Mg2+ translocation across the thylakoid membrane: studies using the ionophore A 23187

1. The degree of the light-dependent Mg2+ efflux across the thylakoid membrane is a function of pH. There is a considerable efflux at pH 7.2 which decreases to a negligible amount at pH 8.6. This conclusion is derived from studies using the divalent cation-specific ionophore A23187. The ionophore is active as an uncoupler at pH 7.2 even in the absence of added Mg2+, and completely inactive at pH 8.6 unless Mg2+ is added. The activity was assayed as the effect on both the rate of electron transport (stimulation at pH 7.2 and inhibition at pH 8.6) and deltapH. 2. Under conditions of maximal Mg2+ efflux, pH 7.2, and in the presence of EDTA, it is shown that Mg2+ is transported across the thylakoid membrane, but it is not diluted in the medium. 3. At high pH, light-induced proton influx is not compensated by Mg2+ efflux in the presence of a relatively permeable anion. However, in the presence of an impermeable anion, where cation efflux is necessary, Mg2+ efflux occurs, indicating its preference to K+ efflux. It is suggested that although light-induced Mg2+ efflux across the thylakoid membrane is evident, its magnitude is small and it is not diluted in the stroma. Thus, it seems hard to visualize how this transport is supposed to play an important role in CO2 fixation.