Conformational and hydrophobic properties of rat and bovine S-100 proteins

The binding of Ca2+ to rat or bovine S-100 proteins, in the absence of ligands, showed a dissociation constant (in 60 mM K+) of 0.5 to 1.0 mM as measured by the effects of Ca2+ on binding of S-100 to phenyl-Sepharose, reactivity of sulfhydryl groups, and difference spectra for PHE, TYR, and TRP residues. Binding of the ligands, "Stainsall" and chlorpromazine lowered the dissociation constant of S-100 for Ca2+ by 2- to 10-fold as measured by the same parameters. The conformational change, in response to Ca2+ binding, probably occurs by exposure to solvent of the hydrophobic region of alpha and beta subunits of S-100 at residue positions 74-93.