Mehler reaction

Besides these external processes, formation of ROS may also take place intrac-ellularly. Photooxidative stress, including UVB, stimulates various cellular processes leading to the production of superoxide radicals and hydrogen peroxide, as well as singlet-oxygen and hydroxyl radicals. The sources and production sites of ROS are mainly related to photosynthetic activities such as the pseudocyclic photophosphorylation and the Mehler reaction, which stimulate the accumulation of hydrogen peroxide (Asada 1994 Elstner 1990). 

Electrons from ferredoxin may also reduce O2, which yields H2O2 and eventually H20 (o2 + 2e + 2H+ -> H202 - H20 +102). (The light-de-pendent consumption of O2, as occurs when electrons from ferredoxin or from one of the iron-plus-sulfur-containing intermediates of Photosystem I move to O2, is termed the Mehler reaction.) Because equal amounts of O2 are evolved at Photosystem II and then consumed using reduced ferredoxin in a separate reaction, such electron flow is termed pseudocyclic (see Figs. 5-18 and 6-4). No net O2 change accompanies pseudocyclic electron flow,... 

Adaptive strategies. Oxygen consumption—The Mehler reaction 1546... 

The Mehler reaction is a photochemical reduction of O2 to H2O2 or H2O in photosystem I (Box 35.2). Mehler activity is thought to be a mechanism for energy dissipation under high tight intensities or when carbon frxation is limited by supply... 

Generation of Superoxide Radicals and Hydrogen Peroxide in Chloroplasts. Isolated, illuminated chloroplast thylakoids slowly take up oxygen in the absence of added electron acceptors. This phenomenon was first observed by Mehler (7) and is often known as the "Mehler reaction. The reaction appears to result from the reduction of O2 to the... 

While NADP is the preferred electron acceptor in photosynthesis, oxygen can also accept electrons from toe photosynthetic electron transport chain (Mehler, 1951 Mehler and Brown, 1952 Allen, 1975, 1992 Egneus et al., 1975 Marsho et al., 1979). Molecular oxygen contains two unpaired electrons with parallel spins. As a consequence Oj is most easily reduced by single electron additions because divalent reduction of Oj (to HjOj) requires a spin inversion (Cadenas, 1989). The addition of an electron to oxygen by the photosynthetic electron transport chain produces superoxide (Asada et al, 1974). The reduction of molecular oxygen by the photosynthetic electron transport system is called the Mehler reaction (Mehler, 1951 Mehler and Brown, 1952) and electron transport from water to molecular is called pseudocyclic electron flow (Allen, 1975, 1977). Reported values for concentrations required to half-saturate the rate of the Mehler reaction are between 2 and 60 juM (Robinson, 1988) which corresponds to 0.17 to 5% Oj in air at 25 °C. 

Another possible site for the increased " 2 2 Production is the acceptor side of PS-II. It is known that the acceptor side of PS-I can utilize as an electron acceptor whereby is produced (Mehler reaction). It is... 

Fig. 4. Photosynthetic transport of electrons from PSII water oxidation is normally used to reduce NADP+ to NADPH for use in Calvin cycle carbon fixation. But, under excess light, when the electron transport chain is over-reduced, electron flow may be diverted at PSI to the Mehler reaction, reducing O2 to superoxide (O2 ). Chloroplast antioxidant systems can further reduce O2 back to water, allowing the water-water cycle to function as a protective alternative electron transport path.

It is well known, that electrons from ferredeoxin reduce oxygen in Mehler reaction. The resulting superoxide-anion participation in desaturation reactions can be proposed, as already suggested by Sreekrishna and Joshi for animal tissue 3. However, superoxide dismutase addition to chloroplast incubation media with 1 acetate (Tabl. 2) has not led to any alteration of stearate desaturase activity. 

Fig. 2 Influence of temperature on the inhibition of the anthraquinone-2-sulfonate-Mehler reaction with DCPiP/ascorbate as electron donors with...

The data shown in Figs. 8 and 21 demonstrate that the changes in the specific activity of the Mehler reaction (calculated as rate of methylvio-logen-mediated oxygen uptake per chlorophyll, using ascorbate-DCIP as... 

Since PSI and PSII are at different locations in stacked thylakoids, it was surprising that proton uptake at both sites showed the same slow kinetics. In presence of methyl viologen (MV) proton uptake at PSI is caused by the Mehler reaction ... 

We asked whether proton uptake induced by PSI was rate limited by the residual amount of superoxide dismutase (SOD), which catalyses the Mehler reaction. 

Rate of proton uptake from the external bulk phase by PSI is limited by. superoxide dismutase. Fig.1 shows the pH -indicating absorption changes of cresol red at 575nm. In the presence of MV proton uptake by PSII and by the Mehler reaction induced by PSI had a half rise time of approximate 120ms (upper trace, left). The rise of the signal was not monoexponential. A detailed analysis of the data will be presented elsewhere. Here we point out qualititative effects. 

Figure 1 Proton uptake from the external bulk phase by PSI and PSII (upper traces) and by the Mehler reaction of PSI only (lower traces). The assay medium contained MV lOuM and where indicated DCMU 5>iM, DAD lOjiM, DTT 4( M, SOD 5 g/ml. Time resolution 1ms/point...

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