2H+:1 malate2− stoichiometry during Crassulacean Acid Metabolism is unaffected by lipophilic cations

Abstract

Abstract. Lipophilic cations inhibit nocturnal malic acid accumulation in leaf cells of the Crassulacean Acid Metabolism plant Kalanchoë tubiflora. perhaps by interacting directly or indirectly with active malic acid transport into the vacuoles. Lipophilic cations do not affect passive efflux of malic acid from the vacuoles. Membrane potentials are depolarized, oxygen uptake is stimulated by lipophilic cations and there may also be stomatal responses. Thus it is striking that lipophilic cations do not alter the stoichiometry of 2 titratable H : 1 enzymatically‐determined malate2− during diurnal malic acid oscillations of Crassulacean Acid Metabolism in Kalanchoë. This suggests that coupling between protons and malate during transport into the vacuole must be tight. Transport as undissociated acid is unlikely because the dissociation equilibrium in the cytoplasm is largely on the side of malate2−. These results appear to suggest an intimate molecular interaction between a proton pump and a presumed malate2− translocator at the tonoplast of leaf cells with Crassulacean Acid Metabolism.