SPECIFIC AFFINITY LABELING OF MU-OPIOID RECEPTORS BY S-ACTIVATED ENKEPHALIN ANALOG CONTAINING P-NITROPHENYLALANINE

p-Nitrophenylalanine (Phe(p-NO2)) was incorporated in place of Phe(4) of [D-Ala(2), Leu(CH2S-Npys)(5)]enkephalin. In the rat brain receptor binding assay, Npys-containing Phe(p-NO2)(4)- and Phe(4)-enkephalins e xhibited almost unchanged affinities for mu receptors (IC50=16-21 nM). However, when the membrane was incubated with each analog, Phe(p-NO2) (4)-enkephalin (EC(50)=6.90 nM) could label the mu receptors about thr ee times more effectively than Phe(4)-enkephalin.