Biochemical characterization of "LAP," a polymorphic aminopeptidase from the blue mussel, Mytilus edulis

A genetically variable naphthylamidase enzyme, previously described as "leucine aminopeptidase," was purified approximately fiftyfold, and its biochemical properties were investigated. The enzyme was renamed "aminopeptidase I." Substrate affinities demonstrate that it is an alpha-aminoacyl peptide hydrolase (E.C. 3.4.11.-). Aminopeptidase I had a monomer molecular weight of 65--68,000, average of pI of pH 4.88, and broad pH optima between 6.5 and 8.0. The enzyme was inactivated rapidly between 40 and 50 C. Antibodies from purified enzyme did not cross-react with other naphthylamidases, but aminopeptidase I activity was inhibited by immune serum. The enzyme exhibited highest naphthylamidase activity for aromatic and hydrophobic aminoacyl naphthylamides. Aminopeptidase activity was highest for aromatic and hydrophobic N-terminal residues of tripeptides. Certain divalent metal cations, p-OH-mercuribenzoate, and N-ethylmaleimide were strongly inhibitory while chelating agents activated the enzyme.