Purification and Characterization of a Novel Serine Aminopeptidase from Lactobacillus casei Ssp. casei IFPL 731

An aminopeptidase showing broad specificity has been purified to homogeneity from the cell-free extract of Lactobacillus casei ssp. casei IFPL 731. Enzyme activity was inhibited by the serine protease inhibitor, phenylmethanesulfonyl fluoride, and reducing agents such as dithiothreitol and β-mercaptoethanol. The metal chelating agent, ethylenediamintetraacetic acid, also reduced enzyme activity. The molecular mass of the purified enzyme was estimated to be 67 kDa by gel filtration and sodium dodecyl sulfate−polyacrylamide gel electrophoresis, indicating that the enzyme exits as a monomer. The purified enzyme hydrolyzed p-nitroanilides of several amino acids and peptides as well as di- and tripeptides. The best substrates were Arg-Pro-p-nitroanilide, Ala-Pro-p-nitroanilide, Phe-Met, Leu-Gly, Phe-Ala, and Leu-Gly-Phe. K m values for Arg-Pro-p-nitroanilide and Leu-Gly were 4.8 and 1.1 mM, respectively. The properties of the enzyme are compared with those of other aminopeptidases isolated from lactic acid bacteria. Keywords: Enzyme purification; aminopeptidase; mesophilic lactic acid bacteria; Lactobacillus casei