Purification and characterization of an aminopeptidase from Plasmodium falciparum

A soluble aminopeptidase from Plasmodium falciparum was purified by high performance liquid chromatography. The enzyme has a molecular weight of 100 000 and pI 6.8. Activity can be monitored conveniently with L-alanine-p-nitroanilide or L-leucine-p-nitroanilide at 405 nm or with L-leucine-7-amido-4-methylcoumarin in a fluorescence assay. The enzyme is inhibited by bestatin and phosphoramidone but not by leupeptin, chymostatin, antipain or pepstatin. pH-rate studies indicated the presence of a group on the free enzyme, pKa = 6.6, which must be in the conjugate base form for activity. The aminopeptidase has an essential sulfhydryl group at the active site which is rapidly modified by Hg2+ or Zn2+, is slowly modified by p-hydroxymercuribenzoate, but is not accessible to iodoacetamide or N-ethylmaleimide. The aminopeptidase is inhibited noncompetitively by chloroquine, mefloquine and quinacrine (Ki = 410, 280 and 20 microM, respectively) but is not inhibited by quinine or primaquine. Hemin does not inhibit. Complexation of hemin with quinacrine prevents inhibition by quinacrine.