A 60–70% inhibition of spinach chloroplast PMS1-catalyzed cyclic photophosphorylation was observed following a 5-min pre-incubation in the dark with either 0.25 mM phloridzin or 10 μM quinacrine. Phloridzin was also shown to severely inhibit the endogenous synthesis of ATP (no catalyst being present). The inhibitors had only a slight effect on the fixation of carbon dioxide. Addition of 3-phosphoglyceric acid was found to inhibit the fixation of CO2 while ribose 5-phosphate stimulated CO2 fixation, even in the presence of 0.25 mM phloridzin.
The effects of exogenous ATP and inorganic pyrophosphate on CO2 fixation were investigated with preparations of intact spinach chloroplasts. Under illumination, both ATP and inorganic pyrophosphate resulted in an increase in the rate of CO2 fixation. These stimuli were generally equivalent to one another, and the degree of stimulation was dependent upon the age and condition of the chloroplasts used. During a reaction the enhancement effect increased with time; the initial kinetics (72 μmoles CO2 fixed per milligram chlorophyll per hour) were unaffected by either ATP or inorganic pyrophosphate. The stimulation of CO2 fixation by ATP or inorganic pyrophosphate was inhibited by a buffer pH of more than 8.0. Concentrations of exogenous magnesium above 0.25 mM were found to inhibit both the ATP- and inorganic pyrophosphate-induced increases in CO2 fixation. Phloridzin was found to partially inhibit the enhancement of CO2 fixation by ATP and inorganic pyrophosphate.
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