Soymorphins, novel mu opioid peptides derived from soy beta-conglycinin beta-subunit, have anxiolytic activities.

Based on the amino acid sequence YPFV found in the soy beta-conglycinin beta-subunit, which is common to an opioid peptide human beta-casomorphin-4, peptides YPFVV, YPFVVN, and YPFVVNA were synthesized according to their primary structure. On guinea pig ileum (GPI) assay, they showed opioid activity (IC50 = 6.0, 9.2 and 13 microM respectively) more potent than human beta-casomorphins, and were named soymorphins-5, -6, and -7, respectively. Their opioid activities on mouse vas deferens (MVD) assay were less potent than on GPI assay, suggesting that they are selective for the mu opioid receptor. Human beta-casomorphin-4 and soymorphin-5 were released from the soy 7S fraction (beta-conglycinin) by the action of gastrointestinal proteases. Soymorphins-5, -6, and -7 had anxiolytic activities after oral administration at doses of 10-30 mg/kg in the elevated plus-maze test in mice.