Chloroalanyl Antibiotic Peptides: Antagonism of Their Antimicrobial Effects by L-Alanine and L-Alanyl Peptides in Gram-Negative Bacteria

Article abstract of DOI:10.1021/jm00160a045

A large number of structurally diverse di- and tripeptides containing the alanine racemase inactivator β-chloro-L-alanine (β-Cl-LAla) have been synthesized, and their antibacterial properties in vitro have been evaluated.The dipeptides 1, 3-6, and 8-17 and the tripeptide 20 are all broad-spectrum antibacterial agents with considerable potency against both Gram-positive and Gram-negative species, but none of the peptides improves dramatically on the antibiotic efficacy of the previously described β-Cl-LAla-β-Cl-LAla, 9 (Cheung, K.S.; Wasserman, S.A.; Dudek, E.; Lerner, S.A.; Johnston, M.J.Med.Chem. 1983, 26, 1733).Gram-negative microorganisms, such as Escherichia coli, Hemophilus influenzae, Shigella flexneri, and Enterobacter species are consistently resistant to any haloalanyl peptide containing an alanyl residue, such as the dipeptide LAla-β-Cl-LAla (2) and the tripeptides LMet-LAla-β-Cl-LAla (7), LAla-LAla-β-Cl-LAla (18), and LVal-LAla-β-Cl-LAla (19).Correspondingly, the same organisms are protected from the bactericidal effects of 9 by supplementation of the growth medium with LAla or LAla-LAla.Escherichia coli JSR-O exposed to 9, but protected from lysis by sucrose stabilization, has only about 10percent the normal level of intracellular alanine racemase activity.But when these cells are cultured in the presence of 9 with LAla supplementation, or in the presence of 2 with no supplementation, the alanine racemase levels are only about 20-30percent below control values.These findings suggest that the resistance of Gram-negative species to chloroalanyl peptides containing alanyl units arises from the ability of LAla to protect the targeted racemase from inactivation by β-Cl-LAla in vivo, an event which otherwise leads to cell death and lysis.Inactivation of alanine racemase in Gram-positive organisms appears not to be the cellular event that confers sensitivity of these species to a haloalanyl peptide.