Abstract
The aqueous extract of the sea anemone Halcurias sp. belonging to the suborder Endocoelantheae was found to be potently lethal to crabs, although it showed neither lethal activity in mice nor hemolytic activity. A polypeptide toxin (named halcurin) with a LD 50 of 5.8 μg/kg against crabs was isolated by gel filtration on Sephadex G-50 and reverse-phase high-performance liquid chromatography on TSKgel ODS-120T. The complete amino acid sequence of halcurin comprising 47 residues was elucidated by sequence analyses of the native molecule and its enzymatic fragment. Comparison with the known sea anemone polypeptide toxins (types 1–3), which are all from members of the suborder Nynantheae, revealed a high sequence homology (49–74%) of halcurin with type 2 toxins. Also, halcurin has several residues conserved for only type 1 toxins. These results, together with the fact that Halcurias sp. is a more primitive species than members of Nynantheae, suggest that type 1 and 2 toxins have evolved from a common ancestor with a sequence more similar to halcurin.
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