Opiorphin highly improves the specific binding and affinity of MERF and MEGY to rat brain opioid receptors

► Opiorphin a peptidase inhibitor is compared to a peptidase inhibitor cocktail. ► We performed displacement experiments of the labeled peptides at two temperatures. ► Opiorphin increased affinity and maximal binding of MERF and MEGY at 0 °C and 24 °C. ► Opiorphin use is suggested in radiolabeled binding experiments of met-enkephalins.