Indolicidin, a novel bactericidal tridecapeptide amide from neutrophils

A potent and structurally novel antimicrobial peptide was purified from the cytoplasmic granules of bovine neutrophils. Suspensions of Staphylococcus aureus and Escherichia coli were virtually sterilized by the peptide at a concentration of 10 μg/ml. The peptide was found to be comprised of 13 amino acids, 5 of which were tryptophan residues, and the carboxylterminal arginine was carboxamidated. The primary structure of the peptide, which we have named indolicidin, is H-Ile-Leu-Pro-Trp-Lys-Trp-Pro-Trp-Trp-Pro-Trp-Arg-Arg-NH₂. The mole percent of tryptophan in indolicidin is the highest observed among known protein sequences. The multiple tryptophan residues presumably play an important role in the function of this unique antibiotic peptide.