Plasma free and total carnitine measured in children by tandem mass spectrometry

Translating the basic knowledge of mitochondrial functions to metabolic therapy: role of L-carnitine

Mitochondria play important roles in human physiological processes, and therefore, their dysfunction can lead to a constellation of metabolic and nonmetabolic abnormalities such as a defect in mitochondrial gene expression, imbalance in fuel and energy homeostasis, impairment in oxidative phosphorylation, enhancement of insulin resistance, and abnormalities in fatty acid metabolism. As a consequence, mitochondrial dysfunction contributes to the pathophysiology of insulin resistance, obesity, diabetes, vascular disease, and chronic heart failure. The increased knowledge on mitochondria and their role in cellular metabolism is providing new evidence that these disorders may benefit from mitochondrial-targeted therapies. We review the current knowledge of the contribution of mitochondrial dysfunction to chronic diseases, the outcomes of experimental studies on mitochondrial-targeted therapies, and explore the potential of metabolic modulators in the treatment of selected chronic conditions. As an example of such modulators, we evaluate the efficacy of the administration of L-carnitine and its analogues acetyl and propionyl L-carnitine in several chronic diseases. L-carnitine is intrinsically involved in mitochondrial metabolism and function as it plays a key role in fatty acid oxidation and energy metabolism. In addition to the transportation of free fatty acids across the inner mitochondrial membrane, L-carnitine modulates their oxidation rate and is involved in the regulation of vital cellular functions such as apoptosis. Thus, L-carnitine and its derivatives show promise in the treatment of chronic conditions and diseases associated with mitochondrial dysfunction but further translational studies are needed to fully explore their potential.

Determination of l-carnitine using enantioselective, potentiometric membrane electrodes based on macrocyclic antibiotics

In order to determine the enantiopurity of l-carnitine three enantioselective, potentiometric membrane electrodes were proposed for the assay of l-carnitine. The electrodes were designed using macrocyclic glycopeptide antibiotics-vancomycin and teicoplanin. Acetonitrile was added to the teicoplanine to design a modified teicoplanine based electrode. The linear concentration ranges for the proposed enantioselective membrane electrodes were 10(-4) to 10(-2)moll(-1) for electrodes based on vancomycin and teicoplanin and 10(-5) to 10(-2)moll(-1) for electrode based on teicoplanin modified with acetonitrile. The slopes of the electrodes were 56.5mV per pl-carnitine; 54.5mV per pl-carnitine and 58.3mV per pl-carnitine for vancomycin-, teicoplanin- and teicoplanin modified with acetonitrile-based electrodes, respectively. The enantioselectivity was determined over d-carnitine. The proposed electrodes could be employed reliably for the assay of l-carnitine raw material and its pharmaceutical formulation, Carnilean((R)) capsules. The surfaces of the electrodes are stable and easily renewable by polishing on alumina paper.

Biochemical characterization of L-carnitine dehydrogenases from Rhizobium sp. and Xanthomonas translucens

Recently, we obtained two L-carnitine dehydrogenases (CDHs) from soil isolates, Rhizobium sp. (Rs-CDH) and Xanthomonas translucens (Xt-CDH). The respective molecular masses of Rs-CDH and Xt-CDH were approximately 50 kDa and 37 kDa. In this study, the genes encoding both enzymes were cloned. Their primary structures exhibited high identities with those of 3-hydroxyacyl-CoA dehydrogenases. In addition, Rs-CDH had a 180-residue long extra sequence in its C-terminal region. Except for the initial 20 residues, the extra sequence exhibited similarity to thioesterase. The activity of Rs-CDH was affected only slightly by deletion of thioesterase domain, but it was eliminated by the deletion of the whole C-terminal extra sequence. A further deletion experiment indicated that the region of Ala330-Pro335 of Rs-CDH has important functions in catalytic activity. Moreover, based on the deletion experiment on Xt-CDH, the five-residue tail is considered to have a function similar to Ala330-Pro335 of Rs-CDH.

Use of diagnostic neutral losses for structural information on unknown aromatic metabolites: an experimental and theoretical study

This work presents the use of neutral losses (NL) for the identification of compounds related to the metabolism of tyrosine. The mass spectra of all the studied compounds, recorded at several collision energies, are compared. The fragmentation mechanism of protonated molecules, MH(+), is explained by combining collision-induced dissociation (CID) mass spectra and density functional theory (DFT) calculations. The results show that the first fragmentation is the elimination from MH(+) of a neutral molecule including a functional group of the linear chain. Three primary neutral losses are observed: 17 u (NH(3)), 18 u (H(2)O) and 46 u (H(2)O+CO) characterizing amino, hydroxyl and carboxylic functions on the linear chain. The presence and abundance of ions corresponding to these losses are dependent on (i) the position of the functional group on the linear chain, (ii) the initial localisation of the protonating hydrogen, and (iii) the substitution of the aromatic ring. For compounds including a functional group on the benzylic carbon atom, the investigation of the other functions requires the knowledge of secondary fragmentations. Among these secondary fragmentations we have retained the loss of NH(3) from [MH-18u](+) and the loss of ketene from [MH-17u](+). Experimentally these fragmentations are detected using losses of 35 u and 59/73 u. In other words, NL35 identifies hydroxy and amino compounds and NL 46 and/or NL59/73 identify carboxylic acids. The search for characteristic neutral losses is used for the analysis of compounds in a mixture and the analysis of biological fluid. We show that selective search of several neutral losses allows also the unambiguous differentiation of isomers and gives the opportunity to identify compounds in biological fluids.

[Determination of normal acylcarnitine levels in a healthy pediatric population as a diagnostic tool in inherited errors of mitochondrial fatty acid beta-oxidation]

INTRODUCTION

Acylcarnitine measurement in blood is a useful test for the diagnosis of inherited errors of mitochondrial fatty acid beta-oxidation. However, there are few data in the literature on the reference ranges of the various acylcarnitines and on whether these reference ranges are age- or sex-dependent.

OBJECTIVES

To draw attention to inherited errors of mitochondrial fatty acid beta-oxidation and to establish reference acylcarnitine values in children.

PATIENTS AND METHODS

A total of 309 blood samples from healthy children divided into four age groups (group A: <1 month; group B: 1-12 months; group C: 1-7 years; group D: 7-18 years) were obtained and analyzed using tandem mass spectrometry.

RESULTS AND CONCLUSION

Reference acylcarnitine values in children are provided. No significant differences were found in relation to age or sex. Our results differ from those reported in the literature reviewed. Importantly, hydroxyacylcarnitines and glutaryl carnitine are absent when normal samples are processed. We review the literature on the main clinical and laboratory findings in mitochondrial fatty acid beta-oxidation deficiencies.

Carnitine-associated encephalopathy caused by long-term treatment with an antibiotic containing pivalic acid

An 18-month-old boy was treated with an antibiotic containing pivalic acid for 6 months for intractable otitis media and then developed repeated convulsions and loss of consciousness. Laboratory data showed hypoglycemia and hypocarnitinemia. Intravenous administration of glucose was ineffective against the seizures and loss of consciousness. However, the patient regained consciousness and recovered soon after intravenous infusion of carnitine. To our knowledge, intravenous carnitine administration that contributed to marked improvements in neurologic deficit caused by administration of an antibiotic containing pivalic acid has not been reported previously. These findings indicate that long-term use of such antibiotics should be avoided.

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.

Determination of free and total carnitine in plasma by an enzymatic reaction and spectrophotometric quantitation spectrophotometric determination of carnitine

OBJECTIVES

Carnitine initiates the beta-oxidation of fatty acids and its deficiency is a problem in several metabolic diseases. This work describes a validated quick and simple enzymatic assay for the determination of free and total carnitine in plasma.

METHODS

Carnitine reacts with acetyl-CoA catalized by carnitine acetyltransferase. The coenzyme A liberated combines with 5,5'-dithiobis-(2-nitrobenzoic acid) and forms thiophenolate ion, measured spectrophotometrically at 412 nm. The method requires precipitation of proteins and the alkaline hydrolysis of acylcarnitines for total carnitine.

RESULTS

The detection limit is 1.7 micromol/L in plasma and inter- and intra-day imprecision is less than 5%. The recovery of spiked plasma samples is 97%. The method was tested with an inter-laboratory assay, yielding excellent correlation (r(2)=0.994), and it was applied to the determination of normal values of carnitine in plasma.

CONCLUSIONS

A reliable spectrophotometric method has been validated with very good precision, with simple instrumental and easy sample preparation.

Determination of total and free plasma carnitine concentrations on the Dade Behring Dimension RxL: Integrated chemistry system

BACKGROUND

L-carnitine is a naturally occurring quaternary ammonium compound present in all mammalian species. Its major function is to facilitate the passage of long-chain fatty acids through the mitochondrial membrane for subsequent beta-oxidation and ketone synthesis. Clinical interest in carnitine disorders relates particularly to possible deficiency states that may result in a phenotypic spectrum that includes cardiomyopathy, skeletal myopathy, hypoglycemia and hyperammonemia. The objective of this study was to develop a method on the Dade Behring Dimension RxL analyzer for measuring free and total carnitine levels in plasma.

METHODS

Plasma samples were deproteinized by ultrafiltration to remove interference by endogenous thiols. Filtrates were measured directly on the RxL for free carnitine or after alkaline hydrolysis for total carnitine by an endpoint enzymatic assay that uses carnitine acetyltransferase.

RESULTS

Within-run imprecision was <5% at high and low levels for both free and total carnitine while between-day imprecision was <15%. Recovery of free carnitine from spiked plasma >90%. The method was linear between 5.0 and 150.0 micromol/l and the limit of quantification was 5.0 micromol/l. Comparison of our method with another automated procedure developed on the Hitachi 917 system using Deming regression analysis resulted in the following equations: Dimension=1.034(Hitachi)-7.44 for total carnitine (r=0.955) and Dimension=0.805(Hitachi)+1.96 for free carnitine (r=0.951), respectively.

CONCLUSIONS

Our method is suitable for analyzer platforms where the level of imprecision is lower and the throughput is higher than manual methods. It also avoids the use of radioisotopes and is appropriate in labs where access to reference methods such as tandem mass spectrometry and HPLC is limited or unavailable.

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.