Removal of dodecyl sulfate ions bound to bovine serum albumin and chymotrypsinogen from the proteins. Effects of reduction of disulfide bridges and cleavage of peptide bonds on the removal extent

The removal of dodecyl sulfate (DS) ions which had bound to proteins, bovine serum albumin (BSA) and chymotrypsinogen, was attempted by precipitating the surfactant ions by the addition of a dipotassium hydrogen and potassium dihydrogen phosphate mixture. The effects of reduction of disulfide bridges and cleavage at peptide bonds on the removal of DS ion were examined. The amount of DS ion remaining on the proteins after the removal treatment sharply depended on the potassium ion concentration up to 150 mM in each case. At high concentrations of potassium ion, the amount of the remaining DS ion was approximately 25 and 10 mol/mol for the intact BSA and the disulfide bridges-reduced BSA, respectively. The amount was approximately 35 and 10 mol/mol for the intact chymotrypsinogen and the disulfide bridges-reduced chymotrypsinogen, respectively. No appreciable amount of DS ion remained on doubly-cleaved BSA and doubly-cleaved chymotrypsinogen, that is, α-chymotrypsin (in both cases the resulting three polypeptide chains produced on cleavage are joined with disulfide bridges). On the other hand, each protein was recovered with no appreciable loss at any potassium ion concentration. The recovery of protein conformation was examined on the intact BSA by circular dichroism measurements. The secondary structure of the protein was recovered to the original one in spite of the remaining DS ion of about 25 mol/mol.