Purification and Characterization of a Substrate-Size-Recognizing Metalloendopeptidase from Streptococcus cremoris H61.

During the ripening of Gouda-type cheese, two kinds of endopeptidases were found to participate in the degradation of alphas1-CN(f1-23), a specific product from alphas1-casein hydrolyzed by chymosin. One of the endopeptidases, lactic acid bacteria endopeptidase (LEP-II), which can recognize the size of its substrates, has already been purified and characterized (T. R. Yan, N. Azuma, S. Kaminogawa, and K. Yamauchi, Eur. J. Biochem. 163:259-265, 1987). The other endopeptidase, LEP-I, was purified to homogeneity by conventional chromatographic techniques from Streptococcus cremoris H61. The enzyme appeared to be monomeric, with an apparent molecular weight of 98,000, and its isoelectric point was 5.1. For the hydrolysis of alphas1-CN(f1-23), the enzyme had an optimum pH and temperature of 7.0 to 7.5 and 40 degrees C, respectively. Its activity was inhibited by such chelating agents as EDTA and 1,10-phenanthrolin, and it could be fully reactivated by Mn. Inhibitors specific for serine and thiol proteases had no effect on the protease activity. The enzyme showed a high affinity toward the Glu-Asn peptide bond of alphas1-CN(f1-23) and alphas1-CN(f91-100) but showed no hydrolysis activity toward alphas1-CN(f1-52), alphas1-CN(61-122), alphas1-CN(136-196), alphas1-casein, beta-casein, kappa-casein, alpha-lactalbumin, and beta-lactoglobulin. The K(m) and V(max) of LEP-I for alphas1-CN(f1-23) were 14.2 pM and 139 U, respectively.