An improved pentafluorobenzylation method was developed for derivatization of L-tryptophan and its acidic metabolites (L-kynurenine, kynurenic acid, anthranilic acid, xanthurenic acid, 3-hydroxyanthranilic acid, picolinic acid, quinolinic acid) present at trace levels in aqueous samples. This method employs lyophilization of aqueous samples in the presence of excess tetrabutylammonium hydrogen sulfate, followed by base-catalyzed anhydrous pentafluorobenzylation. A comparison with other published methods shows the advantage of this modification for the derivatization of kynurenine metabolites. The derivatives were analyzed by gas chromatography/electron capture negative ion mass spectrometry (GC/ECNI-MS) or liquid chromatography/particle beam/ECNI-MS (LC/ECNI-MS). The detection limits for injected standards are in the femtogram range by GC/ECNI-MS and in the low picogram range by LC/ECNI-MS. GC/ECNIMS is 3.6 (xanthurenic acid) to 66 (quinolinic acid) times more sensitive than LC/ECNI-MS. The simultaneous determination of two neuroactive metabolites, quinolinic and kynurenic acids, in culture medium is presented. The minimum measurable concentrations of these metabolites in 100 microL of culture medium are 0.11 nM for quinolinic acid and 0.21 nM for kynurenic acid.
By using ion-exchange chromatography and gas chromatography coupled with mass spectrometry, the content of phenylalanine, tyrosine and their metabolites typical of phenylketonuria (PKU) was determined in the cerebrospinal fluid (CSF) of 8 untreated children with classical PKU and 9 controls. At the same time, plasma and urine were analysed. In PKU the content of phenylalanine is increased on average 23 times in plasma and CSF. The content of phenylalanine and tyrosine in CSF is about 4 times less as compared with plasma. The phenylalanine-to-tyrosine ratio is approximately the same for these fluids both in control and in PKU. This indicates that the transport of phenylalanine and tyrosine through the blood-brain barrier is not disturbed in PKU. Phenylpyruvate and 4-hydroxyphenylpyruvate are either not detected or present in very low concentrations in the CSF of children with PKU; their derivatives, phenyllactate and 4-hydroxyphenyllactate, are present in relatively higher concentrations. This indicates increased metabolic conversion in brain tissues.
In a number of infectious and inflammatory diseases, stimulation of the immune system can lead to increased accumulation of tryptophan metabolites via induction of kynurenine pathway enzymes in extrahepatic tissues. We developed a liquid chromatographic/mass spectrometric (LC/MS) method suitable for tracing the disposition of 13C isotopomers of L-tryptophan and L-kynurenine in various cultured cell, tissue slice, and whole animal model systems used to investigate tryptophan flux through the kynurenine pathway. The method employs extractive derivatization of the analytes and their 2H internal standards with pentafluorobenzyl bromide in order to enhance the negative ion chemical ionization (NICI) mass spectrometric response. Normal-phase liquid chromatographic separation of derivatized analytes was optimized using a silica column with organic solvents, followed by particle beam transfer and NICI-MS. Standard curves were linear over the range 1-250 ng per sample. Particle beam and mass spectrometric operating parameters were optimized with direct flow injections of 1-(methylamino) anthraquinone, which is an ideal test compound for the evaluation of LC/NICI-MS. The developed method was used to quantify the conversion of (13C6)L-tryptophan to (13C6)L-kynurenine by human monocytes (THP-1) stimulated with interferon-gamma, lung and brain tissue slices obtained from gerbils immune-stimulated with pokeweed mitogen. The effect of whole body immune stimulation on the plasma levels of endogenous L-kynurenine in mice stimulated with interferon-gamma was also quantified.
The metabolism of l‐tryptophan to the neuroactive kynurenine pathway metabolites, l‐kynurenine, kynurenate and quinolinate, and the effects of two inhibitors of quinolinate synthesis (6‐chlorotryptophan and 4‐chloro‐3‐hydroxyanthranilate) were investigated by mass spectrometric assays in cultured cells and in vivo. Cell lines obtained from astrocytoma, neuroblastoma, macrophage/monocytes, lung, and liver metabolized l‐[13C6]‐tryptophan to l‐[13C6]kynurenine and [13C6]kynurenate, particularly after indoleamine‐2,3‐dioxygenase induction by interferon‐γ. Kynurenine aminotransferase activity was measurable in all cell types examined but was unaffected by interferon‐γ. These results suggest that many cell types can be sources of kynurenate following immune activation. In vivo synthesis of l‐[13C6]kynurenine and [13C6]kynurenate from l‐[13C6]tryptophan was studied in the CSF of macaques infected with poliovirus, as a model of inflammatory neurologic disease. The effects of 6‐chlorotryptophan and 4‐chloro‐3‐hydroxyanthranilate on the synthesis of kynurenate were different. 6‐Chlorotryptophan attenuated formation of l‐[13C6]kynurenine and [13C6]kynurenate and was converted to 4‐chlorokynurenine and 7‐chlorokynurenate. It may be an effective prodrug for the delivery of 7‐chlorokynurenate, which is a potent antagonist of NMDA receptors. In contrast, 4‐chloro‐3‐hydroxyanthranilate did not reduce accumulation of l‐[13C6]kynurenine and [13C6]kynurenate. 6‐Chlorotryptophan and 4‐chloro‐3‐hydroxyanthranilate are useful tools to manipulate concentrations of quinolinate and kynurenate in the animal models of neurologic disease to evaluate physiological roles of these neuroactive metabolites.
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