Determination of serum physiological concentration of methylmalonic acid by gas chromatography-mass spectrometry with selected ion monitoring

Unusual Derivatization of Methylmalonic Acid with Pentafluorobenzyl Bromide to a Tripentafluorobenzyl Derivative and Its Stable-Isotope Dilution GC-MS Measurement in Human Urine

Methylmalonic acid (MMA) is a very short dicarboxylic acid (methylpropanedioic acid; CH₃CH(COOH)₂; pK_a1, 3.07; pK_a2, 5.76) associated with vitamin B₁₂ deficiency and many other patho-physiological conditions. In this work, we investigated several carboxylic groups-specific derivatization reactions and tested their utility for the quantitative analysis of MMA in human urine and plasma by gas chromatography-mass spectrometry (GC-MS). The most useful derivatization procedure was the reaction of unlabeled MMA (d₀-MMA) and trideutero-methyl malonic acid (d₃-MMA) with 2,3,4,5,6-pentafluorobenzyl bromide (PFB-Br) in acetone. By heating at 80 °C for 60 min, we observed the formation of the dipentafluorobenzyl (PFB) ester of MMA (CH₃CH(COOPFB)₂). In the presence of N,N-diisopropylamine, heating at 80 °C for 60 min resulted in the formation of a tripentafluorobenzyl derivative of MMA, i.e., CH₃CPFB(COOPFB)₂). The retention time was 5.6 min for CH₃CH(COOPFB)₂ and 7.3 min for CH₃CPFB(COOPFB)₂). The most intense ions in the negative-ion chemical ionization (NICI) GC-MS spectra of CH₃CH(COOPFB)₂ were mass-to-charge (m/z) 233 for d₀-MMA and m/z 236 for d₃-MMA. The most intense ions in the NICI GC-MS spectra of CH₃CPFB(COOPFB)₂ were mass-to-charge (m/z) 349 for d₀-MMA and m/z 352 for d₃-MMA. These results indicate that the H at C atom at position 2 is C-H acidic and is alkylated by PFB-Br only in the presence of the base N,N-diisopropylamine. Method validation and quantitative analyses in human urine and plasma were performed by selected ion monitoring (SIM) of m/z 349 for d₀-MMA and m/z 352 for the internal standard d₃-MMA in the NICI mode. We used the method to measure the urinary excretion rates of MMA in healthy black (n = 39) and white (n = 41) boys of the Arterial Stiffness in Offspring Study (ASOS). The creatinine-corrected excretion rates of MMA were 1.50 [0.85–2.52] µmol/mmol in the black boys and 1.34 [1.02–2.18] µmol/mmol in the white boys (P = 0.85; Mann–Whitney). The derivatization procedure is highly specific and sensitive for MMA and allows its accurate and precise measurement in 10-µl of human urine by GC-MS.

BIOMARKERS OF GASTROINTESTINAL DISEASE IN CHEETAHS (ACINONYX JUBATUS)

Gastrointestinal disease is a common clinical problem in captive cheetahs (Acinonyx jubatus). It is reported that gastritis affects the vast majority of the captive population of cheetahs. Pancreatitis and acute and chronic enteritis have also been reported. These issues pose significant long-term health and welfare implications for cheetahs. Cobalamin, folate, methylmalonic acid (MMA), gastrin, feline pancreatic-specific lipase immunoreactivity (fPLI), and feline trypsin-like immunoreactivity (fTLI) immunoassays are important biomarkers of gastrointestinal disease in domestic cats. The goal of this study was to determine if these immunoassays validated in domestic cats could be used clinically in cheetahs, by establishing reference intervals (RI) for these biomarkers in cheetahs. A cohort of 40 clinically healthy cheetahs was selected from three zoological institutions on the basis of being free of clinical gastrointestinal disease and extra-gastrointestinal disease that could affect biomarkers, as well as having banked frozen serum. Cheetah biomarker RI, with domestic cat RI for comparison in parentheses, are as follows: cobalamin 470-618 pg/ml (290-1500 pg/ml), folate 2.2-15.7 ng/ml (9.7-21.6 ng/ml), MMA 365-450 nM/L (139-897 nM/L), fPLI 0.5-1.2 µg/L (0-4 µg/L), and gastrin 30-50 pg/ml (<10^-39.5 pg/ml). This study shows that RI for gastrointestinal biomarkers can be notably different, even between species that are as closely related as the domestic cat and the cheetah. Additionally, it was found that the fTLI assay does not cross-immunoreact with cheetahs. In conclusion, this study emphasizes the importance of developing species-specific RI for biomarker assays and using caution when extrapolating RI from other species.

The relationship of serum cobalamin to methylmalonic acid concentrations and clinical variables in cats

BACKGROUND

Serum cobalamin concentration [CBL] suggests CBL deficiency in cats but serum methylmalonic acid concentration [MMA] more accurately indicates CBL deficiency.

OBJECTIVE

To examine the ability of [CBL] to predict CBL deficiency defined by increased [MMA], and relationships of [CBL] and [MMA] with select clinical and clinicopathological variables.

ANIMALS

One hundred sixty-three client-owned cats with [CBL] measurements, 114 cats with simultaneous [MMA] measurements; 88 cats with medical information.

METHODS

Prospectively collected [CBL] and [MMA] were compared using scatter plots, receiver operating characteristic and correlative analyses with historical [CBL] thresholds and those identified in the study. [CBL] and [MMA] were compared retrospectively to specific clinical and clinicopathological variables.

RESULTS

[CBL] correlated negatively with [MMA] (τ = -0.334, P < .0001). [MMA] ≥ 1,343 nmol/L identified CBL deficiency. [CBL] = 209 pg/mL optimized sensitivity (0.51), specificity (0.96), PPV (0.89), and NPV (0.74) for detecting [MMA] ≥ 1,343 nmol/L. Prevalence of CBL deficiency was 42% (48/114) when defined by [MMA] ≥ 1,343 nmol/L versus 23% (27/114) by [CBL] ≤ 209 pg/mL. Unexpectedly, 23 and 45% of 48 cats with [MMA] ≥ 1,343 nmol/L had [CBL] > 900 pg/mL and 290 pg/mL (historical thresholds). [CBL] correlated with mean corpuscular volume (τ = -0.199, P = .013) and [MMA] with hematocrit (τ = -0.28, P = .006).

CONCLUSIONS AND CLINICAL IMPORTANCE

Cobalamin deficiency ([MMA] ≥ 1,343 nmol/L) occurred in 42% of cats and is predicted with high specificity by [CBL] ≤ 209 pg/mL. CBL status correlates with microcytosis and anemia. Discordance between [CBL] and [MMA] cautions against relying on any single marker for determining CBL status.

A simple high-throughput method for the determination of plasma methylmalonic acid by liquid chromatography-tandem mass spectrometry

BACKGROUND

Cobalamin (Cbl) deficiency is a common clinical phenomenon, in particular among the elderly and possibly also among infants. Methylmalonic acid (MMA) is the most sensitive and specific marker of intracellular Cbl status, but its application is hindered by limited methods available for accurate and high-throughput MMA determination.

METHODS

We developed a non-laborious method for determination of MMA without the need for prior derivatization using HPLC combined with liquid chromatography electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS). Stable isotope-labeled methyl-d(3)-malonic acid (MMA-d(3)) was added to 100 microL of plasma as an internal standard. After deproteinization by ultrafiltration, an acidified aliquot of the eluate was injected into the HPLC system and analyzed by LC-ESI-MS/MS monitoring of the carbonyl loss of MMA and MMA-d(3).

RESULTS

Calibrations between 0.1 and 1.0 microM exhibited consistent linearity and reproducibility. The lower limit of detection for plasma MMA was 0.1 microM (signal-to-noise ratio > or = 10). The intra- and inter-assay CVs of ten determinations of a plasma sample were 1.5% and 6.7%, respectively, at a mean concentration of 0.29 microM. Inter-assay CVs for 25 determinations of low, medium and high concentrations (0.22, 0.45 and 0.94 microM MMA) were 8.3%, 5.9% and 4.6%, respectively. The mean recovery of MMA added to plasma was 100%.

CONCLUSIONS

By avoiding derivatization, we developed a new, non-laborious, simple and reliable high-throughput method for the determination of MMA that is suitable for automation.

Holotranscobalamin and methylmalonic acid as prognostic markers following an acute myocardial infarction

OBJECTIVE

To evaluate whether low levels of holotranscobalamin (holoTC) or elevated levels of methylmalonic acid (MMA), both indicators of vitamin B(12) deficiency, might predispose to new cardiovascular events following an acute myocardial infarction (MI).

DESIGN

A prospective prognostic study.

SETTING

One hospital center in Stavanger, Norway.

SUBJECTS

A total of 300 patients admitted with an acute MI.

METHODS

Registration of new TnT positive coronary events (defined as TnT>0.05 microg/l and a typical MI pattern) and/or cardiac death during a median follow-up time of 45 months.

RESULTS

We compared the recurrence of events in the lowest quartile of holoTC (Q1<73.9 pmol/l) to the event rate above the 25% percentile (Q2-4). For methylmalonic acid (MMA) the same comparison was carried out for the upper quartile (Q4 > or =0.24 micromol/l) as compared with the event rate below the 75% percentile (Q1-3). After 18 and 45 months of follow-up, the odds ratio (OR) for Q1 vs Q2-4 for holoTC was 1.15 (95% confidence interval (CI) 0.91-1.46, P=0.25) and 1.05 (95% CI 0.86-1.29, P=0.64), respectively. For MMA the OR for Q4 vs Q1-3 was 0.95 (95% CI 0.76-1.19, P=0.67) after 18 months and 1.01 (95% CI 0.83-1.23, P=0.90) after 45 months.

CONCLUSION

This study showed no increased risk of future cardiovascular events associated with low levels of holoTC or high levels of MMA following an acute MI.

Automated assay for the determination of methylmalonic acid, total homocysteine, and related amino acids in human serum or plasma by means of methylchloroformate derivatization and gas chromatography-mass spectrometry

BACKGROUND

The combined measurement of methylmalonic acid (MMA) and total homocysteine (tHcy) in serum or plasma is useful in diagnosing and distinguishing between cobalamin and folate deficiencies. We developed and validated an isotope-dilution gas chromatography-mass spectrometry (GC-MS) method with automated sample workup for the determination of MMA, tHcy, and the related amino acids Met, total cysteine (tCys), Ser, and Gly in serum or plasma.

METHODS

Serum or plasma samples (100 microL) were treated with a reductant (dithioerythritol), deproteinized with ethanol, and derivatized and extracted in a single step by the addition of methylchloroformate and toluene. All liquid handling was performed in 96-well (1 mL) microtiter plates by a robotic workstation. The N(S)-methoxycarbonyl ethyl ester derivatives were analyzed by GC-MS in the selected-ion monitoring mode.

RESULTS

Detection limits (signal-to-noise ratio, 5:1) were between 0.03 micromol/L (MMA) and 10 micromol/L (Ser, tCys). The assay was linear to 100 micromol/L for MMA and tHcy and to 1000 micromol/L for Met, tCys, Ser, and Gly. The within-day CVs ranged from 0.7% to 3.6% (n = 20), and the between-day CVs from 2.1% to 8.1% (n = 20). The recovery was between 79% and 99% for the different analytes.

CONCLUSION

This assay combines a simple and automated sample preparation with selective and sensitive GC-MS analysis and is well suited for the combined measurement of MMA, tHcy, and the related amino acids.

An improved assay for plasma methylmalonic acid using chemical ionization gas chromatography mass spectrometry

OBJECTIVES

To develop a precise and sensitive assay for methylmalonic acid (MMA) using positive chemical ionization gas chromatography mass spectrometry (CI GC-MS), and to illustrate its clinical utility.

METHODS

Using the developed assay, reference intervals were determined with 108 ambulatory individuals, and potential clinical utility examined in 178 consecutive patients with possible cobalamin deficiency (serum B12<200 nmol/L).

RESULTS AND CONCLUSIONS

Methylmalonic acid measured by CI GC-MS was precise (CV: 4-5%), and sensitive (limit of quantitation: 37 nmol/L). In a clinical reference set, 37% of individuals with serum B12 less than 200 pmol/L had plasma MMA concentrations within the reference interval (75-378 nmol/L), rendering cobalamin deficiency unlikely. The observation illustrates that MMA assay may be a useful adjunct test in assessing patients with low serum B12.

Metabolism of amino acids in cats with severe cobalamin deficiency

OBJECTIVE

To validate an automated chemiluminescent immunoassay for measuring serum cobalamin concentration in cats, to establish and validate gas chromatography-mass spectrometry techniques for use in quantification of methylmalonic acid, homocysteine, cysteine, cystathionine, and methionine in sera from cats, and to investigate serum concentrations of methylmalonic acid, methionine, homocysteine, cystathionine, and cysteine as indicators of biochemical abnormalities accompanying severe cobalamin (vitamin B12) deficiency in cats.

SAMPLE POPULATION

Serum samples of 40 cats with severe cobalamin deficiency (serum cobalamin concentration < 100 ng/L) and 24 control cats with serum cobalamin concentration within the reference range.

PROCEDURE

Serum concentrations of cobalamin were measured, using a commercial automated chemiluminescent immunoassay. Serum concentrations of methylmalonic acid, methionine, homocysteine, cystathionine, and cysteine were measured, using gas chromatography-mass spectrometry, selected ion monitoring, stable-isotope dilution assays.

RESULTS

Cats with cobalamin deficiency had significant increases in mean serum concentrations bf methylmalonic acid (9,607 nmol/L), compared with healthy cats (448 nmol/L). Affected cats also had substantial disturbances in amino acid metabolism, compared with healthy cats, with significantly increased serum concentrations of methionine (133.8 vs 101.1 micromol/L) and significantly decreased serum concentrations of cystathionine (449.6 vs 573.2 nmol/L) and cysteine (142.3 vs 163.9 micromol/L). There was not a significant difference in serum concentrations of homocysteine between the 2 groups.

CONCLUSIONS AND CLINICAL RELEVANCE

Cats with gastrointestinal tract disease may have abnormalities in amino acid metabolism consistent with cobalamin deficiency. Parenteral administration of cobalamin may be necessary to correct these biochemical abnormalities.

Hyphenated chromatographic methods for biomaterials

The improvement in hyphenated analytical techniques has significantly widened their applications to the analysis of biomaterials. In this article, we discuss recent advances in applications of hyphenated chromatographic techniques including capillary electrophoresis to the analyses of biological samples. As tools of separation, gas chromatography, high-performance liquid chromatography and capillary electrophoresis are considered with special emphasis on applications utilizing the hyphenation of these methods to mass spectrometry. Moreover, applications using other detection methods such as Fourier transform infrared spectroscopy hyphenated to gas chromatography and photodiode array detector combined with high-performance liquid chromatography or capillary electrophoresis are also discussed. Owing to their high sensitivity, luminescence-based detection systems such as laser-induced fluorescence and chemiluminescence are also included in this review.

A GC/MS/MS screening method for multiple organic acidemias from urine specimens

A gas chromatography tandem mass spectrometry method using an ion trap GC/MS system was developed to quickly screen urine samples for 14 organic acids associated with multiple organic acidemias. The following organic acids are used as diagnostic markers: methylmalonic acid, glutaric acid, 2-ketoisocaproic acid, succinylacetone, 3-methylcrotonylglycine, tiglylglycine, isovalerylglycine, fumaric acid, butyrylglycine, propionylglycine, hexanoylglycine, adipic acid, suberic acid, and sebacic acid. 2-ketocaproic acid is used as an internal standard. The samples are prepared using a solid-phase extraction and converted to trimethylsilyl derivatives. The extraction efficiency for the 14 compounds is between 57 and 106%. A derivatized standard mixture of the 14 markers is run prior to the patient samples to determine the accurate absolute and relative retention times. The samples are then injected and the product ion spectra monitored. For data analysis, one characteristic product ion plot is extracted for each of the 14 marker compounds, and the presence of a peak with the expected retention time is determined. The areas of the product ion peaks are compared with the reference range determined from 30 normal controls. Ten samples of patients with known organic acidemias were measured. For all patients, diagnostic peaks at the expected retention times of at least five times the upper limit of the reference range were detected. The method, with its relatively fast sample preparation, short 10.0 min run time and simple data analysis, is suitable for use as a quick metabolic screen of very sick patients in whom there is concern regarding the possibility of a treatable inborn error.