Effects of inorganic phosphate on the photosynthetic carbon reduction cycle in extracts from the stroma of pea chloroplasts

1. Turnover of the photosynthetic carbon reduction cycle has been demon-strated in chlorophyll-free reaction mixtures containing chloroplast stromal extract, as evidenced by the fixation of CO₂ following addition of small amounts of 3-phosphoglycerate.2. The activity of the photosynthetic carbon reduction cycle in this system is inhibited by inorganic phosphate (P_i), with activity reduced to 50% by about 6.5 mM P_i. P_i also increased the lag period which elapsed before a steady rate of CO₂ fixation was obtained.3. The effect of P_i on the rate of 3-phosphoglycerate reduction following the addition of substrate amounts of some cycle intermediates was investigated. Substantial inhibition was observed with fructose 1,6-bisphosphate, sedoheptulose 1,7-bisphosphate and erythrose 4-phosphate as substrates. P_i also affected the activity of ribulose-bisphosphate carboxylase, with stimulation at P_i concentrations below 2.5 mM and inhibition at higher concentrations.4. The results showed that the sedoheptulose bisphosphatase reaction is inhibited more strongly by P_i than the fructose bisphosphatase reaction.5. It is concluded that the previously established inhibitory effects of P_i on photosynthesis by intact isolated chloroplasts may be partly due to these inhibitory effects of P_i on the reactions of the photosynthetic carbon reduction cycle.