Inhibition of Photosynthesis by Oxygen in Isolated Spinach Chloroplasts

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Abstract
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.

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