanide or to photosystem I where, via flavin (or methyl viologen), oxygen is reduced to H,O,.

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“…Stabilization of riboflavin to light in the presence of additives is pharmaceutically important, since certain additives were observed to have considerable influence on the light stability of the system (13)(14)(15)(16)(17)(18)(19)(20)(21)(22). Despite this importance, very few photochemical kinetic studies have been published for riboflavin in the presence of complexing agents.…”

Section: Keyphrasesmentioning

confidence: 99%