Lys-γ₃-Melanotropin Binds with High Affinity to the Rat Adrenal Cortex*

Abstract

Partially purified plasma membrane preparations from the inner zones of the rat adrenal exhibit a specific and high affinity for a pro-gamma MSH peptide, synthetic rat Lys-[125I-iodo-Tyr1] gamma 3MSH, 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 Kd of 0.74 nM. However, this may understate the receptor affinity for native pro-gamma MSH(s), because the binding capacity of Lys-gamma 3MSH is impaired somewhat by iodination. Lys-[125I-iodo-Tyr1] gamma 3MSH exhibits a 100-fold higher affinity for the binding site than ACTH-(1-24), but unlike ACTH, concentrations of Lys-gamma 3MSH up to 10 microM fail to stimulate membrane-associated adenylate cyclase activity. Guanylate cyclase in this subcellular fraction also is unresponsive to Lys-gamma 3MSH. Results obtained with crude membrane fractions from other rat tissues suggest that specific pro-gamma MSH binding may not be unique to the adrenal cortex.