The regulation of starch metabolism by inorganic phosphate

The effects of orthophosphate (Pi) on starch synthesis and degradation in the isolated intact spinach chloroplast are investigated. During CO 2 fixation in the light increasing levels of Pi (0–1.0 mM), which have little effect on the rate of fixation, prolong the lag of starch synthesis and reduce its maximal rate from 14.5 to 6.8 μg-atom C incorporated into starch per milligram chlorophyll per hour. The label in dihydroxyacetone phosphate and other sugar phosphates, especially sedoheptulose-1,7-bis-phosphate, increases up to 10-fold under these conditions. Starch degradation in the dark is monitored in the chloroplast by following the breakdown of the 14C labeled starch. Pi concentrations in the order of 2.5–5.0 mM are optimal for starch mobilization at a rate of about 1 μg-atom C per milligram chlorophyll per hour.