Specific binding of 2-[125I]melatonin by partially purified membranes of rat thymus.
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.
Lopez-Gonzalez
M A
MA
University of Seville School of Medicine, Department of Medical Biochemistry and Molecular Biology, Spain.
Martin-Cacao
A
A
Calvo
J R
JR
Reiter
R J
RJ
Osuna
C
C
Guerrero
J M
JM
D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D007211:Indoles / Q000494:pharmacology; D007457:Iodine Radioisotopes; D008297:Male; D008550:Melatonin / Q000037:antagonists & inhibitors / Q000378:metabolism*; D008566:Membranes / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D013696:Temperature; D013950:Thymus Gland / Q000378:metabolism*; D013997:Time Factors
D007211:Indoles; D007457:Iodine Radioisotopes; D008550:Melatonin