Superpotent synthetic tripeptide mimics of the mud-crab pumping pheromone

Soluble pheromones released by the mud crab, Rhithropanopeus harrisii (Gould) during egg hatching cause the female crab to contract her abdomen rapidly (the pumping response). This stereotypical behavior can be induced in the laboratory by exposing egg-bearing females to solutions containing certain Amino acids or Peptides. Twelve Amino acids exhibited response thresholds of 10(-3)-10(-10) M, the most potent (norleucine, methionine and tryptophan) having a hydrophobic sidechain. Four synthetic tripeptides of the form Gly-X-Arg, where the hydrophobic residue X was isoleucine, norleucine, methionine or phenylalanine, were superpotent agonists over a limited concentration range. Gly-Ile-Arg was a significant agonist at 10(-17) and 10(-16) M, both Gly-Nle-Arg and Gly-Met-Arg at 10(-20) and 10(-19) M, and Gly-Phe-Arg at 10(-21), 10(-20) and 10(-19) M. At the subattomolar concentration of 10(-20) M the superpotent pheromone mimics Gly-Met-Arg and Gly-Phe-Arg produced not only a statistically significant increase in the relative number of pumping mud crabs but also a substantial increase in the pumping rate ratio. In contrast, at 10(-13) M the tripeptide Gly-Met(O)-Arg with an internal residue of methionine sulfoxide blocked the abdominal pumping response of egg-bearing mud crabs.