Requirement of divalent cations for photoactivation of the laten water-oxidation system in intact chloroplasts from flashed leaves

Abstract

The effects of divalent cations on photoactivation of the latent water-oxidation system in intact chloroplasts isolated from wheat ( Triticum aestivum L.) leaves grown under intermittent flash illumination were investigated by using A23187, an ionophore for divalent cations, and the following results were obtained. (a) Photoactivation in the intact chloroplasts was inhibited by A23187, but was restored on addition of a low concentration of Mn 2+ (10 μM). (b) A high concentration of Mn 2+ (70 μM) was inhibitory, in contrast, for photoactivation, but the inhibition was restored by the coexistence of a suitable concentration of Ca 2+ (5 mM). (c) The Ca 2+-dependent restoration was inhibited by a high concentration of Mg 2+ or Sr 2+, but the inhibition was restored by the coexistence of Ca 2+. (d) Kinetic analyses of these competitive effects between divalent cations revealed that: (i) High concentration of Ca 2+ inhibits photoactivation in competition with Mn 2+. (ii) High concentration of Mn 2+ inhibits photoactivation in competition with Ca 2+. (iii) High concentration of Mg 2+ affects photoactivation by inhibiting Ca 2+-dependent restoration in competition with Ca 2+. Based on these results, we propose that the latent water-oxidation center has two binding sites, each specific for Mn 2+ and Ca 2+, and that photoactivation takes place in the center having both Mn 2+ and Ca 2+ on their respective binding sites.