Photosynthetic CO2 Fixation at Air Levels of CO2 by Isolated Spinach Chloroplasts

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Abstract
A system has been developed for the study of photosynthetic CO2 fixation by isolated spinach chloroplasts at air levels of CO2. Rates of CO2 fixation were typically 20 to 60 micromoles/milligrams chlorophyll per hour. The rate of fixation was linear for 10 minutes but then declined to less than 10% of the initial value by 40 minutes. Ribulose 1,5-bisphosphate (RuBP) levels remained unchanged during this period, indicating that they were not the cause for the decline. The initial activity of the RuBP carboxylase in the chloroplast was high for 8 to 10 minutes and then declined similar to the rate of CO2 fixation, suggesting that the decline in CO2 fixation may have been caused by deactivation of the enzyme. In contrast, the addition of 50 micromolar 3-phosphoglycerate or 50 micromolar glycerate doubled the initial rate of CO2 fixation without increasing the activity of the carboxylase. Addition of 4 millimolar MgCl2 plus 2 millimolar pyrophosphate had no effect on the initial rate of CO2 fixation nor its decline but did stop deactivation of the chloroplast carboxylase during the decline. The decline in photosynthesis with isolated chloroplasts, where RuBP is high, may be due to deactivation of the chloroplast RuBP carboxylase; however, measurements of this parameter may not always indicate this. Care must be taken when interpreting measurements of the `activated' RuBP carboxylase as more forms are measured than the enzyme · CO2 · Mg2+ · RuBP form directly involved in carboxylation or oxygenation.