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Lys-₃-Melanotropin Binds with High Affinity to the Rat Adrenal Cortex^*

ROBERT C. PEDERSEN and ALEXANDER C. BROWNIE

Departments of Biochemistry and Pathology, Schools of Medicine and Dentistry, State University of New York at Buffalo Buffalo, New York 14214

Abstract

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

Footnotes

^* This work was supported by USPHS Grants AM-18141 and HL-06975.

To whom requests for reprints should be addressed.

Received July 30, 1982.

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