3‐[(3‐Cholamidopropyl)dimethylammonio]‐1‐Propane Sulfonate‐Solubilized Binding Sites for 2‐[125I]Iodomelatonin in Chick Brain Retain Sensitivity to Guanine Nucleotides
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abstract
Abstract: Binding of 2‐[125I]iodomelatonin to 3‐[(3‐chol‐amidopropyl)dimethylammonio]‐1‐propane sulfonate (CHAPS)‐solubilized sites from chick forebrain was rapid, reversible, saturable, of high affinity, and of pharmacological selectivity. Scatchard analyses showed that 2‐(125I]iodomelatonin binds to a single site with equilibrium dissociation constant (KD) values of 328 ± 22 (n = 4) and 302 ± 26 pM (n = 3) and a maximal number of binding sites (Bmax) of 36.2 ± 2.0 and 49.5 ± 6.6 fmol/mg of protein in solubilized and membrane fractions, respectively. The KD values obtained from the ratio of kinetic constants (k2/k1) in solubilized and membrane preparations were 228 and 216 pM, respectively. Inhibition studies indicated the following order of pharmacological affinities for both membrane and solubilized sites: 2‐iodomelatonin > melatonin > 6‐chloromelatonin ± prazosin > N‐acetylserotonin ± serotonin > metergoline > ketanserin > propranolol > phentolamine > cyproheptadine. Guanyl nucleotides inhibited binding of 2‐[125I]iodomelatonin to solubilized and membrane fractions, by converting binding sites from a high‐affinity to a lowaffinity state. These findings show that solubilized binding sites for melatonin exhibit the specific binding and pharmacological characteristics present in membrane‐bound sites. Moreover, the retention of sensitivity to guanine nucleotides in fractions solubilized with CHAPS suggests that this solubilization procedure is suitable for further studies aimed at the isolation, purification, and molecular characterization of active melatonin binding sites.
