The antimicrobial and hemolytic activities of the 13-residue peptide indolicidin (ILPWKWPWWPWRR-NH2), present in bovine neutrophils, and its analogs have been determined with a view to gaining insight into the structural roles of tryptophan and proline. Peptides where proline was replaced by alanine and tryptophan by phenylalanine showed antibacterial activities comparable to that of indolicidin. The peptides do not exhibit a strong propensity to occur in either helical or β-sheet conformation. The peptides also do not appear to exert their activity by permeabilizing the bacterial plasma membrane unlike other endogenous antibacterial peptides. The presence of tryptophan appears to be essential for hemolytic activity as the phenylalanine analog does not exhibit any hemolytic activity.
