Effects of adenosine triphosphate and inorganic pyrophosphate on carbon dioxide fixation by isolated spinach chloroplasts

Abstract

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.