High-affinity binding of melatonin to hemoglobin.

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.