Reactivities of the cysteine residues of the reduced pancreatic trypsin inhibitor.

The six cysteine residues of the reduced pancreatic trypsin inhibitor have been found to be equally reactive toward iodoacetate under the conditions used for refolding of the protein. The rates of reaction of each residue were comparable to those observed with model thiol compounds. It is concluded that the reduced inhibitor has no stable conformational properties that affect the cysteine residues. The results corroborate the previous conclusion that all six cysteine residues participate in forming the first disulphide bond during refolding of the reduced inhibitor and confirm that disulphide bond formation is an accurate probe of the conformational transitions that occur during protein folding.