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Influence of pH upon the Warburg Effect in Isolated Intact Spinach Chloroplasts

I. Carbon Dioxide Photoassimilation and Glycolate Synthesis ¹

^a Institute for Photobiology of Cells and Organelles, Brandeis University, Waltham, Massachusetts 02154

The influence of pH upon the O₂ inhibition of ¹⁴CO₂ photoassimilation (Warburg effect) was examined in intact spinach (Spinacia oleracea) chloroplasts. With conditions which favored the Warburg effect, i.e. rate-limiting CO₂ and 100% O₂, O₂ 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 ¹⁴C-labeling of glycolate, and a decrease of isotope in some phosphorylated Calvin cycle intermediates, particularly triose-phosphate. The ¹⁴C-labeling of starch was also more inhibited by O₂ at higher pH. The enhanced synthesis of glycolate during ¹⁴CO₂ 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 ¹⁴C-labeling profiles have been interpreted in terms of a "CO₂"-sensitive as well as a "CO₂"-insensitive mechanism for glycolate synthesis. Both mechanisms functioned optimally at the higher pH and both responded to O₂.

³ Present address: Immunological Research Laboratories, Harvard Medical School, Robert Brigham Hospital, Roxbury, Mass.

¹ This research was generously supported by National Science Foundation Grant BMS71-00978 and United States Energy Research and Development Administration Grant ET(11-1)3231.