Cloning and characterization of a leucyl aminopeptidase from three pathogenic Leishmania species

Abstract

Aminopeptidases are emerging as exciting novel drug targets and vaccine candidates in parasitic infections. In this study, we describe for the first time an aminopeptidase from three highly pathogenic Leishmania species. Intronless genes encoding a leucyl aminopeptidase (lap) were cloned from Leishmania amazonensis, Leishmania donovani, and Leishmania major, which encoded 60-kDa proteins that displayed homology to leucyl aminopeptidases from Gram-negative bacteria, plants, and mammals. The lap genes were present as a single copy in each genome, and lap mRNA was detected by reverse transcription-PCR in all life-cycle stages of L. amazonensis. Lap assembled into catalytically competent 360-kDa hexamers and demonstrated potent amidolytic activity against synthetic aminopeptidase substrates containing leucine, methionine, and cysteine residues, representing the most restricted substrate specificity of any leucyl aminopeptidase described to date. Optimal activity was observed against L-leucyl-7-amido-4-methylcoumarin (kcat/Km ≈ 63 s-1·mM-1) with a pH optimum of 8.5. Leishmania Lap activity was inhibited by metal ion chelators and enhanced by divalent manganese, cobalt, and nickel cations, although only zinc was detected in the purified Lap by inductively coupled plasma atomic emission spectroscopy, indicating that zinc is the natural Lap cofactor. Activity was potently inhibited by bestatin and apstatin in a slow binding competitive fashion, with Ki* values of 3 and 44 nM, respectively. Actinonin was a tight binding competitive inhibitor (Ki ≈ 1 nM), whereas arphamenine A (Ki ≈ 70 μM) and Lleucinol (Ki ≈ 100 μM) were non-tight binding competitive inhibitors. Lap was not secreted by Leishmania in vitro and was localized to the parasite cytosol.