Uptake of 32Pi (orthophosphate) by isolated frond-tip chloroplasts of Codium fragile (Suringar) Hariot was light-stimulated, strongly inhibited by 50 μM p-chloromercuriphenyl sulphonic acid (pCMS) and showed an exponential increase with exogenous Pi concentrations above 1 mM, However, [14C] glucose-6-phosphate (G-6-P) uptake showed saturation kinetics, was not light-stimulated, was less sensitive to pCMS and was slower than Pi uptake.
Exogenous Pi stimulated the release of radioactivity from chloroplasts preloaded with [14C]G-6-P and vice versa. 3-Phosphoglycerate (PGA) readily stimulated label release from 32Pi-preloaded chloroplasts, but only slightly stimulated the release of label from [14C]G-6-P-preloaded chloroplasts. The apparent PGA-stimulated release of Pi was inhibited by 50μM pCMS but the exchange of Pi with G-6-P, as indicated by back-exchange studies, showed no consistent sensitivity to pCMS.
The presence of two separate and distinct phosphate translocators in the envelopes of C. fragile frond-tip chloroplasts is postulated: (a) one facilitating the exchange of PGA with Pi, which is relatively fast, light-stimulated and sensitive to inhibition by 50 μM pCMS; (b) another facilitating the relatively slower exchange of Pi with G-6-P which is not light-stimulated and shows little sensitivity to pCMS.