Gas-liquid chromatography of alpha-keto acids: quantification of the branched-chain-alpha-keto acids from physiological sources

Determination of Branched-Chain Keto Acids in Serum and Muscles Using High Performance Liquid Chromatography-Quadrupole Time-of-Flight Mass Spectrometry

Branched-chain keto acids (BCKAs) are derivatives from the first step in the metabolism of branched-chain amino acids (BCAAs) and can provide important information on animal health and disease. Here, a simple, reliable and effective method was developed for the determination of three BCKAs (α-ketoisocaproate, α-keto-β-methylvalerate and α-ketoisovalerate) in serum and muscle samples using high performance liquid chromatography-quadrupole time-of-flight mass spectrometry (HPLC-Q-TOF/MS). The samples were extracted using methanol and separated on a 1.8 μm Eclipse Plus C18 column within 10 min. The mobile phase was 10 mmol L⁻¹ ammonium acetate aqueous solution and acetonitrile. The results showed that recoveries for the three BCKAs ranged from 78.4% to 114.3% with relative standard deviation (RSD) less than 9.7%. The limit of quantitation (LOQ) were 0.06~0.23 μmol L⁻¹ and 0.09~0.27 nmol g⁻¹ for serum and muscle samples, respectively. The proposed method can be applied to the determination of three BCKAs in animal serum and muscle samples.

Quantification and mass isotopomer profiling of α-keto acids in central carbon metabolism

Mass spectrometry has been established as a powerful and versatile technique for studying cellular metabolism. Applications range from profiling of metabolites to accurate quantification and tracing of stable isotopes through the biochemical reaction network. Despite broad coverage of central carbon metabolism, most methods fail to provide accurate assessments of the α-keto acids oxaloacetic acid, pyruvate, and glyoxylate because these compounds are highly reactive and degraded during sample processing and mass spectrometric measurement. We present a derivatization procedure to chemically stabilize these compounds readily during quenching of cellular metabolism. Stable derivatives were analyzed by ultrahigh pressure liquid chromatography coupled tandem mass spectrometry to accurately quantify the abundance of α-keto acids in biological matrices. Eventually, we demonstrated that the developed protocol is suited to measure mass isotopomers of these α-keto acids in tracer studies with stable isotopes. In conclusion, the here described method fills one of the last technical gaps for metabolomics investigations of central carbon metabolism.

Quantification of branched-chain keto acids in tissue by ultra fast liquid chromatography-mass spectrometry

Branched-chain keto acids (BCKAs) are associated with increased susceptibility to several degenerative diseases. However, BCKA concentrations in tissues or the amounts of tissue available are frequently at the limit of detection for standard plasma methods. To accurately and quickly determine tissue BCKAs, we have developed a sensitive ultra fast liquid chromatography-mass spectrometry (UFLC-MS) method. BCKAs from deproteinized tissue extractions were o-phenylenediamine (OPD) derivatized, ethyl acetate extracted, lyophilized in a vacuum centrifuge, and reconstituted in 200 mM ammonium acetate. Samples were injected onto a Shimadzu UFLC system coupled to an AB-Sciex 5600 Triple TOF mass spectrometer instrument that detected masses of the OPD BCKA products using a multiple reaction monitoring method. An OPD-derivatized (13)C-labeled keto acid was used as an internal standard. Application of the method for C57BL/6J (wild-type) and PP2Cm knockout mouse tissues, including kidney, adipose tissue, liver, gastrocnemius, and hypothalamus, is shown. The lowest tissue concentration measured by this method was 20 nM, with the standard curve covering a wide range (7.8-32,000 nM). Liquid chromatography-mass spectrometry run times for this assay were less than 5 min, facilitating high throughput, and the OPD derivatives were found to be stable over several days.

Determination of stable isotopic enrichment and concentration of glycerol in plasma via gas chromatography-mass spectrometry for the estimation of lipolysis in vivo

Measuring glycerol's rate of appearance into the plasma compartment provides an excellent estimation of whole-body lipolysis. The glycerol rate of appearance can be calculated by estimating the plasma dilution of continuously infused stable or radioactive isotopes of glycerol. Previously, determination of glycerol stable isotopic enrichment has required either chemical ionization gas chromatography-mass spectrometry (GC-MS) or electron impact ionization GC-MS in which a fragment containing only a portion of the glycerol molecule was measured. The present method uses tert.-butyldimethylsilyl (tBDMS) derivatization and electron impact ionization to measure a fragment including the entire glycerol molecule. The method determines concentration and enrichment of plasma glycerol in a simple, precise, and cost-efficient manner, providing a basis from which lipid homeostasis can be assessed.

Gas chromatographic-mass spectrometric determination of urinary oxoacids using O-(2,3,4,5,6-pentafluorobenzyl)oxime-trimethylsilyl ester derivatization and cation-exchange chromatography

We introduced a new combined method to isolate, purify and quantify oxoacids in human urine. Preparation of O-(2,3,4,5,6-pentafluorobenzyl) oximes of oxoacids at pH 2 to 3 was followed by cation-exchange column chromatography for removing the biological interferences. The effluent with water was extracted with ethyl acetate and the oxoacids were quantitatively converted into their trimethylsilyl derivatives for detection by gas chromatography-mass spectrometry. Good quality control data were obtained through precision and accuracy tests. Analytical recoveries (53.5-99.8%) were quantitative for a wide variety of oxoacids. This method was used for the measurement of 18 oxoacids in the urine of healthy volunteers.

Removal of branched-chain amino acids and alpha-ketoisocaproate by haemofiltration and haemodiafiltration

Venovenous haemofiltration (VVHF) and haemodiafiltration (VVHDF) were performed with a neonatal haemo(dia)filter (Miniflow 10, Hospal) on 8 anaesthetized rabbits infused with branched-chain amino acids (leucine, isoleucine and valine) and alpha-ketoisocaproate. The branched-chain amino acids (BCAA) and alpha-ketoisocaproate blood levels were close to those previously observed in neonates with maple syrup urine disease when extracorporeal blood purification was required. VVHF and VVHDF performances were assessed with two different blood flows (Qb = 8.3 and 16.6 ml/min). VVHDF was performed with four dialysate flow rates (Qd = 0.5, 1.0, 2.0 and 3.0 L/h). Within each period, clearances of the three BCAA were strictly similar. BCAA clearances obtained by VVHF were similar to ultrafiltration rates (respectively, 0.78 +/- 0.14 and 1.79 +/- 0.28 ml/min at high and low Qb; p < 0.05). The alpha-ketoisocaproate clearances obtained by VVHF were 0.39 +/- 0.17 and 0.92 +/- 0.43 ml/min at low and high Qb (not significantly different). Whatever the Qd value, the VVHDF procedures always allowed higher BCAA and alpha-ketoisocaproate clearances as compared with the corresponding VVHF period with similar Qb. BCAA clearances obtained by VVHDF with a 0.5 L/h dialysate flow were 4.1 +/- 0.5 and 5.4 +/- 0.5 mL/min at low and high Qb, respectively. The concurrent alpha-ketoisocaproate clearances were 2.5 +/- 0.8 and 2.9 +/- 1.0 ml/min.

Reversed-phase high-performance liquid chromatographic analysis of branched-chain keto acid hydrazone derivatives: optimization of techniques and application to branched-chain keto acid balance studies across the forearm

A sensitive method of quantifying branched-chain keto acids in plasma and whole blood samples is described. It is based on the separation by ion-pair reversed-phase liquid chromatography of 2,4-dinitrophenylhydrazine derivatives with ultraviolet detection. The sample clean-up steps that are usually required for reversed-phase high-performance liquid chromatography are eliminated. A reduction in ketoisocaproate isomer formation is obtained by incubation of derivatives in ice. The method is reproducible (coefficient of variation 2%, n = 5, at the 200-pmol level) and the ultraviolet response is linearly related to branched-chain keto acid concentration. Recoveries are high (greater than 95%). Other keto acids do not co-elute with branched-chain keto acids. Because of its sensitivity and precision, this method can be proposed for whole blood branched-chain keto acid balance studies across organs.

Versatile stable isotope technique for the measurement of amino acids and keto acids: comparison with radioactive isotope and its use in measuring in vivo disposal rates

Tracer methods using both carbon-13 and -14 have been utilized for determination of ovine fetal amino acid disposal and the results compared in seven animals. We infused [1-13C]leucine simultaneously with [1-14C]leucine into the fetal circulation of pregnant sheep chronically catheterized during late gestation. Radioactive and stable isotope enrichments of leucine (Leu) and stable isotope enrichments of ketoisocaproic acid (KIC) in the umbilical artery and vein and the maternal artery and uterine vein were measured. Stable isotope enrichments and concentrations of both Leu and KIC were determined from a single 0.2-ml sample by the use of internal standards and electron ionization GC/MS analysis after a simple isolation and derivatization procedure. The KIC/Leu enrichment ratio was measured for the first time in fetal arterial plasma and was 0.66 +/- 0.05 (SE). Fetal leucine disposal rate was 9.0 +/- 0.5 (SE) micron/min/kg. Disposal rates determined by stable isotopes were not different from those determined by radioactive isotopes. The GC/MS stable isotope method provided higher precision in both leucine concentration and enrichment measurements and has been shown to be a general method for the determination of concentration and isotopic enrichment of other amino acids and their corresponding keto acids. Furthermore, this method is ideally suited to clinical studies where large numbers of samples of rather small volume can easily be studied with a short turnaround time.

A convenient enzymatic method for the determination of 4-methyl-2-oxopentanoate in plasma: comparison with high performance liquid chromatographic analysis

A simple and rapid spectrophotometric method for the estimation of 4-methyl-2-oxopentanoate in plasma samples by use of NAD+-dependent D-2-hydroxyisocaproate dehydrogenase from Lactobacillus casei ssp. pseudoplantarum is described. It is based on the kinetic measurement of the decrease of NADH absorbance at 334 nm. Applicability is demonstrated by comparative measurement of 4-methyl-2-oxopentanoate content in plasma of patients with maple syrup urine disease by the enzymatic and a reversed phase high performance liquid chromatographic method.

Determination of leucine metabolism and protein turnover in sheep, using gas-liquid chromatography-mass spectrometry

1. Whole-body protein synthetic rates in non-pregnant ewes were determined by the continuous infusion of L-[15N]- and [1-13C]leucine and measuring the plasma enrichment of leucine, alpha-ketoisocaproate (alpha-KIC) and expired carbon dioxide by gas-liquid chromatography-mass spectrometry. 2. The mean whole-body protein synthesis estimated from plasma leucine flux corrected for oxidation was 5.38 (SE 0.54) g/kg per d. 3. Under the conditions of the present study leucine oxidation was 0.323 (SE 0.067) mmol/kg per d and accounted for 10.71 (SE 2.26) % of plasma [13C]leucine flux. Deamination of leucine was 0.55 (SE 0.035) mmol/kg per d and accounted for approximately 17% of plasma [15N]leucine flux. 4. The rate of alpha-KIC reamination to leucine, calculated by subtracting 13C flux from 15N flux, was 0.228 (SE 0.101) mmol/kg per d. 5. The rate of whole-body protein degradation was 4.49 (SE 0.54) g/kg per d and there was a net protein gain of 0.89 (SE 0.21) g/kg per d.

Amino acid metabolism during exercise in trained rats: the potential role of carnitine in the metabolic fate of branched-chain amino acids

The influence of endurance training and an acute bout of exercise on plasma concentrations of free amino acids and the intermediates of branched-chain amino acid (BCAA) metabolism were investigated in the rat. Training did not affect the plasma amino acid levels in the resting state. Plasma concentrations of alanine (Ala), aspartic acid (Asp), asparagine (Asn), arginine (Arg), histidine (His), isoleucine (Ile), leucine (Leu), lysine (Lys), methionine (Met), phenylalanine (Phe), proline (Pro), serine (Ser), threonine (Thr), and valine (Val) were significantly lower, whereas glutamate (Glu), glycine (Gly), ornithine (Orn), tryptophan (Trp), tyrosine (Tyr), creatinine, urea, and ammonia levels were unchanged, after one hour of treadmill running in the trained rats. Plasma concentration of glutamine (Glu), the branched-chain keto acids (BCKA) and short-chain acyl carnitines were elevated with exercise. Ratios of plasma BCAA/BCKA were dramatically lowered by exercise in the trained rats. A decrease in plasma-free carnitine levels was also observed. These data suggest that amino acid metabolism is enhanced by exercise even in the trained state. BCAA may only be partially metabolized within muscle and some of their carbon skeletons are released into the circulation in forms of BCKA and short-chain acyl carnitines.

Enzymatic determination of the branched-chain alpha-keto acids

A spectrophotometric endpoint assay for determination of branched-chain alpha-keto acids is described. The assay depends on measurement of the NADH produced after addition of branched-chain alpha-keto acid dehydrogenase. Interference by pyruvate and alpha-ketobutyrate was eliminated by pretreating the sample with pyruvate dehydrogenase. The method yielded a peripheral venous plasma value of 59 +/- 5 microM (mean +/- SE) for the branched-chain alpha-keto acids of five overnight fasted healthy humans.

Determination of alpha-keto acids in serum and urine by high-performance liquid chromatography with fluorescence detection

A simple, rapid and highly sensitive high-performance liquid chromatographic method for the determination of alpha-keto acids in serum and urine is described. In dilute hydrochloric acid, alpha-keto acids are converted by 1,2-diamino-4,5-dimethoxybenzene into highly fluorescent quinoxalinone derivatives. The derivatives are isocratically separated simultaneously within 14 min by reversed-phase chromatography on a Radial-Pak cartridge C18 and detected fluorimetrically. The limits of detection are 10-300 fmol in an injection volume of 10 microliters (40-1200 pmol/ml of serum or urine). This sensitivity permits precise determination of several alpha-keto acids in 5 microliter of serum or urine from healthy persons, and also the determination of phenylpyruvic acid in normal urine which cannot be simultaneously determined by other methods.

Gas chromatographic determination of serum branched-chain alpha-keto acids derivatized by extractive alkylation

The procedure presented for gas-liquid chromatographic analysis of alpha-keto acids is relatively simple, requiring only a few steps for the formation of derivatives suitable for measurement. The recoveries of the branched-chain alpha-keto acids varied from 92.7% to 106.7%, being sufficiently good especially when smaller amounts of the alpha-keto acids were added to serum. In addition, the coefficients of variation are satisfactorily small, also for biological samples. The measured values of branched-chain alpha-keto acids correspond well with those presented earlier by different methods. There exists a slight but insignificant difference between women and men, the values being lower in sera of women for the three branched-chain alpha-keto acids studied.

Metabolism of valine and 3-methyl-2-oxobutanoate by the isolated perfused rat kidney

Metabolism of branched-chain amino and 2-oxo acids was studied in the isolated perfused kidney. Significant amounts of 2-oxo acids were released by perfused kidney with all concentrations of amino acids tested (0.1-1.0 mM each), despite the high activity of branched-chain 2-oxo acid dehydrogenase in kidney. As perfusate valine concentration was increased from 0.2 to 1.0 mM, [1-14C]valine transamination (2-oxo acid oxidized + released) increased roughly linearly; [1-14C]valine oxidation, however, increased exponentially. Increasing perfusate concentration of 3-methyl-2-oxo[1-14C]butanoate from 0 to 1.0 mM resulted in a linear increase in the rate of its oxidation and a rise in perfusate valine concentration; at the same time significant decreases occurred in perfusate isoleucine and leucine concentrations, with corresponding increases in rates of release of their respective 2-oxo acids. Comparison of rates of oxidation of [1-14C]valine and 3-methyl-2-oxo[1-14C]butanoate suggests that 2-oxo acid arising from [1-14C]valine transamination has freer access to the 2-oxo acid dehydrogenase than has the 2-oxo acid from the perfusate. The observations indicate that, when branched-chain amino and 2-oxo acids are present in perfusate at near-physiological concentrations, rates of transamination of the amino and 2-oxo acids by isolated perfused kidney are greater than rates of oxidation.

Measurement of alpha-keto acids in plasma using an amino acid analyzer

A method for measuring keto acid concentrations in physiological fluids using an amino acid analyzer was developed. After preliminary deproteinization and removal of amino acids, reduction with sodium cyanoborohydride at 105 degrees C resulted in efficient conversion of the keto acids to their corresponding amino acids. In applying the technique to plasma samples, the use of MeOH for deproteinization was necessary to avoid the large losses of keto acids that occurred during precipitation of proteins with perchloric acid. The method was used to follow plasma ketoisocaproate concentrations in rat plasma after administration of leucine, and was sufficiently sensitive to detect concomitant changes in other branched-chain keto acid concentrations.

Branched-chain alpha-keto acid analysis in biological fluids: preparative clean-up by anion-exchange and analysis by capillary gas chromatography

A method is given for the quantitative analysis of the alpha-keto derivatives of the branched-chain amino acids in physiological fluids. A sample containing alpha-ketovalerate and alpha-ketocaproate as internal standards is passed through a weak anion-exchange resin at neutral pH. After washing the resin with distilled water, the alpha-keto acids are eluted with 4 M hydrochloric acid--ethanol (50:50). Quinoxalinol derivatives are prepared directly in the eluent, extracted with methylene chloride, and trimethylsilylated. Separation of the derivatives is by capillary gas chromatography on a 30 m fused-silica SE-30 column. Chromatographic separation is superior to that reported for packed column methods, thereby permitting the use of alpha-ketovalerate and alpha-ketocaproate as internal standards.

Use of t-butyldimethylsilylation in the gas chromatographic/mass spectrometric analysis of physiologic compounds found in plasma using electron-impact ionization

The use of N-methyl-N-(t-butyldimethylsilyl)trifluoroacetamide to prepare the t-butyldimethylsilyl derivatives of a number of organic compounds (selected amino acids, alpha-keto acids, ketone bodies, free fatty acids, urea, glycerol, lactate, and pyruvate) is reported. These derivatives are particularly useful for gas chromatographic/mass spectrometric analysis involving the use of stable isotopes and selected ion monitoring, since a peak of sufficient abundance at 57 mass/charge units below the molecular ion was always present, and was the result of the loss of one t-butyl group. In each case, this fragment contained the entire skeleton of the original compound, which permitted easy analysis using electron-impact ionization of these compounds alone or when labeled with stable isotopes in any nonexchangeable position.

Valine metabolism in vivo: effects of high dietary levels of leucine and isoleucine

The short-term effects of feeding rats high levels of L-leucine or L-isoleucine on valine metabolism in vivo have been investigated. Consumption of a low-protein diet containing an additional 5% of leucine resulted in depression within one hour of the plasma concentrations of isoleucine, valine, alpha-keto-beta-methylvalerate, and alpha-ketoisovalerate. Concurrently with these changes in blood branched-chain amino acids and branched-chain ketoacids was a rapid increase (51%) in whole-body L-[1-14C]-valine oxidation. Studies with intragastrically administered leucine solutions indicated that the depressions in blood concentrations of valine occurred over the same time period as the stimulation in valine oxidation. In contrast, consumption of a low-protein diet containing an additional 5% of isoleucine had no significant effect on the plasma concentrations of leucine, valine, and alpha-ketoisocaproate; a significant (P less than 0.01) depression in the plasma concentration of alpha-ketoisovalerate was observed three hours after the diet containing excess isoleucine had been consumed. In contrast to the results obtained with excess leucine, consumption of excess isoleucine had no significant effect on the rate of valine oxidation in vivo. As part of an effort to explain the leucine-induced depletion of plasma valine and stimulation of valine oxidation, liver and muscle branched-chain aminotransferase and liver branched-chain ketoacid dehydrogenase activities were measured. Consumption of excess leucine had no significant effect on either muscle or liver aminotransferase activities, but was associated with a greater than two-fold increase in hepatic dehydrogenase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

High-performance liquid chromatographic determination of alpha-keto acids in plasma with fluorometric detection

This paper describes a sensitive high-performance liquid chromatographic method for the quantitative determination of alpha-keto acids in plasma using a fluorescence detector. This method is about ten times more sensitive than that reported in a previous paper. Only 50 microliters of plasma are needed for the determination of alpha-keto acids. However, p-hydroxyphenylpyruvic acid could not be analysed because the quinoxalinol derived from it does not exhibit fluorescence.

Estimation of branched-chain alpha-keto acids in blood by gas chromatography

Plasma or whole blood is treated with o-phenylenediamine dihydrochloride in phosphoric acid under conditions found spectrophotometrically to give maximum yields of the quinoxalinols. The quinoxalinols are extracted and, after removing phosphoric acid, etc., silylated with bis-trimethylsilyltrifluoracetamide in acetonitrile. Other solvents caused instability of the trimethylsilyl(TMS)-quinoxalinols. Gas chromatography on a packed column of trifluoropropyl silicone gave good separation of the TMS-quinoxalinols from one another and from other substances derived from blood. Some representative values for normal arterial and venous human and canine plasma are reported.

Quantitation of 2-ketoacids in biological fluids by gas chromatography chemical ionization mass spectrometry of O-trimethylsilyl-quinoxalinol derivatives

A very sensitive and specific method of 2-ketoacid determination in various biological fluids using gas chromatography chemical ionization mass spectrometry of O-trimethylsilyl-quinoxalinol derivatives is described. After derivatization with o-phenylenediamine in acidic medium and extraction, the 2-ketoacids, and the 2-ketovaleric acid used as internal standard, are silylated and resolved on a capillary column and their concentrations automatically determined by monitoring the m/z values corresponding to their respective protonated molecular ions, using ammonia as reactant gas. The detection limit is below 50 ng ml-1 of each 2-ketoacid. The recoveries of 2-ketoacids from urine and plasma were between 97 and 104%. Problems encountered with interfering substances were tested for and discussed. This method has been applied successfully to various metabolic disorders characterized by an accumulation of aliphatic 2-ketoacids. The metabolic interrelationship of branched chain 2-ketoacids and other compounds, especially pyruvate, is discussed.

Enzymic determination of branched-chain amino acids and 2-oxoacids in rat tissues. Transfer of 2-oxoacids from skeletal muscle to liver in vivo

1. A procedure is described for the purification of leucine dehydrogenase (EC 1.4.1.9) from Bacillus subtilis. 2. The preparation is suitable for the quantitative assay of branched-chain amino acids and their 2-oxoacid analogues. 3. The content of total branched-chain 2-oxoacids in freeze-clamped liver, kidney, heart or mammary gland of fed rats is less than 5 nmol/g fresh wt. Higher amounts are present in skeletal muscle and arterial blood (25 +/- 4 nmol per g fresh wt., and 33 +/- 6 nmol per ml respectively; means +/- S.D. of 3 and 11 animals respectively). The values are not significantly affected by starvation for 24 h. 4. Arteriovenous difference measurements show that considerable amounts of branched-chain 2-oxoacids are released by skeletal muscle into the circulation and similar amounts are removed by the liver (about 1 mmol/24 h in a 400 g rat).