Abstract
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.
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