Effect of N-ethylmaleimide treatment on naloxone binding in frog brain membranes.

Specific binding of (3H) naloxone to opioid receptors in frog (Rana esculenta) brain membranes was irreversibly inactivated by the sulfhydryl group alkylating agent N-ethylmaleimide (NEM). Saturation analysis of (3H) naloxone binding revealed a marked reduction in the number of ligand binding sites after N-ethylmaleimide treatment. Pretreatment of the membranes with unlabelled opioid ligands, i.e. naloxone, morphine, or the kappa selective dynorphin (1-13), and sodium ions resulted in considerable protection of (3H) naloxone binding against the N-ethylmaleimide blockade.