Turnover of the photosynthetic carbon reduction cycle has been demonstrated 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 (Pi), with activity reduced to 50% by about 6.5 mM Pi. Pi also increased the lag period which elapsed before a steady rate of CO2 fixation was obtained. 3. The effect of Pi 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. Pi also affected the activity of ribulose-bisphosphate carboxylase, with stimulation at Pi concentrations below 2.5 mM and inhibition at higher concentrations. 4. The results showed that the sedoheptulose bisphosphatase reaction is inhibited more strongly by Pi than the fructose bisphosphatase reaction. 5. It is concluded that the previously established inhibitory effects of Pi on photosynthesis by intact isolated chloroplasts may be partly due to these inhibitory effects of Pi on the reactions of the photosynthetic carbon reduction cycle.
