Determination of DeltapH in Chloroplasts. 3. Ammonium Uptake as a Measure of DeltapH in Chloroplasts and Sub-Chloroplast Particles

A new method for the determination of ΔpH in chloroplasts and related particles was developed. It is based on measurement, with a cationic electrode of the light-induced uptake of NH4+ in low external concentrations. This, together with a separate determination of the osmotic space by the use of [14C]sorbitol and 3H2O allow the calculation of the internal NH4+ concentration, and the ΔpH. The measurement of ΔpH in various conditions is in general agreement with the values obtained by the [14C]methylamine-distribution method [1,2] and essentially identical to those obtained by the quenching of fluorescent amines [3]. In sub-chloroplast particles the light-induced ΔpH determined by this method was only somewhat lower than in chloroplasts. Furthermore, inhibition of phosphorylation by methylamine in these particles was parallel to that of the ΔpH.