Characterization of a Peptide Released during the Reaction of Human β1–Antitrypsin and Bovine β-Chymotrypsin1

Abstract

The release of a peptide (molecular weight: about 3, 600) was observed during complex formation between human ai-antitrypsin (α₁-AT) and bovine α-chymotrypsin, when monitored by gel-electrophoresis in the presence of sodium lauryl sulfate. Release of the peptide was proportional to the extent of complex formation. Peptides of the same molecular weight were also released during the complex formation of α₁-AT with bovine trypsin or porcine elastase. The peptide released from the complex with bovine a-chymotrypsin was composed of 32 amino acid residues, which did not correspond to the composition of any 32 amino acid segment in the bovine alpha;-chymotrypsin sequence. The N- and C-terminal sequences of the peptide were determined to be H-(Ser)-Ile-Pro-Pro-Glu- and -Gln-Lys-OH, respectively. Though there was some uncertainty as to the N-terminal sequence, it is quite different from that of the original alpha;₁-AT molecule, and showed a similarity to the sequences of the leaving group sides of the reactive sites in some legume proteinase inhibitors. The C-terminal 2 residues were identical with those of native α₁i-AT. These results suggest that the peptide was released from the C-terminal region of α₁AT upon interaction with α-chymotrypsin. It is tempting to suggest that ai-AT inhibits a serine proteinase by the acyl enzyme mechanism at a residue adjacent to the amino group of the N-terminus of this peptide and that this peptide is liberated as a leaving group in the enzymic process.