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

Recently Plieth et al. [Protoplasma (1997) 198: 107-124; 199: 223] gav e a quantitative picture of the Ca2+ and H+ buffers in green algae whi ch we would like to comment. In that paper a mechanistic model was der ived which describes the relationship between cytosolic Ca2+ and H+ as suming that Ca2+ and H+ interact with the same binding site of a Ca2+- H+-exchange buffer. But the increase of the cytosolic free Ca2+ concen tration observed upon acidification can alternatively be described by a co-operative (n = 2) protonation of a Ca2+/H+-binding buffer pointin g to an allosteric mechanism of Ca2+ liberation. Furthermore we presen t evidences that the cytosolic buffer capacities for H+ (90 mM/pH) and Ca2+ (20 mM/pCa) given for Eremosphaera viridis were overestimated by a factor of three and three orders of magnitude, respectively.