Structure-function relationships in novel peptide dodecamers with broad-spectrum bactericidal and endotoxin-neutralizing activities

A series of designed peptide 33-mers (beta pep peptides) are known to be bactericidal [Mayo, Haseman, Ilyina and Gray (1998) Biochim. Biophys. Acta 1425, 81-92]. Here dodecapeptides (SC-1-SC-8), which 'walk through' the sequence of beta pep-25, were investigated for their ability to kill Gram-negative and -positive bacteria and to neutralize endotoxin. SC-4 (KLFKRHLKWKI I-NH2; the -NH2 at the right of each sequence indicates amidation of the C-terminal carboxylate group) is the most effective, more so than beta pep-25, at killing Gram-negative bacteria with nanomolar LD50 values. Against Gram-positive bacteria, SC-4 also shows good activity with submicromolar LD50 values. Leakage studies indicate rapid bacterial membrane permeability, with t(1/2) values of 10-15 min. SC-4 in the micromolar range also effectively neutralizes endotoxin and is not haemolytic below 10(-4) M. For all SC peptides, CD and NMR data indicate the presence of both 3(10)- and alpha-helix. For SC-4, nuclear-Overhauser-effect-based computational modelling yields an amphipathic helix with K1, K4, R5, and K8 arrayed on the same face (K is lysine, R is arginine). Activity differences among SC peptides and single-site variants of SC-4 allow some structure-function relationships to be deduced. Relative to other known bactericidal peptides in the linear peptide, helix-forming category, SC-4 is the most potent broad-spectrum antibacterial identified to date. The present study contributes to the development of agents involved in combating the ever-recurring problem of drug-resistant microorganisms.