Influence of pH upon the Warburg Effect in Isolated Intact Spinach Chloroplasts: I. Carbon Dioxide Photoassimilation and Glycolate Synthesis.

The influence of pH upon the O(2) inhibition of (14)CO(2) photoassimilation (Warburg effect) was examined in intact spinach (Spinacia oleracea) chloroplasts. With conditions which favored the Warburg effect, i.e. rate-limiting CO(2) and 100% O(2), O(2) inhibition was greater at pH 8.4 to 8.5 than at pH 7.5 to 7.8. At pH 8.5, as compared with 7.8, there was an enhanced (14)C-labeling of glycolate, and a decrease of isotope in some phosphorylated Calvin cycle intermediates, particularly triose-phosphate. The (14)C-labeling of starch was also more inhibited by O(2) at higher pH. The enhanced synthesis of glycolate during (14)CO(2) assimilation at higher pH resulted in a diminution in the level of phosphorylated intermediates of the Calvin cycle, and this was apparently a causal factor of the increased severity of the Warburg effect.The (14)C-labeling profiles have been interpreted in terms of a "CO(2)"-sensitive as well as a "CO(2)"-insensitive mechanism for glycolate synthesis. Both mechanisms functioned optimally at the higher pH and both responded to O(2).