A radiolabelled form of deltorphin II was synthesized by catalytic tritiation using [p-IPhe3]-deltorphin II as a precursor. The ligand labels rat brain membranes with a Kd value of 1.9 nM, and the Bmax was found to be 92 fmol/mg protein. This new tritiated ligand exhibits high affinity for the delta opioid binding site, whereas its binding to the mu type is weak and extremely low for the kappa type. Mu/delta and kappa/delta selectivity ratios were about 900 and 10000, respectively. The highly delta selective binding properties of this new radioligand suggest that it could serve as an excellent tool for investigating the delta opioid receptors in various species.
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