Antimicrobial and cytolytic properties of the frog skin peptide, kassinatuerin-1 and its L- and D-lysine-substituted derivatives

Kassinatuerin-1, a 21-amino-acid C-terminally α-amidated peptide first isolated from the skin of the African frog Kassina senegalensis, adopts an amphipathic α-helical conformation in a membrane-mimetic solvent (50% trifluoroethanol) and shows broad-spectrum antimicrobial activity. However, its therapeutic potential is limited by its relatively high cytolytic activity against mammalian cells. The antimicrobial and cytolytic properties of a peptide are determined by an interaction between cationicity, hydrophobicity, α-helicity and amphipathicity. Replacement of the C-terminal α-amide group in kassinatuerin-1 by carboxylic acid decreased both cationicity and α-helicity, resulting in an analog with decreased potency against Escherichia coli (4-fold) and Staphylococcus aureus (16-fold). Low cytolytic activities against human erythrocytes (LD50 > 400 μM) and L929 fibroblasts (LD50 = 105 μM) were also observed. Increasing cationicity, while maintaining amphipathic α-helical character, by progressively substituting Gly⁷, Ser¹⁸, and Asp¹⁹ on the hydrophilic face of the α-helix with L-lysine, increased antimicrobial potency against S. aureus and Candida albicans (up to 4-fold) but also increased hemolytic and cytolytic activities. In contrast, analogs with D-lysine at positions 7, 18 and 19 retained activity against Gram-negative bacteria but displayed reduced hemolytic and cytolytic activities. For example, the carboxylic acid derivative of [D-Lys⁷, D-Lys¹⁸, D-Lys¹⁹]kassinatuerin-1 was active (minimum inhibitory concentration (MIC) = 6-12.5 μM) against a range of strongly antibiotic-resistant strains of E. coli but showed no detectable hemolytic activity at 400 μM and was 4-fold less cytolyic than kassinatuerin-1. However, the reduction in α-helicity produced by the d-amino acid substitutions resulted in analogs with reduced potencies against Gram-positive bacteria and against C. albicans.