Lys-γ₃-melanotropin binds with high affinity to the rat adrenal cortex

Partially purified plasma membrane preparations from the inner zones of the rat adrenal exhibit a specific and high affinity for a pro-γMSH peptide, synthetic rat Lys- [¹²⁵I-iodo-Tyr¹;]γ₃MSH, that is time and temperature dependent, reversible, and saturable. Studies with demedullated adrenals indicate that at least part of this binding is to the adrenal cortex. Scatchard analysis reveals a single class of binding sites (101 fmol/mg membrane protein) for the radioactive ligand, with an apparent K_d of 0.74 nM. However, this may understate the receptor affinity for native pro-γMSH(s), because the binding capacity of Lys-γ₃MSH is impaired somewhat by iodination. Lys-[¹²⁵I-iodo-Tyr¹]γ₃MSH exhibits a 100-fold higher affinity for the binding site than ACTH-(l–24), but unlike ACTH, concentrations of Lys-γ₃MSH up to 10 μM fail to stimulate membrane-associated adenylate yclase activity. Guanylate cyclase in this subcellular fraction also is unresponsive to Lys-γ₃ MSH. Results obtained with crude membrane fractions from other rat tissues suggest that specific pro-γMSH binding may not be unique to the adrenal cortex.