Pep 5 and
nisin are cationic
peptide antibiotics which in addition to their membrane-disruptive action induce
autolysis in staphylococci. To investigate the mechanism of lysis induction, the influence of the
peptides on the activity of the
N-acetylmuramoyl-L-alanine amidase of Staphylococcus simulans 22 was studied. In experiments with isolated cell walls at low ionic strength, the
amidase activity was stimulated by the addition of
Pep 5 and
nisin, as well as by
polylysine,
streptomycin, and mono- and
divalent cations. The concentrations necessary for activation depended on the nature of the
cation and ranged from 5 microM for poly-
L-lysine (n = 17) to 150 mM for Na+ at a cell wall concentration of 100 micrograms of cell walls per ml. No effect was observed if the cell walls were devoid of polyanionic constituents. Kinetic data suggested that the
amidase bound to the teichoic and teichuronic
acids of the cell wall and was thereby inhibited. Cationic molecules reversed this inhibition, most likely by displacing the
enzyme from the
polyanions. If the concentrations of the larger
peptides were high in relation to cell wall concentration, the activation turned into inhibition, presumably by interfering with the access of the
enzyme to its substrate. These experiments demonstrate that the activity of the
amidase is modulated by basic
peptides in vitro and help to explain how
Pep 5 and
nisin may cause lysis of treated cells.