Evaluation of a sulphydryl–disulphide exchange index (SEI) for whey proteins—β-lactoglobulin and bovine serum albumin

Sulphydryl–disulphide (SH–SS) exchange underlies many protein functions in processed foods. There is a need for reliable indicators of SH–SS exchange capacity in protein ingredients. An index for SH–SS exchange (SEI) is described using β-lactoglobulin (BLG) or bovine serum albumin (BSA). The proteins were reacted with 2-pyridine disulphide (PDS Aldrithiol-2TM) or Ellmanʹs reagent (DTNB) in a buffered medium (0.05 M phosphate buffer, pH 6.9) at 25 °C. The 2nd order rate constant for protein SH–SS exchange with PDS or DTNB (k, M−1 s−1) was normalized by dividing by the rate constant for glutathione (GSH) reaction with PDS or DTNB (k*, M−1 s−1) determined under identical conditions. The capacity or SH–SS exchange was inversely related to SEI (=−log k/k*) values of 4.11 for BLG and 1.05 for BSA, based on measurements using PDS. Studies using DTNB yield SEI equal to 4.28 for BLG and 2.20 for BSA. The SH group of BSA was 100–1052 times more reactive than the SH group of BLG. In a medium containing 1.2 M sodium chloride the difference in SH–SS exchange capacity was ∼9000 fold. Differential scanning calorimetry (DSC) results showed that the conformational stability of BLG increased much more substantially than BSA when both proteins were exposed to 0.1–1.2 M sodium chloride concentrations.