Structure-activity relationship of rubiscolins as delta opioid peptides

To study the structure-activity relationship of rubiscolins (YPLDLF and YPLDL), delta opioid peptides derived from the spinach Rubisco, we substituted the amino acid residues and evaluated their activities by mouse vas deferens (MVD) and guinea pig ileum (GPI) assays as well as receptor affinity. Replacement of Leu(3) with Ile and Met in rubiscolin-6 potentiated the delta opioid activity by about four times in MVD assay. Asp(4) cannot be replaced by Ala, Glu or His. The original Leu(5) was optimal, while substitution of Phe(6) with Val potentiated its delta opioid activity by more than 10 times. The most potent derivative we obtained was YPMDLV, which was nearly 20 times more potent than rubiscolin-6 in MVD assay. The derivatives thus obtained showed higher delta receptor affinity and more potent antinociceptive activity than rubiscolins.