Specific binding of 2-[125I]melatonin by partially purified membranes of rat thymus

Results strongly suggest that melatonin is involved in regulation of thymus activity and no other melatonin precursor or derivative was as potent as melatonin in inhibited the binding of [125I]MEL to membranes.

Abstract

Melatonin binding sites were characterized in partially purified rat thymus membranes. The specific binding of 2-[125I]iodomelatonin ([125I]MEL) to thymus membranes was dependent on time and temperature, stable, saturable, and reversible. Concentration-dependent binding of [125I]MEL to thymus membranes was saturable and resulted in a linear Scatchard plot, suggesting binding to a single class of binding sites. The Kd for this single site was 0.47 nM with a binding capacity of 1.01 pM. In competition studies, the specific binding of [125I]MEL to thymus membranes was inhibited by increasing concentrations of native melatonin. Scatchard analysis showed that, unlike in saturation studies with [125I]MEL, data were compatible with the existence of two classes of binding sites: a high-affinity site with a Kd of 1.72 +/- 0.25 nM and a binding capacity of 1.40 +/- 0.18 pM, and a low-affinity site with a Kd of 1226 +/- 325 nM and a binding capacity of 460 +/- 87 pM. Interestingly, Kd and BC values of the high-affinity binding site described by competition studies are similar to those obtained by saturation studies with [125I]MEL. Binding of [125I]MEL to thymus membranes was specific as indicated by the fact no other melatonin precursor or derivative was as potent as melatonin in inhibiting the binding of [125I]MEL to membranes. Results strongly suggest that melatonin is involved in regulation of thymus activity.