Dynorphin A (1-8) is the predominant opioid peptide identified in placental tissue extracts. Dynorphin A (1-8) is the most likely natural ligand of the kappa receptor. The binding of 3H-Bremazocine to the purified kappa receptor is inhibited by Dynorphin A (1-8) (IC50=303 nM).

Sodium Monensin, isolated from Streptomyces cinnamonensis, is a well-known representative of naturally polyether ionophore antibiotics.

Phenylalanylalanine (H-Phe-Ala-OH) is a dipeptide composed of phenylalanine and alanine. Phenylalanylalanine is an incomplete breakdown product of protein digestion or protein catabolism.