CYTOSOLIC CA2-ALGAE - A REPLY( AND H+ BUFFERS IN GREEN)

The explanation of the coupling between butyrate-induced changes in cy tosolic pH and pCa by means of a H+/Ca2+ exchange buffer as proposed b y Plieth et al; [Protoplasma (1997) 198: 107-124: 199: 223] was questi oned by Schonknecht and Bethmann [Protoplasma (1998) 203: 206-209]. Th ey suggested an allosteric control of binding similar to the Hill equa tion. Fitting the measured pH-pCa coupling shows that a distinction be tween the models requires data which would be outside the experimental ly accessible range. Plausibility considerations provide a support for the exchange buffer. The Hill equation is just a phenomenological app roach which does not account for the diversity of allosteric mechanism s. The benefits of the exchanger model are firstly its mechanistic sim plicity and secondly its flexibility as the same set of equations can also be used for an allosteric interaction between H+- and Ca2+- bindi ng sites. The model can easily be extended to include future more deta iled data. Finally, the cytosolic buffer capacities for H+ and Ca2+ ar e discussed.