A secreted aminopeptidase. Acts on free N-terminal amino groups with a very strong preference for Leu- followed by Met- and Ala-, and no activity on Glu- or Gly- peptides (an exhaustive analysis was not performed). Both the AP56 and AP28 forms have activity. Release of an N-terminal amino acid, Xaa-|-Yaa-, in which Xaa is preferably Leu, but may be other amino acids including Pro although not Arg or Lys, and Yaa may be Pro. Amino acid amides and methyl esters are also readily hydrolyzed, but rates on arylamides are exceedingly low. Mature protein inhibited by dithiothreitol and Zn(2+) chelators such as 1,10-phenanthroline, EDTA and EGTA. Not inhibited by serine-protease inhibitors, N-ethylmaleimide or phosphoramidon (which inhibits elastase). Optimum pH is 8.5, active between pH 7 and 9.5. Processing of the 56 kDa pro-protein to mature protein requires elastase (lasB); in vitro it only occurs at greater than 55 degrees Celsius. Several forms of the enzyme have been identified by molecular weight, exactly which part of the sequence they correspond to is not always clear. The AP58 form may be subject to proteolysis to give rise to AP56. Belongs to the peptidase M28 family. M28A subfamily.