Determination of D- and L-amino acids produced by cyanobacteria using gas chromatography on Chirasil-Val after derivatization with pentafluoropropyl chloroformate

Evaluation of a commercial enzyme linked immunosorbent assay (ELISA) for the determination of the neurotoxin BMAA in surface waters

The neurotoxin β-N-methylamino-L-alanine (BMAA) is suspected to play a role in Alzheimer’s disease, Parkinson’s disease and amyotrophic lateral sclerosis. Because BMAA seems to be produced by cyanobacteria, surface waters are screened for BMAA. However, reliable analysis of BMAA requires specialized and expensive equipment. In 2012, a commercial enzyme-linked immunosorbent assay (ELISA) for determination of BMAA in surface waters was released. This kit could enable fast and relatively cheap screening of surface waters for BMAA. The objective of this study was to determine whether the BMAA ELISA kit was suitable for the determination of BMAA concentrations in surface waters. We hypothesised that the recovery of spiked samples was close to 100% and that the results of unspiked sample analysis were comparable between ELISA and liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis. However, we found that recovery was higher than 100% in most spiked samples, highest determined recovery was over 400%. Furthermore, the ELISA gave a positive signal for nearly each tested sample while no BMAA could be detected by LC-MS/MS. We therefore conclude that in its current state, the kit is not suitable for screening surface waters for BMAA.

Improved enantiomer resolution and quantification of free D-amino acids in serum and urine by comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry

The potential of comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry (GC×GC-TOFMS) in the quantitative analysis of amino acid enantiomers (AAEs) as their methyl chloroformate (MCF) derivatives in physiological fluids was investigated. Of the two column sets tested, the combination of an Rt-γDEXsa chiral column with a polar ZB-AAA column provided superior selectivity. Twenty AAEs were baseline resolved including L-Leu and D-Ile, which had failed separation by one-dimensional chiral GC-quadrupole-MS (GC-qMS). Lower limits of quantification (LLOQ) were in the range of 0.03-2 μM. Reproducibility of the analysis of a serum specimen in octaplicate ranged from 1.3 to 16.6%. The GC×GC-TOFMS method was validated by analyzing AAEs in 48 urine and 43 serum specimens, respectively, and by comparing the results with data obtained by a previously validated GC-qMS method. Mean recoveries ranged from 78.4% for D-Leu to 116.4% for D-Pro in urine and 72.2% for L-Thr to 129.4% for L-Ile in serum. The method was applied to the comparison of AAE serum levels in patients suffering from liver cirrhosis to a control group. Significantly increased D-AA concentrations were found for the patient group, whereas L-AA levels were slightly decreased.

Simultaneous determination of serine enantiomers in plasma using Mosher's reagent and stable isotope dilution gas chromatography-mass spectrometry

D-Serine is a co-agonist of the N-methyl-D-aspartate receptor in glutamate neurotransmission and has been proposed as a potential therapeutic agent for schizophrenia. However, D-serine also acts as a nephrotoxic substance in rats at high doses. To investigate the pharmacokinetics and toxicokinetics of D-serine, a method for the stereoselective determination of serine enantiomers in rat plasma was developed using GC-MS with selected ion monitoring (GC-MS-SIM). DL-[(2)H(3)]Serine was used as an internal standard to account for losses associated with the extraction, derivatization and chromatography. Serine enantiomers were purified by cation-exchange chromatography using BondElut SCX cartridge and derivatized with HCl in methanol to form methyl ester followed by subsequent N,O-diacylation with optically active (+)-α-methoxy-α-trifluoromethylphenylacetyl chloride to form epimeric amide. Quantitation was performed by SIM of the molecular-related ions of the epimers in the chemical ionization mode. The intra- and inter-day reproducibility of the assay was less than 5% for D-serine and 3% for L-serine. The method was successively applied to study the pharmacokinetics of D-serine in rats.

Comparison of derivatization and chromatographic methods for GC-MS analysis of amino acid enantiomers in physiological samples

GC-MS analysis of fluorinated and non-fluorinated chloroformate and anhydride derivatives of amino acid (AA) enantiomers on two different chiral columns was compared for the direct quantification of free L- and D-AAs in human serum and urine in a single analytical run. Best sensitivity was achieved with pentafluoropropionic anhydride/heptafluorobutanol derivatives separated on a Chirasil-L-Val column. However, the occurrence of racemization during derivatization precluded accurate quantification of AA enantiomers. Derivatization with methyl chloroformate/methanol and separation on an Rt-gammaDEXsa column did not exhibit racemization and yielded ten baseline separated racemates of proteinogenic AAs with resolution values greater than 2.4. However, protein and peptide hydrolysis occurred in serum and urine during the highly exothermal derivatization reaction under alkaline conditions. Removing serum proteins by precipitation before derivatization and performing the reaction at neutral pH enabled the determination of accurate free AA enantiomer concentrations. Accuracy of quantification was validated by an established nonchiral GC-MS method for AA analysis. Reliable quantification was achieved using stable-isotope labeled L-AAs as internal standards. Limits of detection (LOD) and lower limits of quantification (LLOQ) for the D-AAs were in the range of 3.2-446 nM and 0.031-1.95 microM, respectively. Relative standard deviations (N=6) for the measurement of AAs in urine and serum ranged from 0.49-11.10% to 0.70-3.87%, respectively. The method was applied to the analysis of urine from 19 patients with renal insufficiency. In comparison to healthy probands, D-ratios of Ala, Val, Pro, Thr, Asp, and Asn were significantly increased.

Bacterial attraction and quorum sensing inhibition in Caenorhabditis elegans exudates

Caenorhabditis elegans, a bacterivorous nematode, lives in complex rotting fruit, soil, and compost environments, and chemical interactions are required for mating, monitoring population density, recognition of food, avoidance of pathogenic microbes, and other essential ecological functions. Despite being one of the best-studied model organisms in biology, relatively little is known about the signals that C. elegans uses to interact chemically with its environment or as defense. C. elegans exudates were analyzed by using several analytical methods and found to contain 36 common metabolites that include organic acids, amino acids, and sugars, all in relatively high abundance. Furthermore, the concentrations of amino acids in the exudates were dependent on developmental stage. The C. elegans exudates were tested for bacterial chemotaxis using Pseudomonas putida (KT2440), a plant growth promoting rhizobacterium, Pseudomonas aeruginosa (PAO1), a soil bacterium pathogenic to C. elegans, and Escherichia coli (OP50), a non-motile bacterium tested as a control. The C. elegans exudates attracted the two Pseudomonas species, but had no detectable antibacterial activity against P. aeruginosa. To our surprise, the exudates of young adult and adult life stages of C. elegans exudates inhibited quorum sensing in the reporter system based on the LuxR bacterial quorum sensing (QS) system, which regulates bacterial virulence and other factors in Vibrio fischeri. We were able to fractionate the QS inhibition and bacterial chemotaxis activities, thus demonstrating that these activities are chemically distinct. Our results demonstrate that C. elegans can attract its bacterial food and has the potential of partially regulating the virulence of bacterial pathogens by inhibiting specific QS systems.

Physiological role and regulation of glutamate dehydrogenase in Prochlorococcus sp. strain MIT9313

Glutamate dehydrogenase is an enzyme catalysing a reaction for ammonium assimilation, alternative to those performed by glutamine synthetase and glutamate synthase. In the genus Prochlorococcus, genomic studies have shown the presence of the gdhA gene (encoding glutamate dehydrogenase) in only four of the sequenced strains, including MIT9313. We studied the physiological regulation of glutamate dehydrogenase in this strain, by measuring the expression of gdhA, the intracellular concentration of the enzyme and its activity. Our goal was to clarify the physiological role of glutamate dehydrogenase, in order to understand why it has been selectively conserved in certain strains. Studies performed in cultures under nitrogen starvation, or with inhibitors of the nitrogen assimilation, suggest that the main role of glutamate dehydrogenase is not the assimilation of ammonium. Glutamate dehydrogenase activity and gdhA expression increased along the growth of cultures. Besides, we found a significant upregulation in gene expression when cultures were grown on glutamate as nitrogen source. We suggest that the main physiological role of glutamate dehydrogenase in Prochlorococcus MIT9313 is the utilization of glutamate to produce ammonium and 2-oxoglutarate, and amino acid recycling, thus enabling to use amino acids as nitrogen source. Therefore we propose that glutamate dehydrogenase is present in the genome of strains for whom the utilization of amino acids is most important.