Structure-activity relationship study of antimicrobial dermaseptin S4 showing the consequences of peptide oligomerization on selective cytotoxicity

To understand how peptide organization in aqueous solution might affect the activity of antimicrobial peptides, the potency of various dermaseptin S4 analogs was assessed against human red blood cells (RBC), protozoa, and sev eral Gram-negative bacteria. Dermaseptin 54 had weak antibacterial activity but potent hemolytic or antiprotozoan effects. K4K20-S4 was 2-3 fold more potent against protozoa and RBC, yet K4K20-S4 was more potent by 2 orders o f magnitude against bacteria. K-4-S4 had similar behavior as K4K20-S4, but K-20-S4 and analogous negative charge substitutions were as active as derma septin 54 or had reduced activity. Binding experiments suggested that poten cy enhancement was not the result of increased affinity to target cells. In contrast, potency correlated well with aggregation properties. Fluorescenc e studies indicated that K-20-S4 and all negative charge substitutions were as aggregated as dermaseptin S4, whereas K-4-S4 and K4K20-S4 were clearly less aggregated. Overall, the data indicated that N-terminal domain interac tion between dermaseptin 54 monomers is responsible for the peptide's oligo merization in solution and, hence, for its limited spectrum of action. More over, bell-shaped dose-response profiles obtained with bacteria but not wit h protozoa or RBC implied that aggregation can have dramatic consequences o n antibacterial activity. Based on these results, we tested the feasibility of selectivity reversal in the activity of dermaseptin 54, Tampering with the composition of the hydrophobic domains by reducing hydrophobicity or by increasing the net positive charge affected dramatically the peptide's act ivity and resulted in various analogs that displayed potent antibacterial a ctivity but reduced hemolytic activity. Among these, maximal antibacterial activity was displayed by a 15-mer version that was more potent by 2 orders of magnitude compared with native dermaseptin 54, These results emphasize the notion that peptide-based antibiotics represent a highly modular synthe tic antimicrobial system and provide indications of how the peptide's physi co-chemical properties affect potency and selectivity.