Characterization of new iodinated peptide radioligand for delta opioid receptors

New radioligand [4'-¹²⁵I-[Phe⁴]DPDPE ([¹²⁵I] DPDPE) was prepared from [4'-NH₂-Phe⁴] DPDPE. Its binding characteristics were determined using rat brain membranes. Saturation studies showed one binding site with K(d) = 421 ± 67 pM and B(max) = 36.4 ± 2.7 fmol/mg protein. Kinetic studies showed association rate constant k₊₁ = 5.80 ± 0.88 x 10⁷ M⁻¹ min⁻¹ and dissociation rate constant k-1 = 0.917 ± 0.117 x 10⁻² min⁻¹. Competitive inhibition studies showed high selectivity for delta opioid receptors. Distribution of [¹²⁵I] DPDPE labeled sites in rat brain (autoradiography) is consistent with previous results using other radioligands. The inhibition constants of [¹²⁵I] DPDPE binding by delta selective ligands were similar in mouse brain and mouse vas deferens, and no significantly different values were determined in rat brain. These data indicate that [¹²⁵I] DPDPE due to high specific activity is valuable for characterization of delta opioid receptors