title: High-affinity binding of melatonin to hemoglobin.
abstract: Determination of melatonin by radioimmunoassay in plasma samples from hemolyzed blood often yields flawed values. We studied the possibility that hemoglobin can bind melatonin and the iodinated tracer 125I-melatonin. The specific binding of 125I-melatonin to purified bovine hemoglobin was found to be rapid, saturable, and reversible (Kd = 315 pM, Bmax = 58 pmol/mg protein) and was inhibited by 2-iodomelatonin, serotonin, melatonin, and 5-methoxytryptamine. These data are compatible with the concept that hemoglobin can interfere with melatonin determinations by competing for melatonin and the iodinated tracer. Unlike melatonin receptor binding, the binding of 125I-melatonin to hemoglobin was not inhibited by guanine nucleotide analogs (i.e., GTP gamma S, GTP beta S, and Gpp(NH)p). Sodium cyanide had no effect on 125I-melatonin binding, indicating that 125I-melatonin does not bind to the heme group. On the other hand, 2,3-bisphosphoglycerate, at physiological concentrations (3-4 mM), decreased the apparent Bmax and Kd of 125I-melatonin binding to hemoglobin. These data suggest that 125I-melatonin binding to hemoglobin is conformation-specific and is unfavorable in the deoxyhemoglobin state. Hemoglobin may serve as a carrier protein for melatonin in the blood and discharge it in the target organs. Subsequently, the efficacy of melatonin's action as a hormone or antioxidant in target tissues may be enhanced.
authors.affiliation: Department of Biochemistry, Tel Aviv University, Israel.
authors.forename: E
authors.initials: E
authors.lastname: Gilad
authors.forename: N
authors.initials: N
authors.lastname: Zisapel
mesh_terms: D019794:2,3-Diphosphoglycerate; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism; D002417:Cattle; D004163:Diphosphoglyceric Acids / Q000378:metabolism; D006454:Hemoglobins / Q000378:metabolism*; D007457:Iodine Radioisotopes; D007700:Kinetics; D008550:Melatonin / Q000378:metabolism*; D011485:Protein Binding; D011869:Radioligand Assay
chemical_list: D002352:Carrier Proteins; D004163:Diphosphoglyceric Acids; D006454:Hemoglobins; D007457:Iodine Radioisotopes; D019794:2,3-Diphosphoglycerate; D008550:Melatonin