A peptidase activity from primate liver that inactivates oxytocin in vitro: purification and partial characterization

An aminopeptidase from monkey (Macaca radiata) liver, inactivating oxytocin in vitro and located predominantly in the lysosomal and microsomal fractions, was purified by chromatography on Bio-Gel HTP, DEAE-Sephacel and nickel ion chelate gel and gel filtration on Sephacryl S300. Absence of binding to nickel ion chelate gel indicated the absence of exposed histidine and thiol residues on the enzyme. The enzyme appeared to be a high molecular weight (Mr 106,000) monomeric protein. It was sensitive to inhibition by metal chelators and was found to be a zinc metalloprotein by atomic absorption spectrophotometry. Divalent metal ions Ni2+ and Co2+, and sulphydryl activators glutathione and 2-mercaptoethanol had activating effects, while 4-chloro mercuribenzoate, amino acids with large hydrophobic side chains and L-cystine, beta-lactam antibiotic cloxacillin and peptidase inhibitor amastatin had inhibitory effects on the enzyme activity. The enzyme was most active against S-benzyl L-cysteine 4-nitroanilide substrate. The properties of the enzyme were distinct from those of the well-characterized alanine and leucine aminopeptidases (EC 3.4.11.2 and EC 3.4.11.1 respectively) of liver, and of primate placental cystine aminopeptidases (EC 3.4.11.3).