Characterization of a peptide released during the reaction of human alpha 1-antitrypsin and bovine alpha-chymotrypsin.

The release of a peptide (molecular weight: about 3,600) was observed during complex formation between human alpha 1-antitrypsin (alpha 1-AT) and bovine alpha-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 alpha 1-AT with bovine trypsin or porcine elastase. The peptide released from the complex with bovine alpha-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-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 alpha 1-AT. These results suggest that the peptide was released from the C-terminal region of alpha 1-AT uon interaction with alpha-chymotrypsin. It is tempting to suggest that alpha 1-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.