Dual action of ionophore A23187 on intact chloroplasts

1. Ionophore A23187 induces uncoupling of potassium ferricyanide-dependent O₂ evolution by envelope-free chloroplasts and oxaloacetate-dependent O₂ evolution by intact chloroplasts. The half maximal concentration ($\text{C}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$) for stimulation of oxygen evolution in both cases is approximately 4 μM · 100 μg chlorophyll · ml^?1.2. Ionophore A23187 also induces inhibition of CO₂ and 3-phosphoglycerate-dependent O₂ evolution by intact chloroplasts in the presence of 3 mM MgCl₂. The half maximal concentrations ($\text{C}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$) for inhibition of O₂ evolution are 3 μM and 5 μM respectively · 100 μg^?1 chlorophyll · ml^?1.3. A very high concentration of ionophore A23187 (10 μM · 20 μg^?1 chlorophyll · ml^?1) plus 0.1 mM EDTA lowers the fluorescence yield of intact chloroplasts suspended in a cation-free medium in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, indicating loss of divalent cation from the diffuse double layers of the thylakoid membranes.4. These results are discussed in relation to ionophore A23187-induced divalent cation/proton exchange at both the thylakoid and the envelope membranes of intact chloroplasts.