Melatonin's unique radical scavenging properties - roles of its functional substituents as revealed by a comparison with its structural analogs

Melatonin's O -methyl and N -acetyl residues are not only the basis of its amphilicity enabling the molecule to enter all organs and all subcellular compartments, but are also decisive for its antioxidant properties. We have compared melatonin's redox chemistry with that of several structural analogs: tryptamine, N -acetyltryptamine, serotonin, N -acetylserotonin, 5-methoxytryptamine, 6-chloromelatonin and 2-iodomelatonin. Scavenging of hydroxyl radicals ((.) OH) was measured in a scavenger competition assay based on ABTS cation radical (ABTS(.) (+) ) formation. The capability of undergoing single-electron transfer reactions was studied using an ABTS(.) (+) reduction assay, reflecting the more general property of scavenging organic cation radicals. Direct scavenging of superoxide anions (O-2 (.) (-) ), under non-catalyzed conditions, was investigated in a hematoxylin autoxidation assay. Measurements of chemiluminescence were used for studying scavenging of O-2 (.) (-) under catalyzed conditions, either by hemin-mediated interaction or by combination with the respective indolyl cation radicals. Light emission was determined in the absence or presence of the (.) OH scavenger dimethylsulfoxide and the O-2 (.) (-) scavenger Tiron. Products formed by oxidation of the respective indoles in a moderately alkaline, hemin-catalyzed H-2 O-2 system were analyzed by thin-layer chromatography and fluorometry. Absence of either the O -methyl or the N -acetyl residue causes marked diminutions in the capacities of scavenging (.) OH and ABTS(.) (+) as well as in chemiluminescence emitted during oxidation. The importance of the N -acetyl group is insofar remarkable as it seems, at first glance, to be isolated from the indolic moiety; interactions between side chain and indolic moiety are therefore decisive for melatonin's redox properties. The 5-hydroxylated compounds are not generally more efficient scavengers, but particularly better reducers of ABTS(.) (+) ; in the alkaline H-2 O-2 system generating (.) OH and O-2 (.) (-) , melatonin was much more rapidly oxidized than the 5-hydroxylated and non-substituted analogs. Oxidative products formed from any of the compounds studied contained much less of substituted kynuramines as in the case of melatonin, indicating that radical chain termination by O-2 (.) (-) is considerably more efficient with melatonin. These findings are supported by measurements of chemiluminescence, which largely reflects pyrrole ring cleavage as a result of combination with superoxide anions. In this regard, only 6-chloromelatonin equalled melatonin, whereas the efficiency of 2-iodomelatonin was much lower, another indication for the importance of 2,3-dioxygenation.