Two families of structurally related C-terminally a-amidated antimicrobial peptides have been identified in norepinephrine-stimulated skin secretions of the midwife toad Alytes obstetricans (Alytidae). The alyteserin-1 peptides (Gly-Leu-Lys-(Asp/Glu)-Ile-Phe-Lys-Ala-Gly-Leu-Gly-Ser-Leu-Val-Lys-(Gly/Asn)- Ile-Ala-Ala-His- al-Ala-(Asn/Ser).NH2) show limited structural similarity to the ascaphins from the skins
of frogs of the family Leiopelmatidae. Alyteserin-2a (Ile-Leu-Gly-Lys-Leu-Leu-Ser-Thr-Ala-Ala-Gly-Leu-
Leu-Ser-Asn-Leu.NH2) and alyteserin-2b and -2c (Ile-Leu-Gly-Ala-Ile-Leu-Pro-Leu-Val-Ser-Gly-Leu-Leu-
Ser-(Asn/Ser)-Lys-Leu.NH2) showlimited sequence identity withbombinin H6, present in the skins of frogs
of the family Bombinatoridae. The alyteserin-1 peptides show selective growth inhibitory activity against the Gram-negative bacteria Escherichia coli (MIC 25 mM) whereas alyteserin-2a is more potent against the Gram-positive bacteria Staphylococcus aureus (MIC 50 mM). The hemolytic activity against human erythrocytes of all peptides tested is relatively weak (LC50 > 100 mM). The data demonstrate that the frogs belonging to the family Alytidae are among those producing dermal antimicrobial peptides that may represent a component of the animal’s system of innate immunity.