Spontaneous Alkaloid Formation in Man: Involvement in the Pathogenesis of Neurodegenerative Diseases?
1. Key Words:
In vivo formation and medicinal relevance of endogenous alkaloids in man; exemplarily for chloral-derived b-carbolines (e.g., 1-trichloromethyl-1,2,3,4-tetrahydro-β-carboline, ‘TaClo’): synthesis, stereochemistry, trace analysis, metabolism, and disposition; pharmacodynamic and pharmacokinetic studies; investigations on the neuropharmacological potential of a new group of highly halogenated alkaloids with dopaminergic and serotonergic neurotoxicity; occurrence in humans and presumable involvement as a causative factor in the pathogenesis of neurodegenerative disorders (e.g., Parkinson’s disease).
2. Graphical Abstract:
Subtopic A: Possible Formation of Chloral-Derived Mammalian Alkaloids
Subtopic B: The Neurotoxicological Potential of TaClo
Subtopic C: Occurrence of TaClo in Human Blood after Intake of Chloral Hydrate
Subtopic D: Assignment of the Absolute Configurations of the Two TaClo Enantiomers by Quantum Chemical CD Calculations
3. Brief Description:
The β-carbolines comprise a group of tricyclic indole derivatives. Like other compounds, among them isoquinolines, agrochemicals (e.g., rotenone, paraquat), alkanes, or heavy metals, they have been considered as potential environmental inducers of neurodegenerative processes, due to their occurrence in mammalian organisms. 1-Trichloromethyl-1,2,3,4-tetrahydro-β-carboline (TaClo), which is structurally closely related to the well-established synthetic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), is such a genuine neurotoxic agent for dopaminergic and serotonergic neurons . TaClo is readily formed in the human organism by a Pictet-Spengler type condensation of the endogenously present biogenic amine tryptamine (Ta) and the non-natural aldehyde chloral (Clo). Its spontaneous formation can occur after application of the hypnotic chloral hydrate and upon exposition to the industrial solvent trichloroethylene (TRI), which is known to be metabolized to chloral (see Figure 1).
TaClo has seriously to be taken into account as a causative or supportive agent for the development of idiopathic Parkinson's disease (see Figure 2), because of its ability to easily penetrate the blood-brain barrier [1,2], and to severely affect the striatal dopamine [1,3] and extracellular serotonin  metabolism. TaClo strongly interacts with enyzmes such as tyrosine hydroxylase (TH) related to dopamine biosynthesis and metabolism , and is capable to inhibit dopamine [3,6] and serotonin  uptake. Several in vitro studies revealed TaClo to be toxic to cultured neuroblastoma cells (SK-N-NH) , to the dopaminergic cell line IMR-32 , to neuronal-like clonal pheochromocytoma PC12 cells , to the serotonergic cell line JAR , to dopaminergic SH-SY5Y, and to non-dopaminergic murine Neuro2A neuroblastoma cell lines , most likely by passive diffusion through the cell membrane and not by a selective uptake system [7,10]. Exposure of TaClo to dopaminergic neurons in primary cell cultures from mice led to marked biochemical defects (e.g., inhibition of dopamine uptake, reduction of cell number and size) and to distinct morphological changes (e.g., swelling of dendrites, loss of axons) . While the synthetic MPTP acutely produces an irreversible syndrome in rodents and primates similar to idiopathic Parkinson's disease, TaClo can trigger a slowly developing neurodegeneration in rats with a late onset of first parkinsonian-type symptoms. This is obvious from a distinctly diminished locomotion of the animals observed nine months after the end of a seven-week subchronic exposure to small doses of TaClo [1,11]. The mechanism of TaClo-induced cell death is ascribed to the strong inhibition of complex I (NADH dehydrogenase) and complex II (succinate dehydrogenase) of the mitochondrial respiratory chain [1,3,6], to the formation of reactive oxygen species such as hydroxyl radicals , and to DNA damaging  and apoptotic  processes.
In blood samples obtained from elderly patients who had been treated orally with chloral hydrate for three days up to six months, TaClo was unambiguously identified in concentrations ranging from less than 1 to 35 ng/ml, by applying a sensitive GC/MS or LC-MS/MS device (see Figure 3) [13,14]. An even enhanced TaClo level of ca. 70 ng/g of clot was detected in a young epileptic, obviously as a consequence of a long-term daily intake of 1 g of chloral hydrate over a period of nearly five years . The onset of Parkinson's disease in three chronically TRI-exposed persons showing a continuous release of TRI associated with the presence of TaClo on a ng-scale  gave further hints for the assumption that endogenously originating chloral-derived β-carbolines (such as TaClo), even though formed only on such a small scale, may chronically trigger progressive neurodegenerative lesions in brain that become manifest with age.
In contrast to MPTP, the tetrahydro-β-carboline TaClo is a chiral compound (see Figure 4). Separation and stereochemical attribution of the two TaClo enantiomers was achieved by applying chromatography on a chiral phase HPLC column in hyphenation with circular dichroism (CD) spectroscopy (LC-CD coupling). Assignment of the absolute configuration of TaClo was achieved by quantum chemical CD calculations (see also 'Computational Chemistry') . In a human clot sample, the two TaClo enantiomers were found in equal concentrations (i.e., as a racemate) corroborating a spontaneous, non-enzymatic formation of TaClo from biogenic tryptamine and therapeutically administered chloral. In urine samples of TaClo-treated rats, by contrast, the (S)-enantiomer was found to predominate, hinting at an enantiomer-differentiating metabolism of the compound .
4. Selected Publications:
5. Cooperations and Research Grants:
Within a special research project entitled „Neurotoxins and Neuroprotection: Importance of Radical Mechanisms and of the Inhibition of the Respiratory Chain for the Etiology of the Parkinsonian Syndrome, of Neurodegenerative, and Aging Processes“, sponsored by the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF) (completed).