2-(125I) iodomelatonin binding sites in rat adrenals: pharmacological characteristics and subcellular distribution.

Specific binding sites for 2-[125I] iodomelatonin, a selective radiolabeled melatonin receptor ligand, were detected and characterized in rat adrenal membranes. Saturation studies demonstrated that 2-[125I]iodomelatonin binds to a single class of sites with an affinity constant (Kd) of 541 pM and a total binding capacity (Bmax) of 3.23 fmol/mg protein. Competition experiments revealed that the relative order of potency of compounds tested was as follows: 6-chloromelatonin greater than 2-iodomelatonin greater than melatonin greater than 5-methoxytryptamine greater than 5-methoxytryptophol. The highest density of binding sites was found in membranes from nuclear (0.76 fmol/mg protein) and mitochondrial (1.82 fmol/mg protein) subcellular fractions.