LIMITED TRYPTIC DIGESTION NEAR THE AMINO-TERMINUS OF BOVINE LIVER RHODANESE PRODUCES ACTIVE ELECTROPHORETIC VARIANTS WITH ALTERED REFOLDING

When the enzyme rhodanese was partially digested by immobilized trypsi n, it retained greater than 50% of its original activity although less than 10% of the undigested enzyme remained. The predominant daughter species were two 31-kDa polypeptides whose amino termini corresponded to either residue 44 or 45 of the enzyme's sequence. Following digesti on, charged species were isolated by ion exchange chromatography. Dena turing electrophoresis revealed that a 4-kDa peptide remained associat ed with the 31-kDa fragment. This 4-kDa peptide appears to correspond to the amino-terminal 45 residues of rhodanese. Further proteolysis ga ve a 2.5-kDa peptide that dissociated under non-denaturing conditions without apparent change in migration of the 31-kDa fragment on SDS gel s. Refolding of undigested, urea-denatured rhodanese restored much of its activity. Similar treatment of rhodanese following limited tryptic digestion resulted in no regain of activity. Refolding of a mixture o f intact and digested rhodanese resulted in regain of activity appropr iate for the amount of intact rhodanese in the sample, indicating that clipped rhodanese does not inhibit refolding of intact rhodanese. It is concluded that portions of the amino terminus of rhodanese are impo rtant in the enzyme's folding, but are not essential for the enzyme's sulfurtransferase activity.