Leucine aminopeptidase. VII. Action on long chain polypeptides and proteins.

The availability of highly purified leucine aminopeptidase (LAP)’ from swine kidney (3) has made it possible to study the action of this enzyme on protein and polypeptide substrates. By analogy with its action on amino acid amides and small peptides, the aminopeptidase should possess no endopeptidase activity and should hydrolyze only those peptide bonds which are adjacent to a free a-amino group (4). Although the greatest rate of hydrolysis is observed with N-terminal leucine, the specificity of the enzyme is broad and not, as suggested by its name, restricted to leutine compounds. The rate of hydrolysis should be primarily a function of the nature of the side chain of the N-terminal amino acid residue. Transpeptidation and transamidation do not occur under optimal conditions for the hydrolytic reaction (5). From these considerations the aminopeptidase should be useful for the determination of N-terminal sequences of susceptible proteins and peptides. In addition, the enzyme should be a suitable reagent for the specific degradation of complex, biologically active proteins. It has already been reported that the aminopeptidase will remove a considerable part of the N-terminal sequence of mercuripapain without affecting the activity of the activated enzyme towards synthetic substrates (6). In contrast to this, it has been found that liberation of a few residues from the N-terminal