Inhibition of the α-Ketoglutarate Dehydrogenase and Pyruvate Dehydrogenase Complexes by a Putative Aberrant Metabolite of Serotonin, Tryptamine-4,5-dione

Abstract

A transient energy impairment with resultant release and subsequent reuptake of 5-hydroxytryptamine (5-HT) and NMDA receptor activation with consequent cytoplasmic superoxide (O(2)(-)(*)), nitric oxide (NO(*)), and peroxynitrite (ONOO(-)) generation have all been implicated in a neurotoxic cascade which ultimately leads to the degeneration of serotonergic neurons evoked by methamphetamine (MA) and 3,4-methylenedioxymethamphetamine (MDMA). Such observations raise the possibility that the O(2)(-)(*)/NO(*)/ONOO(-)-mediated oxidation of 5-HT, as it returns via the plasma membrane transporter to the cytoplasm of serotonergic neurons when the MA/MDMA-induced energy impairment begins to subside, may generate an endogenous neurotoxin. In vitro the O(2)(-)(*)/NO(*)/ONOO(-)-mediated oxidation of 5-HT forms tryptamine-4,5-dione (T-4,5-D). When incubated with intact rat brain mitochondria, T-4,5-D strongly inhibits state 3 respiration with pyruvate or alpha-ketoglutarate as substrates at concentrations which do not affect succinate-supported (complex II) respiration. Experiments with freeze-thawed rat brain mitochondria reveal that T-4,5-D inhibits the pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase complexes. These and other properties of T-4,5-D raise the possibility that it may be an endogenously formed intraneuronal metabolite of 5-HT that contributes to the serotonergic neurotoxicity of MA and MDMA.

Molecular and Cellular Mechanisms of Ecstasy-Induced Neurotoxicity: An Overview

“…It has been postulated that under these conditions, oxidation of 5-HT in the cytoplasm of serotonergic neurons by such ROS and reactive nitrogen species (RNS) would be expected to form the toxic metabolite T-4,5-D [417]. T-4,5-D is rapidly conjugated with GSH and reacts with other -SH-containing groups, forming, initially, 7-S-substituted thioethers [420]. This putative cytoplasmic metabolite of 5-HT is extremely electrophilic and reacts with -SH residues of proteins and enzymes [418,421].…”

“…This putative cytoplasmic metabolite of 5-HT is extremely electrophilic and reacts with -SH residues of proteins and enzymes [418,421]. It has been shown, in vitro, that T-4, 5-D irreversibly inactivates rat brain TPH [421], mitochondrial NADH-coenzyme Q1 (CoQ1) reductase (complex I), and cytochrome c oxidase (complex IV) [418] through covalent modification of active site cysteine residues and inhibits the pyruvate dehydrogenase complex (the link of glycolysis to the Krebs cycle) and the α-ketoglutarate dehydrogenase complex (the link of the Krebs cycle to GLU metabolism) [420]. In the event of T-4,5-D formation, its ability to uncouple mitochondrial respiration and to inhibit NADH-CoQ1 reductase, cytochrome c oxidase, PDHC, and KGDHC all together is expected to severely impair mitochondrial ATP production by serotonergic neurons [417,420].…”

“…Whether the hypothesis of intraneuronal ROS/RNSmediated oxidation of 5-HT to T-4,5-D or other toxic 5-HT metabolites is relevant to the serotonergic neurotoxicity of MDMA remains to be established. Additionally, efforts to detect free T-4,5-D in the brains of rats exposed to neurotoxic doses of MDMA are almost certain to be unsuccessful due to the very rapid reaction of this highly electrophilic species with proteins and low-molecularweight nucleophiles like GSH [420].…”

Serotonin as a physiological substrate for myeloperoxidase and its superoxide-dependent oxidation to cytotoxic tryptamine-4,5-dione

“…Stock solutions of serotonin were prepared daily in water. Tryptamine-4,5-dione was synthesized using the method of Jiang and Dryhurst [20]. Briefly, serotonin was allowed to react with a five molar excess of potassium nitrosodisulfonate in water at room temperature (21 • C) for 25 min to form a dark purple solution.…”

Covalent modification of cytoskeletal proteins in neuronal cells by tryptamine-4,5-dione

“…TD was freshly synthesized with minor modifications [7]. Briefly, 1 mg of serotonin was dissolved in 500 µl of water and then added to potassium nitrosodisulfonate (Aldrich), which is known as Fremy's reagent.…”