Characterization of leukotriene C4 binding sites in the brain of the American bullfrog,Rana catesbeiana

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Abstract
In this study, specific binding sites for [3H]‐LTC4 on membrane preparations from American bullfrog (Rana catesbeiana) brain were characterized. Binding assays were done in the presence of serine (5mM) borate (10 mM) for 30 min at 23°C. Under these conditions, no metabolism of LTC4 to LTD4 occurred. Specific binding of [3H]‐LTC4 reached steady state within 10 min, remained constant for 60 min, and was reversible with the addition of 1,000‐fold excess unlabelled LTC4. Scatchard analysis of the binding data indicated a single class of binding sites with an estimated Kd of 89.83 nM and Bmax of 43.79 pmol/mg protein. Competition binding studies demonstrated that LTD4 and LTE4 were ineffective in displacing [3H]‐LTC4 from its binding site. The Ki for LTC4 was 51 nM. S‐decylglutathione, glutathione and hematin had Ki values of 44, 312,602, and 25,576 nM, respectively. The mammalian cysteinyl leukotriene antagonist L‐660,711 inhibited specific binding of [3H]‐LTC4, with a Ki of 87,149 nM. Guanosine‐5′‐0‐3‐thiotriphosphate (GTPγS) did not affect specific binding of [3H]‐LTC4 indicating that, like mammalian LTC4 receptors, a Gi protein is not involved in the transduction mechanism. The LTC4 binding site in bullfrog brain demonstrates both similarities and differences from its mammalian counterpart.