TRANSLOCATION OF ORTHOPHOSPHATE AND GLUCOSE‐6‐phosphate IN CODIUM FRAGILE CHLOROPLASTS

Abstract

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. Copyright © 1983, Wiley Blackwell. All rights reserved