Measurement of carnitine and O-acylcarnitines

Mass spectrometry with derivatization method for concurrent measurement of amino acids and acylcarnitines in plasma of diabetic type 2 patients with diabetic nephropathy

BACKGROUND

Amino acids (AAs) and acylcarnitines play a key role in metabolic disease and can be used as biomarkers of various diseases such as malignancies, type 2 diabetes (T2D), insulin resistance, and cardiovascular diseases, therefore, designing an accurate and simple laboratory method that simultaneously measure both groups of substances, could improve the process of analytes quantification. In this research, a flow injection tandem mass spectrometry (FI-MS/MS) method for simultaneous measurement of AAs and acylcarnitines in addition to results of validation is explained.

METHODS

Samples were mixed with internal standards and after derivatization (with butanolic-HCL), AAs, and acylcarnitines were quantified by tandem mass spectrometry (SCIEX API 3200). Analytical performance studies were designed based on the Clinical and Laboratory Standards Institute (CLSI) guidelines including precision, accuracy, linearity, and limit of detection-quantification (LOD-LOQ) experiments. Samples from patients with T2D in different stages of kidney disease were also analyzed to ensure the clinical usage of the method.

RESULTS

Performance evaluation of the method demonstrated adequate results. The mean of estimated inter-assay precision (reported as a coefficient variation) for AAs and acylcarnitines were less than 8.7% and 12.3%, the estimated mean bias was below 8.8% and 10.2% respectively. LOD of analytes ranged between 0.6-10 μmol per liter (μmol/L) for AAs and 0.02-1 μmol/L for acylcarnitines. LOQ analytes showed a range of 2-25 μmol/L and 0.05-5 μmol/L for AAs and carnitine/acylcarnitines respectively. In diabetic patients sample analysis, a significant increase in acylcarnitines (C2, C4, C5DC, C6, C8, C10, C14) and citrulline with a significant decrease in valine were seen in patients with severely increased albuminuria.

CONCLUSION

FI-MS/MS method with pre-injection derivatization with butanolic-HCL can be used for concurrent measurement of AAs and carnitine/acylcarnitines in a short time and it satisfies the analytical performance requirements. This method is applied for AAs and carnitine/acylcarnitines measurement in patient with T2DM and results show some of the acylcarnitines and AAs can be involved in diabetic nephropathy development.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s40200-021-00786-3.

Advances in Clinical Mass Spectrometry

Although mass spectrometry has been used clinically for decades, the advent of immunoassay technology moved the clinical laboratory to more labor saving automated platforms requiring little if any sample preparation. It became clear, however, that immunoassays lacked sufficient sensitivity and specificity necessary for measurement of certain analytes or for measurement of analytes in specific patient populations. This limitation prompted clinical laboratories to revisit mass spectrometry which could additionally be used to develop assays for which there was no commercial source. In this chapter, the clinical applications of mass spectrometry in therapeutic drug monitoring, toxicology, and steroid hormone analysis will be reviewed. Technologic advances and new clinical applications will also be discussed.

Changes in carnitine octanoyltransferase activity induce alteration in fatty acid metabolism

The peroxisomal beta oxidation of very long chain fatty acids (VLCFA) leads to the formation of medium chain acyl-CoAs such as octanoyl-CoA. Today, it seems clear that the exit of shortened fatty acids produced by the peroxisomal beta oxidation requires their conversion into acyl-carnitine and the presence of the carnitine octanoyltransferase (CROT). Here, we describe the consequences of an overexpression and a knock down of the CROT gene in terms of mitochondrial and peroxisomal fatty acids metabolism in a model of hepatic cells. Our experiments showed that an increase in CROT activity induced a decrease in MCFA and VLCFA levels in the cell. These changes are accompanied by an increase in the level of mRNA encoding enzymes of the peroxisomal beta oxidation. In the same time, we did not observe any change in mitochondrial function. Conversely, a decrease in CROT activity had the opposite effect. These results suggest that CROT activity, by controlling the peroxisomal amount of medium chain acyls, may control the peroxisomal oxidative pathway.

Evaluations of labrador retrievers with exercise-induced collapse, including response to a standardized strenuous exercise protocol

Clinical and metabolic variables were evaluated in 14 Labrador retrievers with exercise-induced collapse (EIC) before, during, and following completion of a standardized strenuous exercise protocol. Findings were compared with previously reported variables from 14 normal Labrador retrievers that participated in the same protocol. Ten of 14 dogs with EIC developed an abnormal gait during evaluation, and these dogs were significantly more tachycardic and had a more severe respiratory alkalosis after exercise compared to the normal dogs. Muscle biopsy characteristics and sequential lactate and pyruvate concentrations were normal. Genetic testing and linkage analysis excluded malignant hyperthermia as the cause of EIC. Common causes of exercise intolerance were eliminated, but the cause of collapse in EIC was not determined.

Pyruvate dehydrogenase deficiency in a Sussex spaniel

A two-year-old, intact female Sussex spaniel was presented with signs of exercise intolerance. Pre- and post-exercise serum lactate and pyruvate concentrations and urinary organic acid screening supported a diagnosis of pyruvate dehydrogenase deficiency, as previously reported in this breed. Dietary therapy was initiated for six months, during which time there was no reported clinical deterioration. A full neurological examination and repeat evaluation of lactate and pyruvate concentrations before and after exercise was conducted one year after diagnosis, at which time the patient had been without dietary modification for six months and had developed more severe exercise intolerance along with evidence of central nervous system dysfunction.

L-2-Hydroxyglutaric aciduria in Staffordshire Bull Terriers

L-2-Hydroxyglutaric aciduria is an inborn error of metabolism, which has been recognized in humans since 1980. The metabolic defect responsible for the disease is unknown, but the disorder can be diagnosed in humans by elevations of the organic acid, L-2-hydroxyglutaric acid in the cerebrospinal fluid (CSF), plasma, and urine of affected patients. The disorder produces a variety of clinical neurological defects in humans including psychomotor retardation, seizures, and ataxia. There have previously been no recognized animal models of the disease. However, 6 Staffordshire Bull Terriers were recently identified with the disorder. The animals presented with a variety of clinical signs, most notably seizures, ataxia, dementia, and tremors. They were all screened for organic acid abnormalities in urine, and CSF and plasma (when available). Levels of L-2-hydroxyglutaric acid were elevated in all body fluids evaluated. The clinical, clinicopathologic, and magnetic resonance imaging (MRI) characteristics associated with L-2-hydroxyglutaric acid in Stafforshire Bull Terriers is reported herein and represents the first veterinary model of this inborn error of metabolism.

Lipid storage myopathy in a cocker spaniel

A six-year-old male cocker spaniel was presented to the Veterinary Medical Teaching Hospital, University of Florida, with a three-week history of generalised weakness and myalgia. Electrodiagnostic evaluation, cerebrospinal fluid analysis and thoracolumbar myelography were unremarkable. Biopsies from vastus lateralis and triceps muscles revealed numerous large lipid droplets within type 1 fibres and to a lesser degree within type 2 fibres. The resting plasma lactate was mildly increased and there was elevated urinary excretion of lactic, pyruvic and acetoacetic acids, increased urinary excretion of carnitine esters, and increased plasma alanine. This pattern of metabolite excretion is consistent with an, as yet undefined, block in oxidative metabolism.

Analysis of organic acids, amino acids, and carnitine in dogs with lipid storage myopathy

Abnormal accumulations of lipid droplets, localized predominantly in histochemical type 1 fibers, were observed in fresh frozen sections of muscle biopsies from 25 dogs with myalgia, weakness, and muscle atrophy. Compared to controls, lactic acidemia, hyperalaninemia, lactic and pyruvic aciduria, variably increased urinary excretion of carnitine esters, and muscle carnitine deficiency were present. These findings support a metabolic block in oxidative metabolism resulting in lactic acidemia in dogs with lipid storage myopathy.

Juvenile-onset distal myopathy in Rottweiler dogs

Two juvenile Rottweiler siblings were presented with the complaint of decreased activity and various postural abnormalities, including plantigrade and palmigrade stance and splayed forepaw digits. The neurologic examinations were otherwise normal. Electromyography revealed rare fibrillation potentials and positive sharp waves. Motor nerve conduction velocities were normal, whereas compound muscle action potentials from the interosseous muscles were decreased. These findings were consistent with a primary myopathy. A 3rd pup from a different litter and a 4th pup from a litter with 3 of 8 affected dogs had similar clinical presentations. Histopathologic changes in fresh-frozen muscle biopsy samples were similar in all pups and consisted of myofiber atrophy with mild myonecrosis, endomysial fibrosis and replacement of muscle with fatty tissue. These changes were more severe in distal muscles than in proximal muscles. Plasma carnitine concentrations (total and free) were decreased in all pups. Muscle carnitine concentrations (total and free) were decreased in 3 of 4 pups and the least affected pup had a borderline low free muscle carnitine concentration. Abnormalities involving major metabolic pathways were not found on quantification of organic and amino acids. Dystrophin immunocytochemistry was normal in 2 dogs tested. Distal myopathies in humans are classified under the dystrophic group of muscle disorders. These 4 cases represent a form of muscular dystrophy apparently not previously reported in dogs.

Methylmalonic and malonic aciduria in a dog with progressive encephalomyelopathy

A 12 week old female Labrador retriever dog with signs of progressive diffuse degeneration of the brain and spinal cord was found to have methylmalonic and malonic aciduria. Over a 5 month period, the dog developed neurologic signs compatible with disease of the central nervous system with predominant diffuse cerebral and right lateralizing brainstem deficits. Gross pathological examination of the brain showed that the lateral, third, and fourth ventricles of the brain were markedly enlarged and associated with white and grey matter atrophy. Syringomyelia and hydromyelia of the central canal into the dorsal funiculus of the spinal cord beginning at the level of the cervical intumescence and extending to the lumbar intumescence was also present. Significant biochemical abnormalities include methylmalonic and malonic aciduria, mild lactic and pyruvic aciduria. There was also accumulation of citric acid cycle intermediates including succinic, aconitic, and fumaric acids. Disordered fatty acid oxidation was suggested by increased excretion of adipic, ethylmalonic, suberic and sebacic acids. Neither ketoacidosis nor hyperammonemia were present, and serum cobalamin levels were normal. Overall, this dog demonstrates an inborn error of metabolism resulting in abnormal organic acid accumulation associated with a neurodegenerative disease.

Carnitine, carnitine acetyltransferase, and glutathione in Alzheimer brain

Glutathione and "total" carnitine (i.e., free carnitine plus acid-soluble carnitine esters) were measured in an affected (superior frontal gyrus; SFG) and unaffected (cerebellum: CBL) region of Alzheimer disease (AD) and control brains. Average glutathione content in AD SFG (n = 13) and AD CBL (n = 7) (7.9 +/- 2.1 and 11.9 +/- 4.0 nmol/mg protein, respectively (mean +/- S.D.)) was similar to that in control SFG (n = 13) and CBL (n = 6) (7.7 +/- 2.0 and 11.6 +/- 2.6 nmol/mg protein, respectively). However, glutathione increased significantly with age in AD brain (p = 0.003) but not in control brain. Average total carnitine in AD SFG (84 +/- 47 pmol/mg protein; n = 10) and AD CBL (108 +/- 86 pmol/mg protein; n = 7) was not significantly different from that in the corresponding regions of control brain (148 +/- 97 (n = 10) and 144 +/- 107 (n = 6) pmol/mg protein, respectively). However, a significant decline of total carnitine with age in both regions was noted for AD brain, but not for control brain. Carnitine acetyltransferase activity in the AD SFG (n = 13) was not significantly different from that of control SFG (n = 13) (1.83 +/- 1.05 and 2.04 +/- 0.82 nmol/min/mg protein, respectively). However, carnitine acetyltransferase activity of AD CBL (n = 7) was significantly lower than that of control CBL (n = 6) (1.33 +/- 0.88 versus 2.26 +/- 0.66 nmol/min/mg protein; p = 0.05).

Organic acidurias and related abnormalities

Organic acid analysis is a powerful technique in the diagnosis of inborn errors of metabolism. Since the development of the technique over twenty-five years ago, it has evolved into a sophisticated and powerful method and is an essential tool in the diagnosis of the organic acidurias. The chemistry and biochemistry of organic acids, as well as sample preparation, instrumentation, and many aspects of the more commonly used methods for the analysis of these compounds, are reviewed. The biochemical and clinical characteristics of each of the primary organic acidurias are described. In addition, the various noninherited causes of secondary organic acidurias that lead to the excretion of abnormal organic acids are also described, and ways of differentiating primary from secondary causes are discussed.

Gas chromatographic profiling and determination of urinary acylcarnitines

A method is described for the routine profiling and determination in urine of most of the acylcarnitines clinically relevant for the diagnosis of organic acidurias. The procedure, which does not require expensive apparatus, involves extraction of the acylcarnitines on strong cation-exchange disposable columns, mild alkaline hydrolysis and gas chromatography of the liberated monocarboxylic acids. The different steps were optimized in order to increase the analytical performance. No significant interferences were encountered, the limit of detection (signal-to-noise ratio = 3:1) ranged from 0.1 to 4 mg/l and the between-day coefficient of variation from 3.6 to 17.7%, depending on the acyl species. The rapidity of the method results from the application of a single solid-phase extraction on disposable columns. The acyl moieties are chromatographed underivatized in order to permit the identification of short-, medium- and long-chain acylcarnitines. The method was assessed by analysing fourteen urine specimens from patients presenting an organic aciduria.

Histopathological findings in a male with late-onset ornithine transcarbamylase deficiency

Late onset of symptoms in a 12 1/2-year-old male with ornithine transcarbamylase (OTC) deficiency were associated with unusual histological features in the liver. The patient presented with an acute onset of hyperammonemia and altered mental status after a 2-day prodrome of vomiting and lethargy. Physical examination showed a combative and disoriented male with icteric sclerae but with no fever or hepatomegaly. The plasma ammonia level was 282 microM. Enzyme assays of liver tissue obtained by percutaneous needle biopsy showed OTC activity of approximately 3% of normal; carbamyl phosphate synthetase was normal. Histopathological findings included severe microvesicular centrilobular steatosis. Hepatic architecture and reticulin framework were well preserved. Many hepatocyte nuclei were filled with glycogen. Electron microscopy showed mitochondria that were rounded and expanded with cristae at the edge of the mitochondrial membrane. In contrast to other reports, only slight variations in size and shape were seen. Megamitochondria and intramatrical paracrystalline inclusions were not identified. The cytoplasm contained scattered fat globules, peroxisomes, and dilated smooth endoplasmic reticulum. The prominent mitochondrial abnormalities commonly found in OTC deficiency were notably absent.

Oxidative stress: free radical production in neural degeneration

It is not yet established whether oxidative stress is a major cause of cell death or simply a consequence of an unknown pathogenetic factor. Concerning chronic diseases, as Parkinson's and Alzheimer's disease are assumed to be, it is possible that a gradual impairment of cellular defense mechanisms leads to cell damage because of toxic substances being increasingly formed during normal cellular metabolism. This point of view brings into consideration the possibility that, besides exogenous factors, the pathogenetic process of neurodegeration is triggered by endogenous mechanisms, either by an endogenous toxin or by inherited metabolic disorders, which become progressively more evident with aging. In the following review, we focus on the oxidative stress theory of neurodegeneration, on excitotoxin-induced cell damage and on impairment of mitochondrial function as three major noxae being the most likely causes of cell death either independently or in connection with each other. First, having discussed clinical, pathophysiological, pathological and biochemical features of movement and cognitive disorders, we discuss the common features of these biochemical theories of neurodegeneration separately. Second, we attempt to evaluate possible biochemical links between them and third, we discuss experimental findings that confirm or rule out the involvement of any of these theories in neurodegeneration. Finally, we report some therapeutic strategies evolved from each of these theories.

Carnitine in adolescents

Carnitine, abundant in muscle and other tissues, is normally synthesized from lysine. Its major metabolic role is transport of long-chain fatty acids into muscle where beta oxidation takes place. Normative data are available for carnitine levels, but few studies define carnitine of specific age groups. This report provides data on normal male and female adolescents in relationship to age and pubertal status. The population consisted of adolescents between 10 and 18 years, seen either for routine health maintenance or minor illnesses. Concentrations of carnitine in plasma were determined by radioisotype enzyme assay. The overall mean was 45.3 +/- 8.9 mumol/L for free carnitine. There was a trend of decreasing free carnitine with increasing Tanner stage. Among the females, there was a statistically significant correlation with both age and Tanner stage. In contrast, the data for males indicated no statistically significant correlation. The data provided standards for the evaluation of carnitine status in normal adolescents. Total carnitine in plasma ranged from 43.2 to 55.9 mumol/L and was virtually all free carnitine. The significant negative correlation of concentration of free carnitine with age and pubertal stage in females was interesting, although not of clinical importance. It suggests the possibility of female hormonal influence associated with puberty.

Synthesis, characterisation and high-performance liquid chromatography of C6-C16 dicarboxylyl-mono-coenzyme A and -mono-carnitine esters

The synthesis and purification of the mono-coenzyme A and mono-carnitine esters of the homologous series of straight-chain even-numbered dicarboxylic acids (C6-C16) is described. The corresponding 3-hydroxyacyl- and 2-enoyl-CoA esters were prepared enzymatically. A reversed-phase high-performance liquid chromatographic (HPLC) system for the analysis of the intact CoA esters is described and their chromatographic behaviour documented. Reversed-phase HPLC systems for the analysis of the 4-bromophenacyl derivatives of the dicarboxylyl-mono-carnitines and the 4-nitrobenzyl derivatives of the free acids are also described. Some preliminary studies of the metabolism of [U-14C]hexadecanedionoyl-mono-CoA by rat liver peroxisomes and rat skeletal muscle mitochondria are described illustrating the application of these methods.

Chromatographic techniques in inborn errors of metabolism

Chromatography has played a pivotal role in the advances made during the last 30 years in our knowledge of inborn errors of metabolism. This review discusses the application of some of these techniques to the analysis of organic acids and acylcarnitines. The separation of organic acids needed a comprehensive approach that would permit all of the many organic acids present in urine or other complex mixtures to be extracted, analysed and identified in a single run. This required analytical methods of great resolving power, wide linear range and universal detectors such as gas chromatography (GC), or GC coupled with mass spectrometry. Sample preparation was another problem that has been tackled by a variety of approaches. Organic solvents have been employed widely for the extraction of organic acids from physiological fluids. Unfortunately, recoveries of the different organic acids by this method are sometimes less than quantitative and variable depending on the compound. Other methods, such as the use of DEAE-Sephadex columns, have the advantage of resulting in close to 100% recoveries, but are more tedious. Liquid partition chromatography on short silicic acid columns has also been recommended as a useful clean-up step prior to GC, permitting both the identification and quantitation of organic acids in urine, plasma or amniotic fluid. Although many derivatization procedure have been used to prepare organic acids for gas chromatography, the most common is trimethylsilylation. Oxo acids are usually reacted with one of several commonly used reagents to form oximes. GC analysis of organic acids was initially done using packed columns with methylsilicone-based, non-polar stationary phases.(ABSTRACT TRUNCATED AT 250 WORDS)

Aminocarnitine and related compounds as inhibitors of carnitine transferases: physiologic implications

beta-Aminocarnitine and its N-acetyl and N-palmitoyl amides were examined as inhibitors of carnitine acetyltransferase, carnitine palmitoyltransferase, and of fatty-acid oxidation in whole animals, tissues, and hepatic microsomal systems. Results were consistent with subsequent findings that aminocarnitine and palmitoylcarnitine have significant antiketogenic and hypoglycemic effects in experimental animals.

Chromatographic and non-chromatographic assay of L-carnitine family components

L-Carnitine and its acyl esters constitute an endogenous pool of the L-carnitine family, involved in the uptake of free fatty acids in the mitochondria by transfer across their membrane of the acyl moieties to fuel the beta-oxidation and the release of the acetyl group from the mitochondria to the cytosol. Therefore acyl-L-carnitine and acyl-L-carnitine transferase are involved in a homeostatic equilibrium with the cells. As most of these substances need to be monitored in foods, chemical and pharmaceutical processes and biological fluids, an overview of the main methods for assaying them is provided here, with specific reference to the intrinsic performance of each analytical procedure and with suggestions on the correct storage and manipulation of analytical samples.

Measurement of L-carnitine and acylcarnitines in body fluids and tissues in children and in adults

We have developed a reliable and validated radio-enzymatic method for the assay of L-carnitine and acylcarnitines, using a modification of existing methods. The sensitivity of the assay is 10 mumol/l using 10 microliters of plasma or urine. It is also suitable for measurements of carnitine in a 10 mg sample of liver or muscle obtained by percutaneous biopsy. The use of N-ethylmaleimide in the reaction mixture together with an excess of [1-14C]acetyl CoA ensures that the reaction proceeds to completion and a linear response is obtained. Using this method control ranges have been established for plasma and urine carnitine concentrations in healthy children and adults, and for the carnitine content of liver and muscle in adults. No significant difference was found between fasting and post-prandial plasma carnitine levels. An age-related increase was found in urinary total carnitine and acylcarnitine concentration throughout childhood. These data provide a reliable basis for studies of patients with abnormal carnitine and acylcarnitine metabolism, distribution and excretion.

Effect of hypoxia on pig heart short-chain acylcarnitines

The right ventricles of pig heart were perfused with hypoxic blood and the left ventricles were perfused with normally ventilated arterial blood. Free carnitine and short-chain acylcarnitines in hypoxic ventricles were lower than in perfused controls, and much lower than in non-perfused heart. Acetylcarnitine levels decreased and the branched-chain acylcarnitines and propionylcarnitine were elevated in the hypoxic perfused ventricles. These data indicate that both hypoxia and anaesthesia caused loss of carnitine and short-chain acylcarnitines from the heart and hypoxia also changed the distribution of short-chain acylcarnitines in the heart.

The effect of intravenous L-carnitine on propionic acid excretion in acute propionic acidaemia

A 6-week-old female infant presented in a severe metabolic crisis from propionic acidaemia. The condition was aggravated by pneumonia and heart insufficiency. In addition to the general supportive measures and caloric intake exclusively from glucose, intravenous L-carnitine treatment (2 g L-carnitine/24 h) was started to enhance propionic acid excretion as a carnitine conjugate. Despite the therapeutic efforts the patient died about 48 h after admission in sudden respiratory arrest and bradycardia. Serum propionic acid concentration was increased to 0.3 mumol/ml. Propionylcarnitine excretion was measured and about 55% of the overall excretion during the 48 h treatment period was attributed to an effect of carnitine administration. 2-methylcitrate and 2-methyl-3-oxovaleric acid excretion decreased during the same period. Obviously carnitine was not able to prevent metabolic deterioration but may provide some additional "buffer capacity" during long-term dietary treatment.