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Inhibition of Photosynthesis by Oxygen in Isolated Spinach Chloroplasts ^1,2

^a Department of Biology, Brandeis University, Waltham, Massachusetts 02154, Department of Biochemistry, Cornell University, Ithaca, New York 14850

The inhibition of photosynthetic CO₂ fixation by O₂, commonly referred to as the Warburg effect, was examined in isolated intact spinach (Spinacia oleracea) chloroplasts. The major characteristics of this effect in isolated chloroplasts are rapid reversibility when O₂ is replaced by N₂, an increased inhibition by O₂ at low concentrations of CO₂ and a decreased effect of O₂ with increased concentrations of CO₂.

Both the DPN- and TPN-linked glyceraldehyde 3-phosphate dehydrogenases but not aldolase were inhibited by O₂. The photoreduction of TPN measured in fragmented chloroplast preparations was similar in N₂ and O₂ down to a concentration of 5 micromolar TPN. The effect of 100% O₂ on ¹⁴CO₂ assimilation was overcome completely by fructose 1,6-diphosphate and by ribose 5-phosphate but not by ascorbate, cysteine, dithiothreitol and reduced lipoate. Glycolate became the major photosynthetic product at high partial pressures of O₂ or at low CO₂ concentrations. It is concluded that O₂ depresses photosynthesis primarily by causing a shift of a major portion of the total carbon into glycolate and impairing the functioning of the photosynthetic carbon reduction cycle. The mechanism whereby O₂ alters the flow of carbon into glycolate remains unknown.

² Much of these studies was submitted to the Graduate School, Cornell University in partial fulfillment of the requirements for the degree of Doctor of Philosophy.