Inhibition of Photosynthesis by Oxygen in Isolated Spinach Chloroplasts

The inhibition of photosynthetic CO(2) fixation by O(2), 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 O(2) is replaced by N(2), an increased inhibition by O(2) at low concentrations of CO(2) and a decreased effect of O(2) with increased concentrations of CO(2).Both the DPN- and TPN-linked glyceraldehyde 3-phosphate dehydrogenases but not aldolase were inhibited by O(2). The photoreduction of TPN measured in fragmented chloroplast preparations was similar in N(2) and O(2) down to a concentration of 5 micromolar TPN. The effect of 100% O(2) on (14)CO(2) 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 O(2) or at low CO(2) concentrations. It is concluded that O(2) 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 O(2) alters the flow of carbon into glycolate remains unknown.