Light-Dependent Ion Translocation in Spinach Chloroplasts

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Abstract
A light-triggered process that brings about ion translocation in spinach chloroplasts in vitro is described. Ca, phosphate and Na were shown to be taken up, using radioactive isotopes, under conditions favoring the hydrolysis of ATP. Similar to light-triggered chloroplast adenosine triphosphatase, Ca uptake could occur in the dark following a light preincubation, was stimulated by compounds containing a sul-fhydryl groups, and was maximum at pH 8. No difference was found in chemical or light requirements for adenosine triphosphatase, previously reported structural changes measured by light-scattering or ion uptake, possibly indicating a common mechanism of action. A number of analogies between ion uptake in chloroplasts and mitochondria were established both substrate and ATP supported ion uptake; Ca uptake was maximum at 3 mM ATP and other nucleoside triphos-phates were relatively ineffective; uptake reached a maximum after about 30 minutes; and both were inhibited by ADP and uncouplers of phosphorylation. In chloroplasts the stoichiometry of light-induced Ca: phosphate uptake was 0.9[plus or minus] 0.2 and the net Ca accumulation was 0.15 [mu]mole/mg chlorophyll/30 minutes. Na accumulation in chloroplasts (from a K-containing medium) was 3.3 times larger under the same conditions. Light did not result in an appreciable uptake of K42, Rb86, Mn54, ZN65, Fe59, S35O4, I131, Br82, and Cl36 in chloroplasts. It is concluded that spinach chloroplasts contain an energy-dependent, light-stimulated mechanism for the regulation of their ionic composition.

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