The y-D-glutamyl-(L)meso-diaminopimelate endopeptidase, or endopeptidase I, from Bacillus sphaericus 9602 was purified to apparent protein homogeneity. The purification was achieved by a six-step procedure: ammonium sulfate fractionation, phenyl-Sepharose chromatography, two consecutive DEAE-Trisacryl chromatographies, chromatofocusing and Sephacryl S-200 permeation chromatography. The enzyme was purified 5000-fold with a 38% recovery of lytic activity. It is an acidic protein (PI 5.4) of hydrophobic nature. Kinetic studies have shown a K , value of 0.57 mM and an apparent V,,, of 8.3 pmol min-' (mg enzyme)-' with N-acetylmuramyl-~-alanyl-y-~-glutamyl-(~)meso-diaminopimelyl(~)-~-[ ''C]alanine as substrate. The enzyme was inhibited by o-phenanthroline and EDTA and was reactivated by zinc, cobalt and manganese ions; thus endopeptidase I is a metallo enzyme, probably a zinc enzyme. Moreover it is a heat-stable protein with an apparent inactivation temperature of 8 0 T . , in which the meso-diaminopimelic acid residues have free o-NH2 and o-COOH groups [6]: It acts on monomeric substrates (peptides and MurNAc-glycopeptides or GlcNAc-MurNAc-glycopeptides) [6] and also on the non-cross-linked peptide chains of the peptidoglycans [7]. The enzymic activity is closely related to sporulation [8]. Apart from its unique action on y-D-glutamyl linkages, the interest in endopeptidase I is due to two of its uses: these are firstly the location of the amide group on the meso-diaminopimelic acid or on the D-glutamic acid residues in monoamidated tetrapeptides of bacterial cell wall peptidoglycans [9] and secondly the preparation of an immunostimulant, GlcNAc-MurNAc-L-Ala-D-Glu-NH2 [lo]. Two forms of the enzyme have been identified: an extracellular one in the sporulation medium [I 11 and a membrane-bound one in the integuments of the forespores and of the spores 131. Whether the soluble and membrane-bound enzymes are different entities or only one enzyme with different localization has not been demonstrated. To elucidate this, we report in this paper the purification and the partial characterization of the extracellular endopeptidase I in order to allow comparison with the membrane-bound enzyme.Correspondence to M. Guinand, Laboratoire de Biochimie Microbienne, Universite Claude Bernard, Lyon I, 43 Boulevard du Onze Novembre 191 8, F-69622 Villeurbanne Cedex, FranceAbbreviations. ms-A2pm, meso-diaminopimelic acid, Glu-NH2, isoglutamine, MurNAc, N-acetylmuramic acid; GlcNAc, N-acetylglucosamine; SDS, sodium dodecyl sulfate; PAGE, polyacrylamide gel electrophoresis.A part of this study, concerning the electrophoretic behavior of the purified endopeptidase I in denaturing and non-denaturing conditions, has been reported recently [ 121.
MATERIALS AND METHODSCrude extracellular endopeptidase I Bacillus sphaericus NCTC 9602 was grown as previously described [13] and the crude extracellular enzyme was obtained as recommended in [14]. Briefly cells of a B. sphaericus culture (67 1) were harvested at the beginning of sporulation (2 -5% ...