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Photosynthetic Oxygen Reduction in Isolated Intact Chloroplasts and Cells in Spinach ¹

Richard J. Radmer^b

^a Department of Biological Sciences, University of Maryland Baltimore County (UMBC), Catonsville, Maryland 21228, ^b Martin Marietta Laboratories, 1450 South Rolling Road, Baltimore, Maryland 21227

The time course of light-induced O₂ exchange by isolated intact chloroplasts and cells from spinach was determined under various conditions using isotopically labeled O₂ and a mass spectrometer. In dark-adapted chloroplasts and cells supplemented with saturating amounts of bicarbonate, O₂ evolution began immediately upon illumination. However, this initial rate of O₂ evolution was counterbalanced by a simultaneous increase in the rate of O₂ uptake, so that little net O₂ was evolved or consumed during the first 1 minute of illumination. After this induction (lag) phase, the rate of O₂ evolution increased 3- to 4-fold while the rate of O₂ uptake diminished to a very low level. Inhibition of the Calvin cycle, e.g. with DL-glyceraldehyde or iodoacetamide, had negligible effects on the initial rate of O₂ evolution or O₂ uptake; both rates were sutained for several minutes, and about balanced so that no net O₂ was produced. Uncouplers had an effect similar to that observed with Calvin cycle inhibitors, except that rates of O₂ evolution and photoreduction were stimulated 40 to 50%.

These results suggest that higher plant phostosynthetic preparations which retain the ability to reduce CO₂ also have a significant capacity to photoreduce O₂. With near-saturating light and sufficient CO₂, O₂ reduction appears to take place primarily via a direct interaction between O₂ and reduced electron transport carriers, and occurs principally when CO₂-fixation reactions are suboptimal, e.g. during induction or in the presence of Calvin cycle inhibitors. The inherent maximum endogenous rate of O₂ reduction is approximately 25 to 50% of the maximum rate of noncyclic electron transport coupled to CO₂ fixation. Although the photoreduction of O₂ is coupled to ion transport and/or phosphorylation, this process does not appear to supply significant amounts of ATP directly during steady-state CO₂ fixation in strong light.

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