In response to epidermal injury, Pavasilurus asotus, a catfish, secreted a strong antimicrobial peptide into the epithelial mucosal layer. The molecular mass of the antimicrobial peptide, named parasin I, was 2000.4 Die, as determined by matrix-associated laser desorption ionization mass spectrometry. The complete amino acid sequence of parasin I, which was determined by automated Edman degradation, was Lys-Gly-Arg-Gly-Lys-Gln-Gly-Gly-Lys-Val-Arg-Ala-Lys-Ala-Lys-Thr-Arg-Ser-Ser. Eighteen of the 19 residues ia parasin I sere identical to the N-terminal of buforin I, ct 39-residue antimicrobial peptide derived from the N-terminal of toad histone H2A [Kim et al, (1996) Biochem. Biophys. Res. Commun. 229, 381-387], which implies that parasin I was cleaved off from the N-terminal of catfish histone H2A, Parasin I showed strong antimicrobial activity, about 12-100 times more potent than magainin 2, against a wide spectrum of microorganisms, without any hemolytic activity, Circular dichroism spectra of parasin I indicated a structural content of 11% alpha-helix, 33% beta-sheet, and 56% random coils. The beta-sheet axial projection diagram of parasin I showed an amphipathic structure. Our results indicate that the catfish may produce parasin I from its histone H2A by a specific protease upon injury to protect against invasion by microorganisms. (C) 1998 Federation of European Biochemical Societies.