Purification and Some Properties of a Membrane-Bound Aminopeptidase A from Streptococcus cremoris

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Abstract
A membrane-bound L-.alpha.-glutamyl (aspartyl)-peptide hydrolase (aminopeptidase A) (EC 3.4.11.7) from Streptococcus cremoris HP has been purified to homogeneity. The free .gamma.-carboxyl group rather than the amino group of the N-terminal L-.alpha.-glutamyl (aspartyl) residue appeared to be essential for catalysis. No endopeptidase activity could be established with this enzyme. The native enzyme is a polymeric, most probably trimeric, metalloenzyme (relative molecular weight, approximately 130,000) which shows on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels apparent high relative molecular weight values due to (lipid?) material dissociable with butanol. The subunit (relative molecular weight, approximately 43,000) is catalytically inactive. The enzyme is inactivated completely by dithiothreitol, chelating agents, and the bivalent metal ions Cu2+ and Hg2+. Of the sulfhydryl-blocking reagents tested, only p-hydroxymercuribenzoate appeared to inhibit the enzyme. Activity lost by treatment with a chelating agent could be restored by Co2+ and Zn2+. The importance of the occurrence of an aminopeptidase A in S. cremoris with respect to growth in milk is discussed.

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