The Serine Protease Inhibitor Family from Ascaris suum: Chemical Determination of the Five Disulfide Bridges

Chymotrypsin/elastase inhibitor-1 is a member of the Ascaris family of serine protease inhibitors. It is characterized by five disulfide bridges in a polypeptide chain of 63 amino acids. The disulfide bridge pairing was resolved by cleavage at methionyl residues with cyanogen bromide followed by a combination of proteolytic digestions with glycyl endopeptidase, Staphylococcal serine proteinase, and submandibular proteinase A. The peptides were separated on a reverse-phase HPLC column. Amino acid analyses and N-terminal microsequencing of the cystine containing peptides revealed the disulfide bridge pairing between residues 5-54, 15-29, 18-38, 22-33, and 40-60. The disulfide bridge pairing of other members of this unique family was also assigned. The major isoform, trypsin inhibitor-1, and chymotrypsin/elastase inhibitor-4 share the same disulfide bridge pattern. These results strongly suggest that all members of the Ascaris family of serine protease inhibitors have the same disulfide bridge pattern which represents a unique motif.