Isolation of peptides of the brevinin-1 family with potent candidacidal activity from the skin secretions of the frog Rana boylii

The emergence of strains of the human pathogen Candida albicans with resistance to commonly used antibiotics has necessitated a search for new types of antifungal agents. Six peptides with antimicrobial activity were isolated from norepinephrine-stimulated skin secretions from the foothill yellow-legged frog Rana boylii. Brevinin-1BYa (FLPILASLAA¹⁰KFGPKLF CLV ²⁰TKKC) was particularly potent against C. albicans [minimal inhibitory concentration (MIC) = 3 μM] and also active against Escherichia coli (MIC = 17 μM) and Staphylococcus aureus (MIC = 2 μM), but its therapeutic potential for systemic use is limited by its strong hemolytic activity (HC₅₀ = 4 μM). The single amino acid substitution (Phe¹² → Leu) in brevinin-1BYb resulted in a fourfold lower potency against C. albicans and the additional amino acid substitutions (Lys¹¹ → Thr, Phe¹⁷ → Leu and Val²⁰ → IIe) in brevinin-1BYc resulted in a ninefold decrease in activity. Two members of the ranatuerin-2 family and one member of the temporin family were also isolated from the secretions but showed relatively low potency against the three microorganisms tested.