Binding of endomorphin‐2 to μ‐opioid receptors in experimental mouse mammary adenocarcinoma

Abstract: Endomorphin‐2 (Tyr‐Pro‐Phe‐Phe‐NH₂) binds with high affinity and selectivity to the μ‐opioid receptor. In the present study, ¹²⁵I]endomorphin‐2 has been used to characterize μ‐opioid‐binding sites on transplantable mouse mammary adenocarcinoma cells. Cold saturation experiments performed with ¹²⁵I]endomorphin‐2 (1 nm ) show biphasic binding curves in Scatchard coordinates. One component represents high affinity and low capacity (K_d = 18.79 ± 1.13 nm , B_max = 635 ± 24 fmol/mg protein) and the other shows low affinity and higher capacity (K_d = 7.67 ± 0.81 μm , B_max = 157 ± 13 pmol/mg protein) binding sites. The rank order of agonists competing for the ¹²⁵I]endomorphin‐2 binding site was d ‐1‐Nal³]morphiceptin > endomorphin‐2 ? d ‐Phe³]morphiceptin > morphiceptin > d ‐1‐Nal³]endomorphin‐2, indicating binding of these peptides to μ‐opioid receptors. The uptake of ¹³¹I‐labeled peptides administered intraperitoneally to tumor‐bearing mice was also investigated. The highest accumulation in the tumor was observed for d ‐1‐Nal³]morphiceptin, which reached the value of 8.19 ± 1.14% dose/g tissue.