Stability of the Putative Neurotoxin Tryptamine-4,5-dione

• Published in: Chemical research in toxicology 2003-04, Vol.16 (4), p.493-501
• Main Authors: Wrona, Monika Z, Jiang, Xiang-Rong, Kotake, Yashige, Dryhurst, Glenn
• Format: Article
• Language: English
• Subjects: 5-Hydroxytryptamine; Benzoquinones - chemistry; Chromatography, High Pressure Liquid; Drug Stability; Free Radicals - chemistry; Indolequinones - chemical synthesis; Indolequinones - chemistry; Iron Chelating Agents - chemistry; Neurotoxins - chemical synthesis; Neurotoxins - chemistry; Reducing Agents - chemistry; Solutions; Spectrum Analysis; Superoxides - chemistry; Time Factors; tryptamine-4,5-dione; Tryptamines - chemical synthesis; Tryptamines - chemistry; Water
• ISSN: 0893-228X; 1520-5010
• DOI: 10.1021/tx020080f

Tryptamine-4,5-dione (1) is formed by oxidation of the neurotransmitter 5-hydroxytryptamine by reactive oxygen and reactive nitrogen species, and on the basis of in vitro and in vivo studies, it has been proposed to be a neurotoxin that may contribute to the selective neurodegeneration in Alzheimer's disease and the serotonergic neurotoxicity of methamphetamine. Several investigators have noted that under the conditions employed in the past to synthesize 1 and explore its in vitro and in vivo biological properties, the dione is somewhat unstable. In the present study, the stability of 1 has been investigated in a number of media employed in previous investigations to synthesize the dione and evaluate its biological properties. At low concentrations (≤200 μM), 1 is most stable in artificial cerebrospinal fluid (aCSF, pH 6−6.5) in which it decomposes ≤10% over 24 h forming primarily 3-(2-aminoethyl)-6-[3‘-(2-aminoethyl)-indol-4‘,5‘-dione-7‘-yl]-5-hydroxyindole-4,7-dione (10). In phosphate buffer or 0.5 M NH4Cl solutions at pH 7.4 and in acidic solution (e.g., 0.01 M HCl), such low concentrations of 1 also decompose to 10 although somewhat more rapidly than in aCSF. As the concentration of 1 is increased in all of these media, its decomposition becomes more rapid and shifts toward formation of 7,7‘-bi-(5-hydroxytryptamine-4-one) (9) and its autoxidation product 7,7‘-bitryptamine-4,5-dione (11). At 20 mM concentrations in aCSF or at pH 7.4, 1 rapidly decomposes to a dark, uncharacterized, presumably polymeric precipitate. However, in 0.01 M HCl solution ≥20 mM, 1 rapidly and almost quantitatively dimerizes to 9. The initial reaction of 1, which leads to the ultimate formation of 9 or 11 and 10, is the nucleophilic addition of water to the C(7) position of the dione to form 4,5,7-trihydroxytryptamine (2). Oxidation of 2 by 1 and/or molecular oxygen forms radical species, the predominant form of which has been detected by electron spin resonance spectroscopy using a spin stabilization method. Subsequent reactions of radical intermediates lead to the formation of 9 or 11 and 10. The results of this investigation are discussed in terms of previous in vitro and in vivo biological properties of 1 and its possible role in the serotonergic neurotoxicity of methamphetamine and neurodegenerative diseases.