Opioid binding properties of the purified kappa receptor from human placenta

A glycoprotein with a molecular weight of 63,000 has been purified, in an active form, from human placental villus tissue membranes. The binding properties of this glycoprotein to opioid alkaloids and peptides indicates that it is the kappa opiate receptor of human placenta. The receptor binds the tritiated ligands etorphine, bremazocine, ethylketocyclazocine and naloxone specifically and reversibly with Kd values of 3.3, 4.4, 5.1 and 7.0nM, respectively. The binding of ³H-Bremazocine to the purified receptor is inhibited by the following compounds with the corresponding Ki values EKC, 1.3×10^-8M; Dynorphin 1-8, 3.03×10^-7; U50,488H, 4.48×10^-9; U69-593, 2.28×10^-8, morphine, 4.05×10^-6 DADLE, 6.47×10^-6 and naloxone, 2.64×10^-8. The purified receptor binds 8 nmole of ³H-Etorphine and 1.7 nmole ³H-BZC per mg protein. The theoretical binding capacity of a protein of this molecular weight is 15.8. Although the iodinated purified receptor appears by autoradiography as one band on SDS-PAGE, yet homogeneity of the preparation is not claimed.