Bioactive substances in nerve-cord extracts from an echiuroid, Urechis unicinctus--II. Peptidic substances

1. Two peptides that showed an inhibitory effect on twitch contraction of the inner circular body-wall muscle of the echiuroid Urechis unicinctus were purified from extracts of the ventral nerve cords of the worms. 2. On the basis of the results of amino acid analysis, amino acid sequence analysis and FAB-MS measurement, the primary structures of the peptides were proposed to be H-Phe-Arg-Val-Phe-OH (FRVF) and H-Phe-Arg-Phe-OH (FRF). 3. FRVF and FRF were synthesized, and their behavior on HPLC and biological activities on the body-wall muscle were confirmed to be identical to those of the native peptides. 4. FRVF was also found to have a potentiating effect on spontaneous contractions of the intestine of the African clawed toad Xenopus laevis.