| Abstrakt: |
Melatonin has been suggested as a physiological antagonist of calmodulin. In this work, we have characterized melatonin binding sites in Xenopus laevisoocyte membranes. Binding of [125I]melatonin by X. laevisoocyte membranes fulfills all criteria for binding to a receptor site. Binding was dependent on time, temperature, and membrane concentration and was stable, reversible, saturable, and specific. The binding site was also pharmacologically characterized. Stoichiometric studies showed a high‐affinity binding site with a Kdof 1.18 nM. These data are in close agreement with data obtained from kinetic studies (Kd=0.12 nM). In competition studies, we observed a low‐affinity binding site (Kd=63.41 µM). Moreover, the binding site was characterized as calmodulin. Thus, binding was dependent on calcium and blocked by anti‐CaM antibodies in a concentration‐dependent manner. Calmodulin inhibitor chlorpromazine also inhibited binding of the tracer. From these results, it is suggested that membrane‐bound calmodulin acts as a melatonin binding site in Xenopus laevisoocytes, where it might couple cellular activities to rhythmic circulating levels of melatonin. This hypothesis correlates with the previous findings describing melatonin as a physiological antagonist of calmodulin.—Romero, M. P., Garcı´a‐Pergan˜eda, A., Guerrero, J. M., Osuna, C. Membrane‐bound calmodulin in Xenopus laevisoocytes as a novel binding site for melatonin. FASEB J.12, 1401–1408 (1998) |
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