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An aminopeptidase was purified to homogeneity from a crude cell extract ofLactococcus lactis subsp.cremoris Wg2 by a procedure that included diethyl-aminoethane-Sephacel chromatography, phenyl-Sepharose chromatography, gel filtration, and high-performance liquid chromatography over an anion-exchange column. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified enzyme showed a single protein band with a molecular weight of 95,000. The aminopeptidase was capable of degrading several peptides by hydrolysis of the N-terminal amino acid. The peptidase had no endopeptidase or carboxypeptidase activity. The aminopeptidase activity was optimal at pH 7 and 40C. The enzyme was completely inactivated by thep -chloromecuribenzoate mersalyl, chelating agents, and the divalent cations Cu2+ and Cd2+ . The activity that was lost by treatment with the sulfhydryl-blocking reagents was restored with dithiothreitol or β-mercapto-ethanol, while Zn2+ or Co2+ restored the activity of the 1,10-phenantroline-treated enzyme. Kinetic studies indicated that the enzyme has a relatively low affinity for lysyl-p -nitroanilide (Km , 0.55 mM) but that it can hydrolyze this substrate at a high rate (V max , 30 μmol/min per mg of protein).

Purification and Characterization of an Aminopeptidase from Lactococcus lactis subsp. cremoris Wg2