L-Carnitine moiety assay: an up-to-date reappraisal covering the commonest methods for various applications

Trimethylamine N-Oxide: The Good, the Bad and the Unknown

Trimethylamine N-oxide (TMAO) is a small colorless amine oxide generated from choline, betaine, and carnitine by gut microbial metabolism. It accumulates in the tissue of marine animals in high concentrations and protects against the protein-destabilizing effects of urea. Plasma level of TMAO is determined by a number of factors including diet, gut microbial flora and liver flavin monooxygenase activity. In humans, a positive correlation between elevated plasma levels of TMAO and an increased risk for major adverse cardiovascular events and death is reported. The atherogenic effect of TMAO is attributed to alterations in cholesterol and bile acid metabolism, activation of inflammatory pathways and promotion foam cell formation. TMAO levels increase with decreasing levels of kidney function and is associated with mortality in patients with chronic kidney disease. A number of therapeutic strategies are being explored to reduce TMAO levels, including use of oral broad spectrum antibiotics, promoting the growth of bacteria that utilize TMAO as substrate and the development of target-specific molecules with varying level of success. Despite the accumulating evidence, it is questioned whether TMAO is the mediator of a bystander in the disease process. Thus, it is important to undertake studies examining the cellular signaling in physiology and pathological states in order to establish the role of TMAO in health and disease in humans.

Separation of carnitine and acylcarnitines in biological samples: a review

Carnitine and its acylesters are a family of compounds that can be used in the early diagnosis of many diseases. Carnitine and acylcarnitines have a crucial role in fatty acid transportation. The increased level of free carnitine, total carnitine, or the acylesters can act as biomarkers for many metabolic disorders, including diabetes, encephalopathy and cardiomyopathy. The determination of these compounds is difficult owing to the simple aliphatic structure, the chiral center and the permanent positive charge. Although MS detection can be enough to differentiate between some carnitine derivatives, closely related structural isomers of the acylcarnitines must be separated before detection because they form the same base peak and second most abundant ion peak. Different separation methods are discussed in this review, including reversed-phase, hydrophilic interaction, ion exchange, ion pairing, mixed mode liquid chromatography, gas chromatography and electrophoresis. Representative example chromatograms are shown. The sample preparation and the different derivatization reactions are also covered. A table that summarizes the most important analytical methods by detailing the analyte mixture, the sample matrix, the separation mode and the detection method is provided.

A liquid chromatography and tandem mass spectrometry method for the determination of potential biomarkers of cardiovascular disease

A simple, accurate and sensitive liquid chromatography tandem mass spectrometry (LC-MS/MS) method was developed and validated for the quantitation of α-ketoglutaric acid (α-KG), L-carnitine (L-CAR) and acetyl-L-carnitine (acetyl-L-CAR) in human urine as potential biomarkers of cardiovascular disease. The separation was performed using an isocratic elution of 0.1% formic acid in water and acetonitrile (97:3, v/v) on an Acclaim 120 C8 column (150 mm × 4.6 mm, 3.0 μm). The flow rate of the mobile phase was 1.2 mL/min and the total assay run time was 3 min. Detection was performed on a triple-quadrupole mass spectrometer in selected reaction monitoring (SRM) mode via an electrospray ionization (ESI) source in positive and negative ion modes. This method covered a linearity range of 0.1-500 ng/mL for L-CAR and acetyl-L-CAR and 1-1000 ng/mL for α-KG with lower limits of quantification (LLOQ) of 0.08 ng/mL for L-CAR, 0.04 ng/mL for acetyl-L-CAR and 0.8 ng/mL for α-KG. The intra-day and inter-day precision and accuracy of the quality control samples exhibited relative standard deviations of less than 5.54% and relative error values from -5.95% to 3.11%. Analyte stability was evaluated under various sample preparation, analysis and storage conditions and varied from -9.89% to -0.47%. A two-step solid-phase extraction (SPE) procedure using silica gel and quaternary amine cartridges was used for urine sample cleanup. The average recoveries for all analyzed compounds were better than 86.64% at three concentrations. The method was successfully applied for the quantitation of α-KG, L-CAR and acetyl-L-CAR in human urine samples.

Single dose administration of L-carnitine improves antioxidant activities in healthy subjects

L-carnitine has been used as a supplement to treat cardiovascular or liver disease. However, there has been little information about the effect of L-carnitine on anti-oxidation capability in healthy human subjects. This study was designed to investigate the correlation between plasma L-carnitine concentration and antioxidant activity. Liquid L-carnitine (2.0 g) was administered orally as a single dose in 12 healthy subjects. Plasma concentration of L-carnitine was detected by HPLC. The baseline concentration of L-carnitine was 39.14 ± 5.65 µmol/L. After single oral administration, the maximum plasma concentration (C(max)) and area under the curve (AUC(0-∞)) were 84.7 ± 25.2 µmol/L and 2,676.4 ± 708.3 µmol/L·h, respectively. The half-life and the time required to reach the C(max) was 60.3 ± 15.0 min and 3.4 ± 0.46 h, respectively. There was a gradual increase in plasma concentrations of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), catalase and total antioxidative capacity (T-AOC) in the first 3.5 h following L-carnitine administration. The plasma concentrations of SOD, GSH-Px, catalase and T-AOC returned to baseline levels within 24 h. A positive correlation was found between L-carnitine concentration and the antioxidant index of SOD (r = 0.992, P < 0.01), GSH-Px (r = 0.932, P < 0.01), catalase (r = 0.972, P < 0.01) or T-AOC (r = 0.934, P < 0.01). In conclusion, L-carnitine increases activities of antioxidant enzymes and the total antioxidant capacity in healthy subjects. It may be useful as a supplementary therapy for chronic illnesses involving excessive oxidative stress.

Profiling of acylcarnitines and sterols from dried blood or plasma spot by atmospheric pressure thermal desorption chemical ionization (APTDCI) tandem mass spectrometry

Free carnitine and acylcarnitines play an important role in the metabolism of fatty acids. Sterols are structural lipids found in the membranes of many eukaryotic cells, and they also have functional roles such as the regulation of membrane permeability and fluidity, activity of membrane-bound enzymes and signals transduction. Abnormal profiles of these compounds in biological fluids may be useful markers of metabolic changes. In this review, we describe the subset of the lipidome represented by acylcarnitines and sterols, and we summarize how these compounds have been analyzed in the past. Over the last 50years, lipid mass spectrometry (MS) has evolved to become one of the most useful techniques for metabolic analysis. Today, the introduction of new ambient ionization techniques coupled to MS (AMS), which are characterized by the direct desorbing/ionizing of molecules from solid samples, is generating new possibilities for in situ analysis. Recently, we developed an AMS approach called APTDCI to desorb/ionize using a heated gas flow and an electrical discharge to directly analyze sterols and indirectly investigate acylcarnitines in dried blood or plasma spot samples. Here, we also describe the APTDCI method and some of its clinical applications, and we underline the common complications and issues that remain to be resolved.

Determination of highly soluble L-carnitine in biological samples by reverse phase high performance liquid chromatography with fluorescent derivatization

This study was performed in order to validate an effective high performance liquid chromatograpy (HPLC) method to determine L-carnitine in biological samples such as plasma, milk and muscle in cows. An L-carnitine derivative for fluorescence absorption was synthesized with 1-aminoanthracene (16 mg/mL in acetone) and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDAC; 160 mg/mL in 0.01 M NaH2PO4 buffer) as a precolumn fluorescent derivative reagent. gamma-Butyrobetaine HCI was used as an internal standard. A reversed-phase column with fluorescence detection at the excitation and emission wavelengths of 248 and 418 nm were used. The mobile phase consisted of 30% acetonitrile with 0.1 M ammonium acetate in water (pH 3.5) adjusted with acetic acid and delivered at a flow rate of 1.5 mL/ min. The L-carnitine concentration in plasma, milk and muscle samples of cows after oral feeding with 24 g L-carnitine/day for 2 months was then determined. All biological samples were deproteinated by barium hydroxide and zinc sulfate heptahydrate before the derivative reaction. Blank cow plasma was dialyzed using cellulose membrane for standard calibration. The calibration curve showed good linearity (r2 > 0.999) over the concentration range of 50 to 5000 ng/mL. The precision and accuracy were also satisfactory with less than 15% intra- and interday coefficiency of variations. The peaks of L-carnitine and internal standard in HPLC chromatography were successfully separated in plasma, milk and muscle samples of cows. The current derivatization method of L-carnitine for fluorescence detection was simple and adequately sensitive and could be applied to determine L-carnitine in biological samples.

Rapid determination of short chain carnitines in human plasma by electrospray ionisation-ion trap mass spectrometry using capillary electrophoresis instrument as sampler

A capillary electrophoresis apparatus was used as sampler for flow injection analysis (FIA) in tandem mass spectrometry of L-carnitine and its acetyl- and propionyl-metabolites in human plasma. The capillary electrophoresis instrument was coupled to the ion trap mass spectrometer by an electrospray ionization coaxial sheath liquid interface. The electrophoresis capillary introduced the sample directly into the source by applying a prolonged sample injection. The use of the capillary electrophoresis apparatus miniaturised the FIA procedure, substantially reducing the quantities of solvents and samples used, and allowed rapid automated sequential analyses. The method was optimised and validated using a dialyzed human plasma matrix. The plasma samples were analysed after a simple, rapid deproteinisation procedure with acetonitrile and diluted 70 times before direct injection into the mass spectrometer for product ion scan MS/MS analysis in positive ionisation. The total analysis time was 5 min, including capillary preconditioning and acquisition time (3 min). The method was sensitive, allowing the determination of L-, L-acetyl- and L-propionyl-carnitines at 140, 14 and 3.6 nM concentrations (injected values) corresponding to lower limit of quantitation values in plasma of 10, 1 and 0.25 microM, respectively. The method was processed for full validation and applied to the analysis of L-carnitine and its short chain derivatives in human plasma samples.

Strategy for the isolation, derivatization, chromatographic separation, and detection of carnitine and acylcarnitines

A strategy for detection of carnitine and acylcarnitines is introduced. This versatile system has four components: (1) isolation by protein precipitation/desalting and cation-exchange solid-phase extraction, (2) derivatization of carnitine and acylcarnitines with pentafluorophenacyl trifluoromethanesulfonate, (3) sequential ion-exchange/reversed-phase chromatography using a single non-end-capped C8 column, and (4) detection of carnitine and acylcarnitine pentafluorophenacyl esters using an ion trap mass spectrometer. Recovery of carnitine and acylcarnitines from the isolation procedure is 77-85%. Derivatization is rapid and complete with no evidence of acylcarnitine hydrolysis. Sequential ion-exchange/reversed-phase HPLC results in separation of reagent byproducts from derivatized carnitine and acylcarnitines, followed by reversed-phase separation of carnitine and acylcarnitine pentafluorophenacyl esters. Detection by MS/MS is highly selective, with carnitine pentafluorophenacyl ester yielding a strong product ion at m/z 311 and acylcarnitine pentafluorophenacyl ester fragmentation yielding two product ions: (1) loss of m/z 59 and (2) generation of an ion at m/z 293. To demonstrate this analytical strategy, phosphate buffered serum albumin was spiked with carnitine and 15 acylcarnitines and analyzed using the described protein precipitation/desalting and cation-exchange solid-phase extraction isolation, derivatization with pentafluorophenacyl trifluoromethanesulfonate, chromatography using the sequential ion-exchange/reversed-phase chromatography HPLC system, and detection by MS and MS/MS. Successful application of this strategy to the quantification of carnitine and acetylcarnitine in rat liver is shown.

Determination of l-carnitine in food supplement formulations using ion-pair chromatography with indirect conductimetric detection

A novel method for the determination of L-carnitine in food supplement formulations was developed and validated, using ion-pair chromatography with indirect conductimetric detection. The chromatographic method was based on a non-polar (C18) column and an aqueous octanesulfonate (0.64 mM) eluent, acidified with trifluoroacetic acid (5.2 mM). The retention time was 5.4 min and the asymmetry factor 0.65. A linear calibration curve from 10 to 1000 microg/ml (r= 0.99998), with a detection limit of 2.7 microg/ml (25 microl injection volume), a repeatability %RSD of 0.8 (40 microg/ml, n = 5) and reproducibility %RSD of 2.6 were achieved. The proposed method was applied for the determination of carnitine in oral solutions and capsules. No interference from excipients was found and the only pretreatment step required was the appropriate dilution with the mobile phase. Recovery from spiked samples was ranged from 97.7 to 99.7% with a precision (%RSD, n = 3) of 0.01-2.1%.

Acetyl-L-carnitine permeability across the blood-brain barrier and involvement of carnitine transporter OCTN2

OCTN2 (SLC22A5), an organic cation/carnitine transporter, is widely distributed throughout the body, including the brain. In the present study, the involvement of OCTN2 in acetyl-L-carnitine (ALCAR) permeation across the blood-brain barrier (BBB) was examined using a microdialysis method in mouse. OCTN2 function was examined by comparison of wild-type mice with jvs mice, which express defective OCTN2 and are considered a model for primary systemic carnitine deficiency. Zero-net-flux method analysis indicated higher in vivo recovery of ALCAR and lower physiological ALCAR concentration in thalamus extracellular fluid (ECF) in jvs mice compared with wild-type mice. Externally added ALCAR showed significantly slower initial uptake across the BBB in jvs mouse. These results indicated that OCTN2 is functionally involved in ALCAR transfer across the BBB. Total radioactivity in ECF after i.v. administration of radiolabelled ALCAR remained constant for the rest of the experimental period. Accordingly, our results indicate that ALCAR is transported from blood to brain ECF by OCTN2 at least in part, and its concentration in brain ECF is regulated by other events such as protein binding and anabolic reactions in the brain, as well as by transport across the BBB.

Use of tandem mass spectrometry for multianalyte screening of dried blood specimens from newborns

BACKGROUND

Over the past decade laboratories that test for metabolic disorders have introduced tandem mass spectrometry (MS/MS), which is more sensitive, specific, reliable, and comprehensive than traditional assays, into their newborn-screening programs. MS/MS is rapidly replacing these one-analysis, one-metabolite, one-disease classic screening techniques with a one-analysis, many-metabolites, many-diseases approach that also facilitates the ability to add new disorders to existing newborn-screening panels.

METHODS

During the past few years experts have authored many valuable articles describing various approaches to newborn metabolic screening by MS/MS. We attempted to document key developments in the introduction and validation of MS/MS screening for metabolic disorders. Our approach used the perspective of the metabolite and which diseases may be present from its detection rather than a more traditional approach of describing a disease and noting which metabolites are increased when it is present.

CONTENT

This review cites important historical developments in the introduction and validation of MS/MS screening for metabolic disorders. It also offers a basic technical understanding of MS/MS as it is applied to multianalyte metabolic screening and explains why MS/MS is well suited for analysis of amino acids and acylcarnitines in dried filter-paper blood specimens. It also describes amino acids and acylcarnitines as they are detected and measured by MS/MS and their significance to the identification of specific amino acid, fatty acid, and organic acid disorders.

CONCLUSIONS

Multianalyte technologies such as MS/MS are suitable for newborn screening and other mass screening programs because they improve the detection of many diseases in the current screening panel while enabling expansion to disorders that are now recognized as important and need to be identified in pediatric medicine.

Carnitine biosynthesis in mammals

Carnitine is indispensable for energy metabolism, since it enables activated fatty acids to enter the mitochondria, where they are broken down via beta-oxidation. Carnitine is probably present in all animal species, and in numerous micro-organisms and plants. In mammals, carnitine homoeostasis is maintained by endogenous synthesis, absorption from dietary sources and efficient tubular reabsorption by the kidney. This review aims to cover the current knowledge of the enzymological, molecular, metabolic and regulatory aspects of mammalian carnitine biosynthesis, with an emphasis on the human and rat.

Carnitine biosynthesis in mammals

Carnitine is indispensable for energy metabolism, since it enables activated fatty acids to enter the mitochondria, where they are broken down via beta-oxidation. Carnitine is probably present in all animal species, and in numerous micro-organisms and plants. In mammals, carnitine homoeostasis is maintained by endogenous synthesis, absorption from dietary sources and efficient tubular reabsorption by the kidney. This review aims to cover the current knowledge of the enzymological, molecular, metabolic and regulatory aspects of mammalian carnitine biosynthesis, with an emphasis on the human and rat.

Trimethylamine oxide accumulation in marine animals: relationship to acylglycerol storagej

Trimethylamine oxide (TMAO) is a common and compatible osmolyte in muscle tissues of marine organisms that is often credited with counteracting protein-destabilizing forces. However, the origin and synthetic pathways of TMAO are actively debated. Here, we examine the distribution of TMAO in marine animals and report a correlation between TMAO and acylglycerol storage. We put forward the hypothesis that TMAO is derived, at least in part, from the hydrolysis of phosphatidylcholine, endogenous or dietary, for storage as diacylglycerol ethers and triacylglycerols. TMAO is synthesized from the trimethylammonium moiety of choline, thus released, and is retained as a compatible solute in concentrations reflecting the amount of lipid stored in the body. A variation on this theme is proposed for sharks.

Determination of L-carnitine by flow injection analysis with NADH fluorescence detection

A flow injection analysis method for determining L-carnitine is reported. The system uses the enzyme L-carnitine dehydrogenase covalently immobilized to Eupergit C. The NADH produced by the action of the enzyme, which is proportional to the L-carnitine concentration, is quantified using fluorescence detection. The system response was rapid and had a wide range of linearity. At a flow rate of 0.2 ml/min, a detection limit of 1 microM (20 pmol) was obtained for L-carnitine, peak areas were linear up to 100 microM, and samples could be injected every 4 min. The method performed well as a routine assay, showing high sensitivity (54,000 AU/microM), a precision of 0.96%, and the ability to carry out 144 consecutive assays with an RSD of 1.47% (good stability). Comparisons were made with other known methods for L-carnitine determination. Presence of D-carnitine had no effect on L-carnitine assay. The analysis was valid for determining L-carnitine concentrations in commercial pharmaceutical preparations.

Determination of carnitine and acylcarnitines in biological samples by capillary electrophoresis-mass spectrometry

Free carnitine and acylcarnitines (carnitine esters) play an important role in the metabolism of fatty acids. Metabolic disorders can be detected by abnormal levels of these compounds in biological fluids. Capillary electrophoresis-mass spectrometry has the advantage of combining an efficient separation technique with highly selective detection. Therefore, we have developed a method for the determination of carnitine and several of its esters implementing electrospray capillary electrophoresis-mass spectrometry in the positive ion selected reaction monitoring mode. A sheath-flow interface with a mixture of 2-propanol or methanol, water and acetic acid as sheath liquid and nitrogen as nebulizing gas was used. The zwitterionic analytes migrated as cations in the applied electric field using ammonium acetate-acetic acid or formic acid electrolytes. Separations were performed in aqueous, mixed organic-aqueous and non-aqueous media. The influence of the electrolyte composition on the separation efficiency was investigated. The electrospray conditions have been optimized regarding ion current stability and sensitivity. Ammonium acetate (10 mmol/l)-0.8% formic acid in water or 6.4% formic acid in acetonitrile-water (1:1) were used as running buffers for the determination of carnitine and acylcarnitines in human biological samples. Methanol extracts of dried blood spots were analyzed as well as urine and plasma following sample preparation via solid-phase or liquid-liquid extraction. Recoveries approaching 100% were achieved depending on the analytes and sample preparation procedures employed. Endogenous carnitine and acetylcarnitine were determined at concentrations between 2.7 and 108 nmol/ml in normal human urine and plasma. Other acylcarnitines were detected at levels of below the limit of detection to 12 nmol/ml. Good precision (0.8 to 14%) and accuracy (85 to 111%) were obtained; the achieved limits of quantitation (0.1 to 1 nmol/ml) are sufficient to characterize carnitine and acylcarnitine levels occurring as markers for metabolic disorders.

Direct chromatographic resolution of carnitine and O-acylcarnitine enantiomers on a teicoplanin-bonded chiral stationary phase

R-(-)-Carnitine (vitamin B(T)) plays an important role in human energy metabolism, by facilitating the transport of long-chained fatty acids across the mitochondrial membranes. Its (S)-enantiomer acts as a competitive inhibitor of carnitine acetyltransferase, causing depletion of the body R-(-)-carnitine stock. Consequently, the separation of carnitine enantiomers is very important both to study their biological activities and to control the enantiomeric purity of pharmaceutical formulations. In the present paper we describe an easy, fast and convenient procedure for the separation of the enantiomers of carnitine and O-acylcarnitines by enantioselective HPLC on a laboratory-made chiral column containing covalently bonded teicoplanin as selector. High enantioselectivity factors (alpha values ranging from 1.31 to 3.02) and short-time analyses characterize the analytical procedure; in addition, analytes are easily detected by evaporative light scattering with no need for preliminary derivatization. The effects of pH and ionic strength of the mobile phase and of the nature of the organic modifier on the enantioselective separations were also investigated.