Reactions of the melatonin metabolite <I>N</I><SUP>1</SUP>-acetyl-5-methoxykynuramine (AMK) with the ABTS cation radical: identification of new oxidation products

Abstract

The melatonin metabolite N1-acetyl-5-methoxykynuramine (AMK; 1), which was previously shown to be a potent radical scavenger, was oxidized using the ABTS cation radical [ABTS = 2,2'-azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid)]. Several new oxidation products were obtained, which were separated by repeated chromatography and characterized by spectroscopic methods such as mass spectrometry (ESI-MS and ESI-HRMS), 1H-NMR and 13C-NMR, HMBC, HSQC, H,H COSY correlations and IR spectroscopy. The main products were oligomers of 1 (3 dimers, 1 trimer and 2 tetramers). In all cases, the amino group N2 was involved in the reactions. Two of the dimers turned out to be cis (2a) and trans (2b) isomers containing an azo bond. In the other dimer (3a), the nitrogen atom N2 was attached to atom C5 of the second aromatic amine, with loss of the methoxy group. In the trimer (5), one N2 formed a bridge to C5 of unit B, as in the respective dimer, while this one of C had bridged to C6 of B. One of the tetramers (6) was composed of a trimer 5 attached to N2 of a fourth 1 molecule via an azo bond as in 2a/b. In the other tetramer (7), an additional C-C bond between rings B and C in 6 is assumed. Although oligomers of AMK may only attain low in vivo concentrations, the types of reactions observed shed light on the physiologically possible metabolism of AMK once reacted with a free radical. The displacement of a methoxy group, rarely seen in the oxidation of methoxylated biomolecules, underlines the reactivity of AMK (1). Preliminary data show that, in the presence of ABTS cation radicals, AMK (1) can interact with side chains of aromatic amino acids, a finding which may be crucial for understanding to date unidentified protein modification by a melatonin metabolite detected earlier in experiments with radioactively labeled melatonin.

Melatonin is synthesised by yeast during alcoholic fermentation in wines

“…doi:10.1016/j.foodchem.2010.12.038 antioxidant enzymes (Hardeland & Pandi-Perumal, 2005;Reiter, Tan, & Maldonado, 2005). Moreover, the biological activities of MEL metabolites (N 1 -acetyl-N 2 -formyl-5-methoxykynuramine (AFMK) and N 1 -acetyl-5-methoxykynuramine (AMK)) have been described previously (Guenther et al, 2005;Schaefer & Hardeland, 2009;Than, Heer, Laatsch, & Hardeland, 2006). MEL has been reported to be present in foods (De la Puerta et al, 2007;Iriti et al, 2006;Maldonado, Moreno, & Calvo, 2009).…”

Melatonin Metabolism in the Central Nervous System

“…In chemical systems, several dimers and oligomers have been identified [144], which are, however, presumably physiologically irrelevant, because of much lower educt concentrations present in the biological material. It seems more likely that AMK intermediates formed by oxidants rather interact with other aromates, which has, in fact been demonstrated with tyrosine [50,89] and tryptophan [50].…”

Melatonin and Synthetic Melatonergic Agonists: Actions and Metabolism in the Central Nervous System

“…An additional, photochemical mechanism [267] may not be relevant to the brain, but occur in UV-exposed tissues. AMK (10) was reported to be a potent scavenger of free radicals [268,269] and singlet oxygen [270], but also to form adducts with aromatic amino acid residues [44,271]. Moreover, AMK is an efficient scavenger of various reactive nitrogen species [45,272,273].…”