Methionine Antagonizes para-Aminosalicylic Acid Activity via Affecting Folate Precursor Biosynthesis in Mycobacterium tuberculosis

para-Aminosalicylic acid (PAS) is a second-line anti-tubercular drug that is used for the treatment of drug-resistant tuberculosis (TB). PAS efficacy in the treatment of TB is limited by its lower potency against Mycobacterium tuberculosis relative to many other drugs in the TB treatment arsenal. It is known that intrinsic metabolites, such as, para-aminobenzoic acid (PABA) and methionine, antagonize PAS and structurally related anti-folate drugs. While the basis for PABA-mediated antagonism of anti-folates is understood, the mechanism for methionine-based antagonism remains undefined. In the present study, we used both targeted and untargeted approaches to identify factors associated with methionine-mediated antagonism of PAS activity. We found that synthesis of folate precursors as well as a putative amino acid transporter, designated MetM, play crucial roles in this process. Disruption of metM by transposon insertion resulted in a ≥30-fold decrease in uptake of methionine in M. bovis BCG, indicating that metM is the major facilitator of methionine transport. We also discovered that intracellular biotin confers intrinsic PAS resistance in a methionine-independent manner. Collectively, our results demonstrate that methionine-mediated antagonism of anti-folate drugs occurs through sustained production of folate precursors.

Synthesis of N-Chloroamino Acids and Their Biological Activities in LPS Stimulated RAW 264.7 Cells

Amino acids (AAs) are essential for protein synthesis, neurotransmission and macro molecule biosynthesis. Ala, Gln, Gly, Lys, Val and taurine (Tau) are the most abundant free AAs in mammals, and some of these react with hypochlorite (HOCl/OCl-) produced by myeloperoxidase in activated phagocytes to form N-chloroamino acids (NCAA). In this study, we reacted 20 AAs and Tau with sodium hypochlorite (NaOCl), then classified the products into five types (I-V) based on the change in their absorbance. Type I AAs (Ala, Arg, Gln, Gly, Ile, Lys, Phe, Ser, Tau, Thr and Val) generated a typical monochloramine peak at 252 nm, while Type II AAs (Asn and Tyr) and Type III AAs (Glu and Leu) produced peaks at 275 nm and 225 nm, respectively. The Type IV AAs (His, Met and Trp) did not show any distinct absorption peak, and Type V AAs (Asp, Cys and Pro) did not appear to react with NaOCl. The ArgCl and TauCl were stable, while GlnCl, GlyCl, IleCl, LysCl, PheCl and ValCl were less stable and AlaCl, SerCl and ThrCl were the least stable. Tau is the most abundant non-proteinogenic free AA in cellular fluid and has many physiological functions in the nervous, cardiovascular, renal and immune systems. Tau reacts with HOCl to form TauCl, which inhibits the production of proinflammatory mediators such as superoxide, nitric oxide (NO) and interleukins, while increasing the antioxidant proteins in macrophages. We determined the effects of Type I NCAA on cell viability, NO and TNF-α production in LPS-activated RAW 264.7 cells. All Type I NCAA showed dose-dependent cytotoxicity and inhibited LPS-induced NO production. However, only GlnCl, GlyCl, IleCl, LysCl, SerCl and TauCl inhibited LPS-induced TNF-α production. In summary, Type I NCAA showed dose-dependent cytotoxicity and inhibited NO production, while their effects on TNF-α varied. Our results suggest that Type I NCAA may serve as biological regulators similar to TauCl during inflammation.

An Aldol Reaction-Based Iridium(III) Chemosensor for the Visualization of Proline in Living Cells

A long-lived aldol reaction-based iridium(III) chemosensor [Ir(ppy)2(5-CHOphen)]PF6 (1, where ppy = 2-phenylpyridine and 5-CHOphen = 1,10-phenanthroline-5-carbaldehyde) for proline detection has been synthesized. The iridium(III) complex 1, incorporating an aldehyde group in N^N donor ligand, can take part in aldol reaction with acetone mediated by proline. The transformation of the sp2-hybridized carbonyl group into a sp3-hybridized alcohol group influences the metal-to-ligand charge-transfer (MLCT) state of the iridium(III) complex, resulting in a change in luminescence in response to proline. The interaction of the iridium(III) complex 1 with proline was investigated by 1H NMR, HRMS and emission titration experiments. Upon the addition of proline to a solution of iridium(III) complex 1, a maximum 8-fold luminescence enhancement was observed. The luminescence signal of iridium(III) complex 1 could be recognized in strongly fluorescent media using time-resolved emission spectroscopy (TRES). The detection of proline in living cells was also demonstrated.

The role of red blood cell S-nitrosation in nitrite bioactivation and its modulation by leucine and glucose

Previous work has shown that red blood cells (RBCs) reduce nitrite to NO under conditions of low oxygen. Strong support for the ability of red blood cells to promote nitrite bioactivation comes from using platelet activation as a NO-sensitive process. Whereas addition of nitrite to platelet rich plasma in the absence of RBCs has no effect on inhibition of platelet activation, when RBCs are present platelet activation is inhibited by an NO-dependent mechanism that is potentiated under hypoxia. In this paper, we demonstrate that nitrite bioactivation by RBCs is blunted by physiologically-relevant concentrations of nutrients including glucose and the important signaling amino acid leucine. Our mechanistic investigations demonstrate that RBC mediated nitrite bioactivation is largely dependent on nitrosation of RBC surface proteins. These data suggest a new expanded paradigm where RBC mediated nitrite bioactivation not only directs blood flow to areas of low oxygen but also to areas of low nutrients. Our findings could have profound implications for normal physiology as well as pathophysiology in a variety of diseases including diabetes, sickle cell disease, and arteriosclerosis.

Branched-chain amino acid supplementation promotes aerobic growth of Salmonella Typhimurium under nitrosative stress conditions

Nitric oxide (NO) inactivates iron-sulfur enzymes in bacterial amino acid biosynthetic pathways, causing amino acid auxotrophy. We demonstrate that exogenous supplementation with branched-chain amino acids (BCAA) can restore the NO resistance of hmp mutant Salmonella Typhimurium lacking principal NO-metabolizing enzyme flavohemoglobin, and of mutants further lacking iron-sulfur enzymes dihydroxy-acid dehydratase (IlvD) and isopropylmalate isomerase (LeuCD) that are essential for BCAA biosynthesis, in an oxygen-dependent manner. BCAA supplementation did not affect the NO consumption rate of S. Typhimurium, suggesting the BCAA-promoted NO resistance independent of NO metabolism. BCAA supplementation also induced intracellular survival of ilvD and leuCD mutants at wild-type levels inside RAW 264.7 macrophages that produce constant amounts of NO regardless of varied supplemental BCAA concentrations. Our results suggest that the NO-induced BCAA auxotrophy of Salmonella, due to inactivation of iron-sulfur enzymes for BCAA biosynthesis, could be rescued by bacterial taking up exogenous BCAA available in oxic environments.

Essential roles of methionine and S-adenosylmethionine in the autarkic lifestyle of Mycobacterium tuberculosis

Multidrug resistance, strong side effects, and compliance problems in TB chemotherapy mandate new ways to kill Mycobacterium tuberculosis (Mtb). Here we show that deletion of the gene encoding homoserine transacetylase (metA) inactivates methionine and S-adenosylmethionine (SAM) biosynthesis in Mtb and renders this pathogen exquisitely sensitive to killing in immunocompetent or immunocompromised mice, leading to rapid clearance from host tissues. Mtb ΔmetA is unable to proliferate in primary human macrophages, and in vitro starvation leads to extraordinarily rapid killing with no appearance of suppressor mutants. Cell death of Mtb ΔmetA is faster than that of other auxotrophic mutants (i.e., tryptophan, pantothenate, leucine, biotin), suggesting a particularly potent mechanism of killing. Time-course metabolomics showed complete depletion of intracellular methionine and SAM. SAM depletion was consistent with a significant decrease in methylation at the DNA level (measured by single-molecule real-time sequencing) and with the induction of several essential methyltransferases involved in biotin and menaquinone biosynthesis, both of which are vital biological processes and validated targets of antimycobacterial drugs. Mtb ΔmetA could be partially rescued by biotin supplementation, confirming a multitarget cell death mechanism. The work presented here uncovers a previously unidentified vulnerability of Mtb-the incapacity to scavenge intermediates of SAM and methionine biosynthesis from the host. This vulnerability unveils an entirely new drug target space with the promise of rapid killing of the tubercle bacillus by a new mechanism of action.

Inhibitors of amino acids biosynthesis as antifungal agents

Fungal microorganisms, including the human pathogenic yeast and filamentous fungi, are able to synthesize all proteinogenic amino acids, including nine that are essential for humans. A number of enzymes catalyzing particular steps of human-essential amino acid biosynthesis are fungi specific. Numerous studies have shown that auxotrophic mutants of human pathogenic fungi impaired in biosynthesis of particular amino acids exhibit growth defect or at least reduced virulence under in vivo conditions. Several chemical compounds inhibiting activity of one of these enzymes exhibit good antifungal in vitro activity in minimal growth media, which is not always confirmed under in vivo conditions. This article provides a comprehensive overview of the present knowledge on pathways of amino acids biosynthesis in fungi, with a special emphasis put on enzymes catalyzing particular steps of these pathways as potential targets for antifungal chemotherapy.

Optimization of a free separation of 30 free amino acids and peptides by capillary zone electrophoresis with indirect absorbance detection: a potential for quantification in physiological fluids

This report describes a rapid, single-run procedure, based on the optimization of capillary electrophoresis (CE) and indirect absorbance detection capabilities, which was developed for the separation and quantification of 30 underivatized physiological amino acids and peptides, usually present in biological fluids. p-Aminosalicylic acid buffered with sodium carbonate at pH 10.2+/-0.1 was used as the running electrolyte. Electrophoresis, carried out in a capillary (87 cm x 75 microm) at 15 kV potential (normal polarity), separated the examined compounds within 30 min. Limits of detection ranged from 1.93 to 20.08 micromol/l (median 6.71 micromol/l). The method was linear within the 50-200 micromol/l concentration range (r ranged from 0.684 to 0.989, median r=0.934). Within run migration times precision was good (median C.V.=0.7%). Less favorable within run peak area precision (median C.V.=6.6%) was obtained. The analytical procedure presented was successfully tested for separation and quantification of amino acids in physiological fluids, such as plasma or supernatant of macrophage cultures. Sample preparations require only a protein precipitation and dilution step.

Amino acid determination in biological fluids by automated ion-exchange chromatography: performance of Hitachi L-8500A

The Hitachi L-8500A is a newly available apparatus for amino acid (AA)analysis that allows automatic on-line mixing of the ninhydrin reagent. The within-run precision (human plasma pools at three different concentrations) showed CVs <3.8% except for the lowest concentration of citrulline (4.4%), Tyr (4.5%), and α-aminobutyric acid (7.6%), and for the intermediate concentration of Asp (8.7%). Between-run precision (CV) was <3.1% for 17 AAs and <8.0% for 24 of 25 AAs (CV Asp = 12.0%). For retention times, within-run precision was <0.4% and between-run precision <1.8%. Excellent relations were found between the results from the Hitachi L-8500A and the widely used Beckman 6300 analyzer (0.929≤ r ≤0.999). The detection was still linear at 5 μmol/L except for Pro and hydroxyproline (20 μmol/L). The upper limit was at least 2500 μmol/L for 13 AAs and at least 1000 μmol/L for 27 of 29 AAs (anserine = 500, Val = 600 μmol/L). Values from 100 human plasma samples agreed with previously published data. We conclude that the results obtained with the Hitachi L-8500A are satisfactory when compared with those of other AA analyzers utilizing the same method. Furthermore, the Hitachi L-8500A displays several advantages including programming flexibility, microsample capacity, low noise plotting, ammonia filtering, and manual repacking of the analytical column.

Gas chromatographic analysis of amino acids in physiological fluids: a critique

The available methodology for sample purification prior to gas chromatographic analysis of amino acids in physiological fluids and tissue extracts is analysed. It would appear that over the past ten years the method of choice is that of an ion-exchange purification step, and little, if any, progress has been achieved in sample purification procedures. The inherent disadvantages of such a methodology are not only perpetuated but also cast some considerable doubt on the accuracy of the quantitative analysis of amino acids.

Profiles of strongly polar and less polar acids obtained from human blood, plasma and serum by two-step ultrafiltration

A two-step ultrafiltration method combined with anion-exchange chromatography is described for the separation of lipophilic and strongly lipophobic acids occurring in human blood, serum and plasma. After treatment with diazomethane, the acid fractions are separated further by gas chromatography. The acids were identified by their mass spectra. Profiles obtained from the strongly acidic fraction of blood samples of healthy individuals are characterized by main peaks corresponding to organic phosphoric acid esters. The peaks are absent in plasma and serum.

Gas-liquid chromatographic determination of N(O,S)-trifluoroacetyl n-propyl esters of protein and non-protein amino acids

Chromatographic conditions for the determination of protein and non-protein amino acids as their N(O,S)-trifluoroacetyl n-propyl esters are given which allow the separation of about 30 amino acids within 19 min using nitrogen or within 17 min using helium as the carrier gas. Retention times and responses are given for 42 amino acids or related compounds. Only few of them elute together when using a 2 m x 2 mm I.D. glass column filled with 0.65% ethylene glycol adipate on Chromosorb W AW (80-100 mesh). The acylation of the n-propyl esters was investigated with respect to optimal reaction conditions and was found to be best performed at 150 degrees C for 5 min.