Characterization of zeta (ζ): a new opioid receptor involved in growth

Endogenous opioid systems (i.e., opioids and opioid receptors) are known to play a role in neural cancer. Using [³H]-[Met⁵]enkephalin, a potent ligand involved in growth, specific and saturable binding was detected in homogenates of S20Y neuroblastoma transplanted into A/Jax mice; the data fit a single binding site. Scatchard analysis yielded a K_d of 0.49 nM and a binding capacity of 5.32 fmol/mg protein. Binding was dependent on protein concentration, time, temperature, and pH, and was sensitive to Na⁺ and guanine nucleotides. Optimal binding required protease inhibitors, and pretreatment of the tumor homogenates with trypsin markedly reduced [³H]-[Met⁵]enkephalin binding, suggesting that the binding site was proteinaceous in character. Displacement experiments indicated that [Met⁵]enkephalin was the most potent displacer of [³H]-[Met⁵]enkephalin; other ligands selective for μ, δ, κ, ε, and σ were not highly competitive. Given the functional significance of [Met⁵]enkephalin as a potent regulator of normal and abnormal growth, and that the receptor recognized by [Met⁵]enkephalin does not resemble any previously described, the present study has demonstrated the presence of a new opioid receptor termed zeta (ζ) (from the Greek 'Zoe', life) related to the proliferation of cells and tissues.