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

Endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) binds with high affinity and selectivity to the mu-opioid receptor. In the present study, [I-125]endomorphin-2 has been used to characterize mu-opioid-binding sites on transplantable mouse mammary adenocarcinoma cells. Cold saturation experiments performed with [I-125]endomorphin-2 (1 n(M)) show biphasic binding curves in Scatchard coordinates. One component represents high affinity and low capacity (K-d = 18.79 +/- 1.13 n(M), B-max = 635 +/- 24 fmol/mg protein) and the other shows low affinity and higher capacity (K-d = 7.67 +/- 0.81 mu(M), B-max = 157 +/- 13 pmol/mg protein) binding sites. The rank order of agonists competing for the [I-125]endomorphin-2 binding site was [D-1-Nal(3)]morphiceptin > endomorphin-2 > [D-Phe(3)]morphiceptin > morphiceptin > [D-1-Nal(3)]endomorphin-2, indicating binding of these peptides to mu-opioid receptors. The uptake of I-131-labeled peptides administered intraperitoneally to tumor-bearing mice was also investigated. The highest accumulation in the tumor was observed for [D-1-Nal(3)]morphiceptin, which reached the value of 8.19 +/- 1.14% dose/g tissue.