Is the rate of sulfur-disulfide exchange between the native β-Lactoglobulin and PDS related to protein conformational stability?

The sulfhydryl (SH) group of beta-lactoglobulin (beta-Lg) affects many of its functional properties. Native beta-Lg was treated with pyridine disulfide (PDS) at pH 2.6-8.5 (25 degrees C). SH-disulfide exchange was monitored by spectrophotometry. The kinetics of beta-Lg sulphydryl-disulphide exchange was compared with the same reaction for reduced glutathione (GSH). From such results we estimate, Delta G(ex), the free energy change for exposing the SH-group of beta-Lg to solvent. The numerical value for Delta G(ex) is equal to the free energy change for beta-Lg dissociation at pH 7. At neutral pH, the rate of sulphydryl-disulphide exchange appears to be controlled by the dimer-monomer dissociation equilibrium. At other solvent pHs, beta-Lg sulphydryl reactivity towards a small disulphide compound like PDS may not involve the dissociation of native beta-Lg.