Quantification of carnitine esters by high-performance liquid chromatography. Effect of feeding medium-chain triglycerides on the plasma carnitine ester profile

Octanoate and nonaoate oxidation increases 50-80% over the first two days of life in piglet triceps brachii and gracilis muscle strips

An in vitro muscle strip incubation system was developed to measure the rate of catabolism of 1 mmol/L [1-(14)C]octanoate, 1 mmol/L [1-(14)C]nonanoate, 1 mmol/L [9-(14)C]nonanoate, and 10 mmol/L [U-(14)C]glucose by measuring the recovery of (14)CO(2). Muscle strips (13 mm × 1.5 mm, ~50 mg) were isolated from triceps brachii and gracilis muscles of newborn and 2-d-old, small (<950 g) and large (>1450 g) piglets. The position of the (14)C label in the substrate affected the rate and amount of recovery in (14)CO(2). Therefore, comparisons were made between age groups (0 vs. 2 d old) within substrates but limited across substrates to comparisons of [1-(14)C]-labeled fatty acids. The medium-chain fatty acid (MCFA) oxidation rates [pmol/(h · mg)] in muscle strips isolated from piglets from the 2 weight groups (<950 and >1450 g) did not differ (P > 0.99), there was a trend towards a difference between triceps brachii and gracilis muscle (P = 0.09; data not shown), and there were no significant interactions involving pig weight or muscle type; therefore, results were pooled across these factors. During the first 2 d of life, MCFA oxidation [pmol/(h • · mg muscle strip)] increased (P < 0.05) 50-80%, but the glucose oxidation rate did not change (P > 0.82). By d 2, the oxidation rate of nonanoate as represented by the one carbon was 25% greater than for octanoate (P < 0.05). The conversion of [9-(14)C]nonanoate to (14)CO(2) indicated that muscle had the capacity to oxidize the propionyl-CoA produced by β-oxidation of nonanoate and that odd-chain C-9 MCFA provided anabolic carbon to the citric acid cycle.

Determination of carnitine, its short chain acyl esters and metabolic precursors trimethyllysine and gamma-butyrobetaine by quasi-solid phase extraction and MS/MS detection

For the investigation of the metabolism and biosynthesis of carnitine, sensitive determination of carnitine and its metabolic precursors, trimethyllysine and gamma-butyrobetaine, is required. We present here a new simplified method for the analysis of carnitine, its acetyl- and propyl esters, as well as trimethyllysine and gamma-butyrobetaine without need for derivatization reactions by means of normal-phase LC and electrospray ionization tandem mass spectrometry. The limits of quantification were between 5 nM for acetyl carnitine and 70 nM for carnitine. Relative standard deviations in a fivefold determination of standard solutions were between <2% for carnitine and <10% for trimethyllysine. Quantifying the formation of deuterated carnitine from deuterated gamma-butyrobetaine, this method is also suitable for the determination of the activity of gamma-butyrobetaine dioxygenase in tissues.

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.

Carnitine deficiency and supplementation do not affect the gene expression of carnitine biosynthetic enzymes in rats

Starved male weanling rats supplemented with 20 mmol/L pivalate in their drinking water exhibit significantly depressed concentrations of carnitine in tissues and plasma. In addition, pivalate supplementation has been linked with increased renal and hepatic trimethyllysine hydroxylase (TMLH) activity, whereas carnitine supplementation has been associated with significantly decreased hepatic gamma-butyrobetaine hydroxylase (BBH) activity. The purpose of this study was to determine whether pivalate or carnitine supplementation affects the activity and genetic expression of 2 enzymes of carnitine (Cn) biosynthesis, TMLH and BBH, expressed as mRNA abundance, relative to the abundance of beta-actin mRNA. Male weanling rats were administered the control treatment (C; n = 6), the pivalate treatment (P; n = 7), or the pivalate treatment plus supplemental dietary carnitine (P+Cn; n = 7). Rats in group P had elevated renal TMLH activity, relative to the other groups (P < 0.05). The groups did not differ in the abundance of renal or hepatic TMLH or BBH mRNA. A previously unreported finding was the quantifiable level of renal BBH mRNA, which was verified by direct sequencing of the BBH cDNA product amplified from kidney RNA. The groups did not differ in renal BBH mRNA abundance and renal BBH enzyme activity was not detected. Thus, the alterations in enzyme activities in the pivalate-treated rats are not regulated at the transcriptional level, and are apparently related to post-transcriptional effects on the enzymes themselves.

Quantitation of long-chain acylcarnitines by HPLC/fluorescence detection: application to plasma and tissue specimens from patients with carnitine palmitoyltransferase-II deficiency

BACKGROUND

Carnitine palmitoyltransferase-II deficiency (CPT-II deficiency) is a rare disorder of lipid metabolism, in which the accumulation of long-chain acylcarnitines is a diagnostic marker. HPLC with fluorescence detection is an attractive analysis method due to its favorable combination of sensitivity, specificity, ease of analysis and minimal capital equipment costs.

METHODS

Long-chain acylcarnitines were isolated from tissue homogenates (0.5-2 mg wet weight) or plasma (50 microl) using silica gel columns and derivatized with 2-(2,3-naphthalimino)ethyl trifluoromethanesulfonate. Quantitation was by HPLC and fluorescence detection with standard curves (0.0-5.0 nmol/ml) for myristoyl-, palmitoleoyl-, palmitoyl-, oleoyl- and stearoylcarnitine using heptadecanoylcarnitine as the internal standard.

RESULTS

Significantly greater amounts of long-chain acylcarnitines were quantified in patients with CPT-II deficiency when compared to controls; e.g. (nmol/ml in patient plasma, controls mean+/-standard deviation): myristoylcarnitine (0.3, not detectable), palmitoleoylcarnitine (0.5, 0.1+/-0.1), palmitoylcarnitine (0.9, 0.1+/-0.0), oleoylcarnitine (3.0, 0.2+/-0.1), stearoylcarnitine (0.4, not detectable).

CONCLUSIONS

This method can be used to quantitate long-chain acylcarnitines, illustrating their accumulation in CPT-II deficiency. The analysis was accomplished using inexpensive and widely available instrumentation and is appropriate for research investigators who require precise quantitation of long-chain acylcarnitines in complex biological samples.

Pharmacokinetics of L-carnitine

L-Carnitine is a naturally occurring compound that facilitates the transport of fatty acids into mitochondria for beta-oxidation. Exogenous L-carnitine is used clinically for the treatment of carnitine deficiency disorders and a range of other conditions. In humans, the endogenous carnitine pool, which comprises free L-carnitine and a range of short-, medium- and long-chain esters, is maintained by absorption of L-carnitine from dietary sources, biosynthesis within the body and extensive renal tubular reabsorption from glomerular filtrate. In addition, carrier-mediated transport ensures high tissue-to-plasma concentration ratios in tissues that depend critically on fatty acid oxidation. The absorption of L-carnitine after oral administration occurs partly via carrier-mediated transport and partly by passive diffusion. After oral doses of 1-6g, the absolute bioavailability is 5-18%. In contrast, the bioavailability of dietary L-carnitine may be as high as 75%. Therefore, pharmacological or supplemental doses of L-carnitine are absorbed less efficiently than the relatively smaller amounts present within a normal diet.L-Carnitine and its short-chain esters do not bind to plasma proteins and, although blood cells contain L-carnitine, the rate of distribution between erythrocytes and plasma is extremely slow in whole blood. After intravenous administration, the initial distribution volume of L-carnitine is typically about 0.2-0.3 L/kg, which corresponds to extracellular fluid volume. There are at least three distinct pharmacokinetic compartments for L-carnitine, with the slowest equilibrating pool comprising skeletal and cardiac muscle.L-Carnitine is eliminated from the body mainly via urinary excretion. Under baseline conditions, the renal clearance of L-carnitine (1-3 mL/min) is substantially less than glomerular filtration rate (GFR), indicating extensive (98-99%) tubular reabsorption. The threshold concentration for tubular reabsorption (above which the fractional reabsorption begins to decline) is about 40-60 micromol/L, which is similar to the endogenous plasma L-carnitine level. Therefore, the renal clearance of L-carnitine increases after exogenous administration, approaching GFR after high intravenous doses. Patients with primary carnitine deficiency display alterations in the renal handling of L-carnitine and/or the transport of the compound into muscle tissue. Similarly, many forms of secondary carnitine deficiency, including some drug-induced disorders, arise from impaired renal tubular reabsorption. Patients with end-stage renal disease undergoing dialysis can develop a secondary carnitine deficiency due to the unrestricted loss of L-carnitine through the dialyser, and L-carnitine has been used for treatment of some patients during long-term haemodialysis. Recent studies have started to shed light on the pharmacokinetics of L-carnitine when used in haemodialysis patients.

Simple method for the routine determination of betaine and N,N-dimethylglycine in blood and urine

A simple and convenient method using commercially available derivatization reagents is described for the measurement of betaine and N,N-dimethylglycine (DMG) in blood and urine. Precolumn derivatization of plasma or urine is performed directly in acetonitrile without extraction with p-bromophenacyl bromide and crown ether as catalyst. The p-bromophenacyl ester derivatives are then separated by high-performance liquid chromatography, using an isocratic system of acetonitrile and water containing choline. Effluent was monitored at 254 nm. The limit of detection was 5 μmol/L for betaine and 2 μmol/L for DMG. Analytical recovery was &gt;97% for both analytes. Total and within-day CVs were 2.0–4.4% and 0.9–2.2% for DMG. For betaine, the total and within-day CVs were 1.3–5.3% and 0.4–3.8%, respectively. The method is precise and cost-effective and has been used successfully to determine the concentrations of DMG and betaine in human plasma and urine.

New insights into the utilization of medium-chain triglycerides by the neonate: observations from a piglet model

Because of their unique digestive and metabolic properties, medium-chain triglycerides (MCT) are used in a variety of nutritional settings, including use as a readily digestible energy source for the neonate. This review examines recent findings from our laboratory related to MCT digestion and metabolism that are drawn from a neonatal piglet model, but which may be clinically relevant to human infants. We have shown that MCT utilization improves rapidly with postnatal age (within 24 h), which is likely due to the ontogeny of pancreatic lipase. Additional data delineate the dramatic effects of emulsification and fatty acid chain length (within the medium-chain family) on utilization, with the suggestion that triacylhexanoate is utilized at the highest rate. Again, these effects are likely mediated via an increase in the kinetics of digestion rather than metabolism. Indeed, using both in vitro and in vivo radiotracer techniques, we were unable to detect metabolic differences among even-chain fatty acid homologues. However, studies with isolated hepatocytes have shown greater oxidation rates of odd-chain fatty acids compared with even-chain homologues, in part as a result of the anaplerotic potential of propionyl-CoA arising from odd-carbon fatty acid oxidation. In vivo radiotracer studies also showed an improvement in octanoate oxidation to CO2, with a concomitant reduction in urinary dicarboxylic acid excretion when colostrum-deprived piglets were supplemented with L-carnitine. Further metabolic research led to the novel finding that piglets have a very limited hepatic capacity to synthesize ketone bodies, and that acetate may be a relatively important product of hepatic fatty acid oxidation in this species.

Synthesis of [9-14C]nonanoic acid via 2-thienyl(14CH3)(cyano)cuprate and its oxidation by newborn piglet muscle strips

To determine if newborn piglet muscle could oxidize propionyl-CoA formed by catabolism of odd-chain fatty acids, an odd-chain fatty acid labeled in the terminal three carbons was needed. The synthetic scheme described is based upon the displacement of a primary alkyl iodide, ethyl 8-iodooctanoate, by a [14C]methyl group via an activated 2-thienyl(14CHa)(cyano)cuprate intermediate, forming ethyl [9-14C]nonanoate. Ethyl [9-14C]nonanoate was hydrolyzed in 6 N KOH and [9-14C]nonanoic acid recovered by ion-exchange chromatography. The yield of [9-14C]nonanoic acid was 40%, based on the initial amount of [14C]methyl iodide. The cuprate and other precursors were commercially available or readily synthesized from available precursors. Mass spectroscopy of commercial and synthesized nonradioactive nonanoate determined an m/z of 159 for the product molecular ion, as expected. The 14C-labeled product phenacyl ester was found to cochromatograph in a C-18 reverse-phase HPLC system with similarly derivatized commercially obtained nonanoic acid. The synthesis should be generally applicable to labeling of compounds by displacement of primary alkyl iodides, where other reactive groups (e.g., carboxylic acid), if present, can be protected (e.g., converted to an ester). Muscle strips isolated from the triceps muscle of newborn piglets oxidized [9-14C]nonanoic acid to 14CO2. Newborn piglet muscle can oxidize propionyl-CoA produced during odd-chain fatty acid oxidation.

Acylcarnitines in intermediary metabolism

From the time of its discovery in 1905 until the first description of its deficiency in 1973, the role of carnitine in intermediary metabolism was decidedly vague. Identification of carnitine acyl transferases and their products, acylcarnitines, have paved the way to the confirmation of the importance of carnitine in the transfer of fatty acid CoAs into the mitochondrion for beta-oxidation and energy production. The elucidation of defects in fatty acid oxidation together with the concept of carnitine therapy in certain organoacidaemias have given a new meaning to the term acylcarnitine. Not only are these compounds of diagnostic importance, their formation may be part of a secondary carnitine depletion which may be brought about as a result of various medications. Recent evidence suggests that long-chain acylcarnitines are responsible for cardiac arrhythmias and other effects, both good and bad, will certainly be found. This review will attempt to highlight the importance of acylcarnitines, from their production, the difficulties in analysis, the diagnostic possibilities and their positive and negative effects on intermediary metabolism.

Carnitine affects octanoate oxidation to carbon dioxide and dicarboxylic acids in colostrum-deprived piglets: in vivo analysis of mechanisms involved based on CoA- and carnitine-ester profiles

Newborn, colostrum-deprived piglets (n = 21) were used to study the effects of L-carnitine supplementation on the in vivo oxidation of [1-14C]octanoate to CO2 and dicarboxylic acids. Pigs were fitted with arterial and bladder catheters and were infused with octanoate (supplying 35-100% of piglets' energy expenditure) and with or without valproate for a period of 24 h. After achieving steady-state octanoate oxidation, carnitine was coinfused [50 mumol/kg 0.75 prime plus 20 mumol/h.kg 0.75)], and deviations in the octanoate oxidation rate, dicarboxylic acid excretion rate, and carnitine metabolism were monitored. At the end of the 24-h infusion, samples of liver and muscle were analyzed for carnitine- and CoA-esters by HPLC. Carnitine stimulated octanoate oxidation by 7% (P < 0.05) and decreased dicarboxylic acid excretion by 45% (P < 0.05). Carnitine supplementation increased (P < 0.05) concentrations of carnitine and acetyl carnitine in hepatic tissue (three- and 55-fold, respectively) and plasma (seven- and 11-fold); whereas, muscle-carnitine concentration doubled upon carnitine supplementation, but acetyl carnitine concentration remained unaltered. Urinary excretion of acetyl and free carnitine also increased with carnitine supplementation, but accounted for < 10% of carnitine infused. Hepatic total CoA and CoA esters increased with carnitine supplementation, whereas muscle acetyl-CoA decreased. Valproate had only marginal effects on octanoate metabolism. These data confirm the hypothesis that carnitine effects the in vivo oxidation of octanoate in colostrum-deprived piglets and suggest that the effects may be mediated by aiding the export of excess acetyl groups from muscle or by enhancing uptake of octanoate into liver mitochondria.

General strategies and selection of derivatization reactions for liquid chromatography and capillary electrophoresis

The general strategies, reasons and the different possibilities for the derivatization of biomedically important compounds are reviewed. Different approaches apply for small versus large analyte molecules, different advantages and disadvantages are visualized with pre- and post-column arrangements. Particular interest is focused upon solid-phase derivatization reagents.

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.