The interaction of alpha 1-antitrypsin with trypsin, chymotrypsin and human leukocyte elastase as revealed by end group analysis.

Complexes of alpha 1-antitrypsin with trypsin, alpha-chymotrypsin, and human leukocyte elastase were purified and examined for amino-terminal sequences. These complexes were shown to possess the expected N-terminal sequences for alpha 1-antitrypsin and the corresponding enzymes; no newly generated amino groups could be detected. Each of these three complexes was dissociated at pH 10, and the inhibitor component was isolated. When the latter was subjected to sodium dodecyl sulfate gel electrophoresis a single band was obtained in all cases, and its molecular weight was judged to be 45 000 compared to 52 000 for alpha 1-antitrypsin. Examination of the N-terminal sequence of these modified inhibitors, however, disclosed the presence of two molecular species with different N-termini. The predominant species had the N-terminal sequence previously reported for post-complex alpha 1-antitrypsin (Johnson, D. and Travis, J. (1978) J. Biol. Chem. 253, 7142-7144) and the same carboxyl sequence as alpha 1-antitrypsin. Present in lesser amounts was a species which had retained the same N-terminal sequence as alpha 1-antitrypsin, but of which the C-terminus was resistant to the action of carboxypeptidases A and B. From these results it is concluded that (1) alpha 1-antitrypsin is a double-headed inhibitor with identical but overlapping binding sites; (2) binding of the enzyme may occur at one of these two sites but not at both simultaneously, and (3) peptide cleavage does not occur as a consequence of the binding process but can be demonstrated only if the complex is dissociated.