Endogenous alkaloids in man: XXXVIII. “Chiral” and “achiral” determination of the neurotoxin TaClo (1-trichloromethyl-1,2,3,4-tetrahydro-β-carboline) from blood and urine samples by high-performance liquid chromatography–electrospray ionization tandem mas

An improved sensitive assay for the determination of the dopaminergic and serotonergic neurotoxin 1-trichloromethyl-1,2,3,4-tetrahydro-β-carboline (TaClo) is presented, based upon on-line coupling of high-performance liquid chromatography with electrospray ionization tandem mass spectrometry (HPLC–ESI-MS–MS). Applying synthetic [D4]TaClo as a fourfold deuterated internal standard, TaClo was detected and reliably quantified as a trace constituent of blood samples (0.5 up to 70 ng g−1 of clot) obtained from six patients orally treated with the hypnotic chloral hydrate. Unambiguous identification of this tricyclic “endogenous alkaloid” was achieved by selected reaction monitoring (SRM) experiments. The molecular ion peaks of TaClo, m/z 289 (for [35Cl3]TaClo) and m/z 291 (for its [37Cl35Cl2]isotopomer), were both monitored to undergo a retro-Diels–Alder fragmentation by loss of a CH2NH portion (−29 u) as typical of a tetrahydropyrido ring system of tetrahydro-β-carbolines. Detection of the resulting fragments, m/z 260 and m/z 262, with the expected statistical chlorine isotopic intensities of 100:96 confirmed the identity of the TaClo molecule. In addition, an enantiomer-specific device was developed for TaClo, by employing a chiral reversed-phase HPLC column in combination with circular dichroism (CD) spectroscopy and MS–MS analysis (LC–CD and LC–MS–MS coupling). In a human clot sample, both TaClo enantiomers were found in equimolar concentration (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)-antipode was found to predominate, hinting at an enantiomer-differentiating metabolism of the compound.