Cytosolic Ca2+ and H+ buffers in green algae: A reply

The explanation of the coupling between butyrateinduced changes in cytosolic pH and pCa by means of a H+/Ca2+ exchange buffer as proposed by Plieth et al. [Protoplasma (1997) 198: 107–124; 199: 223] was questioned by Schönknecht and Bethmann [Protoplasma (1998) 203: 206–209]. They suggested an allosteric control of binding similar to the Hill equation. Fitting the measured pH-pCa coupling shows that a distinction between the models requires data which would be outside the experimentally accessible range. Plausibility considerations provide a support for the exchange buffer. The Hill equation is just a phenomenological approach which does not account for the diversity of allosteric mechanisms. The benefits of the exchanger model are firstly its mechanistic simplicity and secondly its flexibility as the same set of equations can also be used for an allosteric interaction between H+- and Ca2+-binding sites. The model can easily be extended to include future more detailed data. Finally, the cytosolic buffer capacities for H+ and Ca2+ are discussed.