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 I-125-mel atonin. The specific binding of I-125-melatonin to purified bovine hem oglobin was found to be rapid, saturable, and reversible (K-d = 315 pM , B-max = 58 pmol/mg protein) and was inhibited by 2-iodomelatonin, se rotonin, melatonin, and 5-methoxytryptamine. These data are compatible with the concept that hemoglobin can interfere with melatonin determi nations by competing for melatonin and the iodinated tracer. Unlike me latonin receptor binding, the binding of I-125-melatonin to hemoglobin was not inhibited by guanine nucleotide analogs (i.e., GTP gamma S, G TP beta S, and Gpp(NH)p). Sodium cyanide had no effect on I-125-melato nin binding, indicating that I-125-melatonin does not bind to the heme group. On the other hand, 2,3-bisphosphoglycerate, at physiological c oncentrations (3-4 mM), decreased the apparent B-max and K-d of I-125- melatonin binding to hemoglobin. These data suggest that I-125-melaton in binding to hemoglobin is conformation specific and is unfavorable i n the deoxyhemoglobin state. Hemoglobin may serve as a carrier protein for melatonin in the blood and discharge it in the target organs. Sub sequently, the efficacy of melatonin's action as a hormone or antioxid ant in target tissues maybe enhanced. (C) 1995 Academic Press,Inc.