Determination of choline, betaine, and dimethylglycine in plasma by a high-throughput method based on normal-phase chromatography-tandem mass spectrometry.
The quaternary quaternary /qua·ter·nary/ (kwah´ter-nar?e)
1. fourth in order.
2. containing four elements or groups.
1. Consisting of four; in fours. ammonium compounds, choline choline: see vitamin.
Organic compound related to vitamins in its activity. It is important in metabolism as a component of the lipids that make up cell membranes and of acetylcholine. and Betaine betaine /be·ta·ine/ (be´tah-en) the carboxylic acid derived by oxidation of choline; it acts as a transmethylating metabolic intermediate and is used in the treatment of homocystinuria. , and N,N-dimethylglycine (DMG (Disk iMaGe) The file format used in the Macintosh for distributing Mac software. Mac install packages appear as a virtual disk drive on the Mac as if you had inserted a CD or floppy disk. ),1 have a variety of biological effects and are closely connected metabolically. Choline is obtained from the diet or by sequential methylation methylation,
n a phase-II detoxification pathway in the liver; methyl groups combine with toxins to rid the body of various substances.
(meth´ of phosphatidylethanolamine. It is a precursor of membrane and lipoprotein lipoprotein (lĭp'əprō`tēn), any organic compound that is composed of both protein and the various fatty substances classed as lipids, including fatty acids and steroids such as cholesterol. phospholipids and the neurotransmitter acetylcholine acetylcholine (əsēt'əlkō`lēn), a small organic molecule liberated at nerve endings as a neurotransmitter. It is particularly important in the stimulation of muscle tissue. ; it thus is important for the integrity of cell membranes, lipid metabolism, and cholinergic cholinergic /cho·lin·er·gic/ (ko?lin-er´jik)
1. parasympathomimetic; stimulated, activated, or transmitted by choline (acetylcholine); said of the sympathetic and parasympathetic nerve fibers that liberate acetylcholine at a nerve function (1, 2). An alternative metabolic route for choline is oxidation to Betaine, catalyzed by the sequential action of choline oxidase oxidase /ox·i·dase/ (ok´si-das) any enzyme of the class of oxidoreductases in which molecular oxygen is the hydrogen acceptor.
n. (EC 188.8.131.52) and Betaine aldehyde dehydrogenase (EC 184.108.40.206). Betaine, in turn, is the methyl donor and DMG a product of the enzyme Betaine-homocysteine methyltransferase (EC 220.127.116.11). This enzyme catalyzes the remethylation of homocysteine Homocysteine Definition
Homocysteine is a naturally occurring amino acid found in blood plasma. High levels of homocysteine in the blood are believed to increase the chance of heart disease, stroke, Alzheimer's disease, and osteoporosis. to methionine methionine (mĕthī`ənēn), organic compound, one of the 20 amino acids commonly found in animal proteins. Only the L-stereoisomer appears in mammalian protein. , a metabolic conversion that is also carried out by the folate-dependent enzyme, methionine synthase synthase /syn·thase/ (-thas) a term used in the names of some enzymes, particularly lyases, when the synthetic aspect of the reaction is dominant or emphasized.
n. (EC 18.104.22.168) (1, 3) (Fig. 1). The betainedependent homocysteine remethylation in this way connects Betaine to the metabolism and function of folates, homocysteine, and methionine. It also explains the homocysteine-lowering and therapeutic effect of Betaine in patients with homocystinuria (4) and the moderate reduction in plasma total homocysteine in healthy individuals (5) and renal patients (6).
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Plasma choline has been determined in several small human studies (2). The concentration in healthy fasting humans is 7-10 [micro]mol/L (2), is remarkably high in neonates (7, 8), is influenced by diet (2) and exercise (9), and is increased in renal patients (10). Data on Betaine and DMG in human plasma/serum in health and disease are sparse. Betaine and DMG concentrations have been reported in small numbers of healthy individuals (11) and in patients with vitamin B vitamin B
1. Vitamin B complex.
2. A member of the vitamin B complex, especially thiamine.
vitamin B, vitamin B complex
a group of water-soluble substances described separately. deficiencies (11) or renal disease Renal disease
Mentioned in: Glycogen Storage Diseases
hypertension High blood pressure Cardiovascular disease An abnormal ↑ systemic arterial pressure, corresponding to a systolic BP of > 160 mm Hg (12). Notably, in HIV-1-infected patients, markedly increased DMG, which is normalized after antiviral therapy, has recently been demonstrated (13). This metabolic abnormality may reflect tissue wasting, including membrane degradation, which generates choline, which is further metabolized to DMG.
The published data would appear to strongly motivate larger epidemiologic and clinical studies on plasma choline, Betaine, and DMG and their relationships with nutritional status and disease risk, but no such investigations have been conducted. This is probably explained by the inconvenient methods for the determination of the quaternary ammonium compounds choline and Betaine. Various methods have been reported for the measurement of plasma free choline, including radioenzymatic assays (14,15), gas chromatography (8,16), HPLC HPLC high-performance liquid chromatography.
high performance liquid chromatography.
HPLC High-performance liquid chromatography Lab instrumentation A highly sensitive analytic method in which analytes are placed with electrochemical detection (17), and very recently, HPLCmass spectrometry (MS) (18). The radioenzymatic assay is cumbersome with variable recovery (8), whereas the chromatographic chro·mat·o·graph
An instrument that produces a chromatogram.
tr.v. chro·mat·o·graphed, chro·mat·o·graph·ing, chro·mat·o·graphs
To separate and analyze by chromatography. assays require complex extraction or derivatization procedures. A novel HPLC method for DMG and Betaine in blood requires derivatization and is characterized by low sensitivity and limited throughput (19). Betaine in serum has been assayed by gas chromatography with M5, but Betaine has to be enzymatically converted to DMG to be detected by this method (11). In this report, we describe a sensitive method based on liquid chromatography-tandem MS (LC-MS/MS) for the simultaneous determination of choline, Betaine, and DMG in human plasma and serum. The sample processing is a simple protein precipitation step carried out by a robotic workstation, followed by separation by a fast HPLC procedure. This method is suitable for large-scale epidemiologic studies.
Materials and Methods
Choline chloride, Betaine glycine glycine (glī`sēn), organic compound, one of the 20 amino acids commonly found in animal proteins. Glycine is the only one of these amino acids that is not optically active, i.e. , and DMG were obtained from Sigma Chemicals. [d.sub.9]-Choline chloride was from Aldrich Chemical, and [d.sub.9]-Betaine was from Isotec. Methanol, acetonitrile acetonitrile /ac·e·to·ni·trile/ (as?e-to-ni´tril) a colorless liquid with an etherlike odor used as an extractant, solvent, and intermediate; ingestion or inhalation yields cyanide as a metabolic product. , and formic acid were HPLC grade and purchased from Merck Chemicals. Other reagents were of the highest purity available. Pooled human EDTA EDTA: see chelating agents. plasma for assay calibration and linearity experiments was dialyzed three times (i.e., until the concentrations of choline, Betaine, and DMG were below the detection limit) against 10 volumes of phosphate-buffered saline containing 4 mmol/L EDTA. We used phosphate-buffered saline to prevent substantial changes in the salt content of the plasma samples.
SAMPLE COLLECTION AND PROCESSING
EDTA plasma was obtained by collecting blood into Vacutainer Tubes (Becton Dickinson); the final EDTA concentration in the samples was 4 mmol/L. The EDTAblood samples were centrifuged within 60 min. Serum was obtained by collecting blood into Vacutainer Tubes with no additive. Blood was allowed to clot at room temperature for 30 min before the serum fraction was transferred to an empty glass vial. Plasma and serum were either processed immediately or stored at -20 [degrees]C until use.
Plasma or serum samples (30 [micro]L) were mixed with three volumes of acetonitrile containing 10 [micro]mol/L each of [d.sub.9]-choline and [d.sub.9]-Betaine. Samples were then centrifuged for 2 min at 58008. The supernatant supernatant /su·per·na·tant/ (-na´tant) the liquid lying above a layer of precipitated insoluble material.
the liquid lying above a layer of precipitated insoluble material. was then transferred to sealed polypropylene microtiter plates (Costar) or sealed autosampler glass vials (Chromacol). The mixing of plasma/serum with acetonitrile and the transfer of supernatant were carried out by a robotic device (Plato 7; RoSyst Anthos).
[FIGURE 2 OMITTED]
An Agilent series HPLC system (Agilent Technologies) equipped with a thermostated autosampler and a degasser were used for solvent delivery and sample introduction. Serum samples deproteinized with acetonitrile were placed in a cooled (4 [degrees]C) sample tray and injected (2 [micro]L) into a normal-phase column (10 x 2.1 cm) packed with 5-[micro]m diameter particles of Hypersil silica (Agilent Technologies) and equilibrated with 25% solution A (15 mmol/L ammonium formate, pH 3.5) and 75% solution B (acetonitrile). The column was eluted at ambient temperature at a flow rate of 0.6 mL/min and developed with gradient elution elution /elu·tion/ (e-loo´shun) in chemistry, separation of material by washing; the process of pulverizing substances and mixing them with water in order to separate the heavier constituents, which settle out in solution, from the as follows: 0-0.1 min, 25% A/75% B; 3.5 min, 80% A/20% B; 3.6 min, 25% A/75% B; and 5.6 min, 25% A/75% B. All gradient steps were linear. The column effluent was split at a ratio of 1:4, delivering the eluate eluate /el·u·ate/ (el´u-at) the substance separated out by, or the product of, elution or elutriation.
The solution of solvent and dissolved matter resulting from elution. at a flow rate of 150 [micro]L/min into the mass spectrometer. The injection interval was 6 min.
[FIGURE 3 OMITTED]
We used an API 3000 (Applied Biosystems/MDS SCIEX) triple-quadrupole tandem mass spectrometer with Turbo Ion SprayTM interface in the positive-ion mode. Nitrogen was used as the drying gas at a flow rate of 6 L/min and for collision-activated dissociation. The collision energy was 28 eV, the declustering potential was 31 V, and the ion source temperature was 350 [degrees]C. For development work, the product-ion spectra for choline, [d.sub.9]-choline, betaine, [d.sub.9]-betaine, and DMG were acquired in the continuous flow injection mode, with use of a Harvard Mode111 syringe pump connected directly to the ion source by PEEK tubing. For signal optimization, we dissolved each compound at a concentration of 10 [micro]mol / L in a mixture of 15 mmol/L ammonium formate and acetonitrile (75:25 by volume), infused at a rate of 10 [micro]L/min.
The quaternary ammonium compounds and DMG were detected in the multiple-reaction monitoring mode of the tandem mass spectrometer with the following transitions: choline, m/z 104[right arrow]60; [d.sub.9]-choline, m/z 113[right arrow]69; betaine, m/z 118[right-arrow]59; [d.sub.9]-betaine, m/z 127[right-arrow]68; and DMG, m/z104[right arrow]58.
Analyst software (Applied Biosystems/MDS SCIEX) was used for HPLC system control, data acquisition, and data processing.
[FIGURE 4 OMITTED]
LINEARITY AND ASSAY CALIBRATION
We assessed the linear range and sensitivity of the assay by adding 0.01-400 [micro]mol/L choline, betaine, and DMG to dialyzed plasma. The peak-area ratios of choline to [d.sub.9]-choline, betaine to [d.sub.9]-betaine, and DMG to [d.sub.9]-betaine were determined and plotted against the concentrations of choline, betaine, and DMG.
The assay was routinely calibrated by analyzing four samples containing added choline (3, 7.5, 15, and 30 [micro]mol/L), betaine (6, 15, 30, and 60 [micro]mol/L) and DMG (0.6, 1.5, 3, and 6 [micro]mol/L) in dialyzed EDTA plasma.
RECOVERY AND ANALYTICAL VARIATION
Pooled EDTA plasma was dialyzed against two volumes of phosphate-buffered saline containing 4 mmol/L EDTA. This was done to obtain a sample with low-normal concentrations of choline (2.7 [micro]mol/L), betaine (11.4 [micro]mol/L), and DMG (0.91 [micro]mol/L). The samples were divided into three portions, and medium or high concentrations of choline (10 or 30 [micro]mol/L), betaine (20 or 60 [micro]mol/L), and DMG (2 or 6 [micro]mol/L) were added to two of the portions. At each analyte concentration, 18 replicates were analyzed in one analytical run. Recovery (%) was calculated as:
Measured concentration-Endogenous concentration x 100 Concentration added
To determine within-day precision, we used the data from the recovery experiments. Between-day precision was determined by assaying the same samples on 20 different days over a period of 3 weeks.
STABILITY IN EDTA PLASMA AND SERUM
EDTA-plasma and serum samples were collected from the same individual. Serum and plasma samples were incubated at 0 or 25 [degrees]C for up to 72 h. The incubation times were 10, 20, and 60 min and 2, 6,12, 24, 48, and 72 h. Each sample was run in triplicate.
CHOLINE, BETAINE, AND DMG IN PLASMA AND SERUM FROM HEALTHY INDIVIDUALS
Plasma and serum samples were collected from 120 healthy blood donors (mean age, 43.3 years; range, 20-65 years). One-half of the donors, 22 males and 38 females, were fasting. The other half, 42 males and 18 females, had eaten a light breakfast 2-3 h before blood sampling.
The protonated molecular [M+H]+ ions for choline (m/z 104), [d.sub.9]-choline (m/z 113), betaine (m/z 118), [d.sub.9]-betaine (m/z 127), and DMG (m/z 104) were the predominant Q1 ions obtained by electrospray MS in the positive-ion mode by infusion of the respective compounds (data not shown). The product ion spectra obtained by Q3 transmissions of the protonated molecular [M+H]+ ions are shown in Fig. 2. Under relatively mild collision-activated dissociation conditions, each compound broke down to give essentially one major fragment (m/z 60, 69, 59, 68, and 58 for choline, [d.sub.9]-choline, betaine, [d.sub.9]-betaine, and DMG, respectively).
Choline, [d.sub.9]-choline, betaine, [d.sub.9]-betaine, and DMG were analyzed separately by normal-phase chromatography on a silica gel column equilibrated with a mixture of 15 [micro]mol/L ammonium formate and acetonitrile (25:75 by volume), and eluted with a gradient of increasing portion of ammonium formate, as detailed in the Materials and Methods. Choline and [d.sub.9]-choline coeluted at 2.8 min, betaine and [d.sub.9]-betaine coeluted at 1.3 min, and DMG eluted at 1.15 min. Each compound showed a single peak (data not shown).
Shown in Fig. 3 is an extracted multiple-reaction monitoring chromatogram chromatogram /chro·mato·gram/ (kro-mat´o-gram) the record produced by chromatography.
The pattern of separated substances obtained by chromatography. for choline, betaine, and DMG and the two deuterated internal standards in EDTA plasma and the same EDTA plasma to which each analyte had been added at a concentration of 50 [micro]mol/L. Addition of authentic choline, betaine, and DMG to plasma caused a comparable increase in absolute abundance of the respective product ions. The chromatograms for the mass ion transitions of choline, [d.sub.9]-choline, betaine, and [d.sub.9]-betaine each showed one main peak, whereas three peaks were detected for the mass transitions of DMG. The DMG peak (retention time, 1.15 min) was increased 30-fold in the plasma to which analyte had been added, and a second peak coeluting with choline (retention time, 2.8 min) increased moderately (Fig. 3).
Total run time, which included column equilibration equilibration /equi·li·bra·tion/ (e-kwil?i-bra´shun) the achievement of a balance between opposing elements or forces.
occlusal equilibration , was 6 min.
PERFORMANCE OF THE ASSAY
The sensitivity and linearity of the assay were assessed by analyzing a dialyzed plasma to which choline, betaine, and DMG had been added at concentrations of 0.01-400 [micro]mol/L. The lower limits of detection, defined as a signal-to-noise ratio of 5, were 0.1 [micro]mol/L for choline, 0.3 [micro]mol/L for betaine, and 0.2 [micro]mol/L for DMG. A linear dynamic range was demonstrated from the lower limit of detection up to 400 [micro]mol/L for choline (y = 0.035x 0.0053 [micro]mol/L; [R.sup.2] = 0.9996), betaine (y = 0.029x + 0.014 [micro]mol/L; [R.sup.2] = 0.9935), and DMG (y = 0.0021x + 0.017 [micro]mol/L; [R.sup.2] = 0.9989; Fig. 4).
The precision and recovery of the assay were investigated at low- to high-normal concentrations of choline (2.7, 12.7, and 32.7 [micro]mol/L), betaine (11.4, 31.4, and 71.4 [micro]mol/L), and DMG (0.9, 2.9, and 6.9 [micro]mol/L) in dialyzed plasma. The analytical recovery was 87-105% for all compounds and concentrations. The within- and between-day imprecision (CVs) was 2.1-8.8%, with the highest imprecision for DMG (Tables 1 and 2). Notably, similar values for recovery were obtained in nondialyzed plasma (data not shown).
Preparation of samples for LC-MS/MS analysis, which includes mixing samples with acetonitrile containing deuterated internal standard, and removal of the supernatant after centrifugation Centrifugation
A mechanical method of separating immiscible liquids or solids from liquids by the application of centrifugal force. This force can be very great, and separations which proceed slowly by gravity can be speeded up enormously in centrifugal are carried out by a robotic liquid handler that can process >200 samples in 60 min. The chromatographic run time of the current assay format is 6 min and limits the capacity of the assay, which is --200 samples per day. The required sample volume is 30 [micro]L.
STABILITY OF METABOLITES
Serum and EDTA-plasma samples were incubated at 0 or 25 [degrees]C for up to 72 h (Fig. 5). Choline, betaine, and DMG were essentially stable in EDTA plasma at both temperatures and in serum at the lower temperature. In serum incubated at 25 [degrees]C, DMG increased moderately and Choline increased linearly as a function of time, reaching a sevenfold increase after 72 h (Fig. 5).
In serum and EDTA plasma mixed with three volumes of acetonitrile, all three metabolites were completely stable for at least 72 h at both temperatures (data not shown).
METABOLITES IN PLASMA AND SERUM IN FASTING AND NONFASTING INDIVIDUALS
We measured Choline, betaine, and DMG in both serum and EDTA plasma from 60 fasting and 60 nonfasting blood donors (Table 3). The median (25th-75th percentiles) plasma concentrations in fasting donors were 8.0 (7.0-9.3) [micro]mol/L for Choline, 31.7 (27.0-41.1) [micro]mol/L for betaine, and 1.66 (1.30-2.02) [micro]mol/L for DMG. In donors who had eaten a light breakfast, plasma concentrations of all three metabolites were significantly (25-30%) higher than in fasting donors (Table 3). The prandial prandial /pran·di·al/ (pran´de-il) pertaining to a meal.
Of or relating to a meal.
pertaining to a meal. status influenced the whole distribution curve for all metabolites, which were displaced to the right in nonfasting individuals, demonstrating that food intake increased both low- and high-normal metabolite concentrations (Fig. 6).
For betaine and DMG, concentrations were not significantly different in plasma vs serum, whereas Choline was (13-22%) higher in serum than in plasma (Table 3). The fasting plasma concentrations of Choline, betaine, and DMG were significantly correlated (Fig. 7; Pearson r = 0.5-0.6; P <0.001).
We have developed a LC-MS/MS method for the determination of free Choline, betaine, and DMG in human plasma or serum. The sample processing involves mixing with acetonitrile, carried out by a robotic workstation, followed by centrifugation of the samples. The sample processing is fast and simple, requiring no derivatization, and all steps except for centrifugation are fully automated. This is the first published method that allows the simultaneous analysis of these three metabolites with use of the same preparation procedure and detection method.
The samples were deproteinized and metabolites were extracted by mixing with three volumes of acetonitrile. Preconditions for this convenient sample preparation are no covalent co·va·lent
Of or relating to a chemical bond characterized by one or more pairs of shared electrons. binding of analytes to plasma proteins (requiring steps such as reduction or acid hydrolysis hydrolysis (hīdrŏl`ĭsĭs), chemical reaction of a compound with water, usually resulting in the formation of one or more new compounds. ), solubility of analytes at high percentages of acetonitrile, and no adsorption adsorption, adhesion of the molecules of liquids, gases, and dissolved substances to the surfaces of solids, as opposed to absorption, in which the molecules actually enter the absorbing medium (see adhesion and cohesion). of analytes to precipitated proteins or protein pellet. The metabolites are all stable under these mild conditions, but the extracted samples must be stored in sealed or cooled tubes to avoid evaporation. Furthermore, acetonitrile has been recommended for the extraction of choline because it is highly polar and a poor lipid solvent that does not support the hydrolysis of choline esters (20). The analytes in 750 mL/L acetonitrile are retained on the silica stationary phase and eluted by a short gradient of ammonium formate. Thus, the acetonitrile step combines five components of the assay procedure: protein precipitation, analyte extraction, inhibition of choline ester hydrolysis, addition of deuterated internal standard, and an injection matrix that enables analyte retention.
[FIGURE 5 OMITTED]
The sample throughput of the present version of the assay is --200 samples per 24 h, which matches the capacity of two 96-well microtiter plates. If required, the capacity can be substantially increased by reduction in column dimensions or by increasing the flow rate of the mobile phase. However, this may increase predisposition to high back-pressures and column clogging, which will decrease the ruggedness of the method.
The electrospray ionization mass spectra of choline, betaine, and DMG all produced prominent base peaks corresponding to their protonated molecular [[M+H].sup.+] ions. The product ion spectra (Fig. 2) were obtained by exciting the protonated molecules with an equivalent collision energy, which fragments the ions into still smaller pieces. For choline (m/z 104), the main product ion at m/z 60 corresponds to [(CH.sub.3).sub.3][NH.sup.+]. This was confirmed by the Q3 spectrum of the [d.sub.9]-analog (m/z 113), which produced a base peak at m/z 69, i.e., 9 mass units higher than the unlabeled compound. Similarly, for betaine (m/z 118) and [d.sub.9]-betaine (m/z 127), the main product ions of m/z 59 and 68 are consistent with the structures [(CH.sub.3).sub.3][N.sup.+] and [(CD.sub.3).sub.3][N.sup.+], respectively. DMG (m/z 104) produced one base peak at m/z 58. Thus, DMG and choline have one common molecular transition of m/z 104[right arrow]58 (Fig. 2).
Detection of the analytes by MS/MS affords sufficient sensitivity. Specificity, on the other hand, is related to the selectivity of the molecular transitions and the chromatographic separation. In plasma and serum, at least two compounds exist with the same transition as DMG (Fig. 3). One coelutes with choline, probably arises from the common transition of m/z 10458, and is easily separated from DMG. Some chromatographic optimization, however, was required to obtained resolution of DMG and the material eluting immediately before DMG (Fig. 3).
Stable-isotope-labeled internal standards are preferable in MS/M5, but deuterated analogs were available only for choline and betaine. We therefore quantified DMG by measurement of the peak-area ratios of DMG to [d.sub.9]betaine. Using [d.sub.9]-betaine as internal standard may not correct for the possible matrix effect on DMG ionization ionization: see ion.
Process by which electrically neutral atoms or molecules are converted to electrically charged atoms or molecules (ions) by the removal or addition of negatively charged electrons. . This may partly explain the somewhat higher between and within-day CVs for DMG compared with choline and betaine (Tables 1 and 2).
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
The median (25th-75th percentiles) plasma concentrations of choline [8.0 (7.0-9.3) [micro]mol/L], betaine [31.7 (27.0-41.1) [micro]mol/L], and DMG [1.66 (1.30-2.02) [micro]mol/L] determined by the present LC-MS/MS method are the same as reported by several other authors (11,12,15, 2123). There is one report on higher plasma DMG in the range 4-13 [micro]mol/L, but this result was based on an assay with a detection limit of 2 [micro]mol/L (19).
The plasma concentrations of choline, betaine, and DMG were 25-30% higher in donors 2-3 h after a small meal (breakfast) compared with donors who had fasted overnight (Table 3). There are no data in the literature on postprandial postprandial /post·pran·di·al/ (-pran´de-al) occurring after a meal.
Following a meal, especially dinner. effects on betaine or DMG concentrations in plasma or serum. The effect on choline, however, is in agreement with published data demonstrating a moderate (10%)increase in plasma 1 h after a typical meal (15). A diet low in choline has no effect on plasma concentrations (24). Marked increases (two- to fourfold increase) have been reported after the intake of a diet with high choline content or supplemented with lecithin lecithin
Any of a class of phospholipids (also called phosphatidyl cholines) important in cell structure and metabolism. They are composed of phosphate, choline, glycerol (as the ester), and two fatty acids. Various fatty acids pairs distinguish the various lecithins. (21, 24), whereas fasting (23) or a choline-deficient diet (25) cause a moderate (30%) reduction in plasma choline.
The concentration of free choline increased markedly ([micro]1 [micro]mol/L per hour) in serum incubated at room temperature (Fig. 5). Others have demonstrated that on incubation at room temperature, choline content increases in plasma but not in heated plasma and ultrafiltrate (8,14,15). Such in vitro formation of choline may account for the higher choline concentrations in serum than in plasma (Table 3) and could be explained by enzymatic cleavage of choline esters. Conceivably, enzymatic formation of choline may occur during the time interval when serum is left at room temperature and may be inhibited in plasma by the EDTA in the collections tubes.
In conclusion, the postprandial effect on all of the metabolites, the pronounced in vitro formation of choline but not betaine and DMG, and the lower choline concentration in plasma than in serum (Table 3 and Fig. 6) have implications for determination of these metabolites in clinical and epidemiologic research. On the basis of the observations listed above, we recommend the use of fasting EDTA-plasma samples for the collective measurement of choline, betaine, and DMG. Our results also indicate that variable storage of samples at room temperature and nonfasting samples may cause bias, leading to underestimation of "true" associations between metabolite concentrations and biological traits. Finally, we believe that the strong correlations between plasma concentrations of choline, betaine, and DMG (Fig. 7), probably related to their close metabolic connection (Fig. 1), indicate a need for future investigation of the biological and clinical relevance of metabolite ratios.
This work was supported by grants from the Norwegian Cancer Society and the Foundation to promote research into functional vitamin [B.sub.12] deficiency.
Received October 2, 2002; accepted November 15, 2002.
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(20.) Green JP, Szilagyi PIA pi·a
The pia mater.
pial adj. . Measurement of acetylcholine by pyrolysis py·rol·y·sis
Decomposition or transformation of a chemical compound caused by heat.
n gas chromatography. In: Hanin I, ed. Choline and acetylcholine: handbook of chemical assay methods. New York: Raven Press, 1974:151-62.
(21.) Hirsch MJ, Growdon JH, Wurtman RJ. Relations between dietary choline or lecithin intake, serum choline levels, and various metabolic indices. Metabolism 1978;27:953-60.
(22.) Lever M, Sizeland PC, Bason LM, Hayman CM, Robson RA, Chambers ST. Abnormal glycine betaine content of the blood and urine of diabetic and renal patients. Clin Chim Acta 1994;230: 69-79.
(23.) Savendahl L, Mar MH, Underwood LE, Zeisel SH. Prolonged fasting in humans results in diminished plasma choline concentrations but does not cause liver dysfunction. Am J Clin Nutr 1997;66:622-5.
(24.) Zeisel SH, Growdon JH, Wurtman RJ, Magil SG, Logue M. Normal plasma choline responses to ingested lecithin. Neurology 1980; 30:1226-9.
(25.) Zeisel SH, Da Costa KA, Franklin PD, Alexander EA, Lamont JT, Sheard NF, et al. Choline, an essential nutrient for humans. FASEB FASEB Federation of American Societies for Experimental Biology J 1991;5:2093-8.
PAL I. HOLM, PER MAGNE UELAND, * GRY (puzzle) gry - "Angry" and "hungry" are two words that end in "gry". What is the third word. Everyone knows what it means and everyone uses it every day. Look closely and I have already given you the third word. What is it?
Answer: "what". KVALHEIM, and ERNST A. LIEN
LOCUS for Homocysteine and Related Vitamins, University of Bergen The University of Bergen (Universitetet i Bergen) is located in Bergen, Norway. Although founded as late as 1946, academic activity had taken place at Bergen Museum as far back as 1825. The university today caters for more than 16,000 students. , N-5021 Bergen, Norway.
* Address correspondence to this author at: LOCUS for Homocysteine and Related Vitamins, Department of Pharmacology, University of Bergen, N-5021 Bergen, Norway. Fax 47-55-973115; e-mail firstname.lastname@example.org.
 Nonstandard non·stan·dard
1. Varying from or not adhering to the standard: nonstandard lengths of board.
2. abbreviations: DMG, N, N-dimethylglycine; MS, mass spectrometry; and LC, liquid chromatography.
Table 1. Analytical recovery of the assay. (a) Concentration, [micro] mol/L Added (b) Expected (b) Analyte Endogenous Low High Low High Choline 2.7 10 30 12.7 32.7 Betaine 11.4 20 60 31.4 71.4 DMG 0.91 2 6 2.9 6.9 Concentration, [micro] mol/L Measured (b) Recovery, (b) % Analyte Low High Low High Choline 13.2 (0.3) 33.8 (1.0) 105.2 (2.9) 103.7 (3.3) Betaine 32.0 (1.4) 67.6 (2.7) 102.9 (6.8) 93.7 (4.4) DMG 2.8 (0.14) 6.2 (0.3) 92.9 (6.8) 87.4 (4.3) (a) n = 18 for all concentrations. (b) Concentrations are given as the mean (SD). Table 2. Precision of the assay. Within-day Between-day Analyte Concentration, (b) CV, % Concentration, (b) CV, % [micro] mol/L [micro] mol/L Choline 2.7 (0.2) 6.2 2.7 (0.2) 8.8 13.2 (0.3) 2.1 12.7 (0.5) 3.5 33.8 (1.0) 2.9 31.2 (1.4) 4.3 Betaine 11.4 (0.5) 4.3 11.1 (0.6) 5.5 32.0 (1.4) 4.3 32.0 (1.3) 4.1 67.6 (2.7) 3.9 63.9 (3.3) 5.2 DMG 0.91 (0.07) 7.2 0.91 (0.08) 8.8 2.8 (0.1) 4.9 2.7 (0.2) 8.6 6.2 (0.3) 4.2 5.9 (0.5) 8.0 (a) n = 18 for within-day precision and 20 for between-day precision for all concentrations. (b) Concentrations are given as the mean SDI. Table 3. Concentrations of choline, betaine, and DMG in plasma and serum from healthy donors. Analyte Fasting Plasma Serum (n = 60) (n = 59) P (b) Choline, [micro] mol/L 8.0 (7.0-9.3) 9.8 (8.8-10.8) <0.001 Betaine, [micro] mol/L 31.7 (27.0-41.1) 32.8 (28.3-42.1) NS (d) DMG, [micro] mol/L 1.66 (1.30-2.02) 1.88 (1.39-2.19) NS Analyte Nonfasting Plasma Serum (n = 60) (n = 46) P (b) Choline, [micro] mol/L 10.5 (9.0-12.3) 11.9 (10.7-13.3) 0.003 Betaine, [micro] mol/L 41.7 (35.9-46.8) 41.5 (37.4-47.9) NS DMG, [micro] mol/L 2.07 (1.60-2.45) 1.93 (1.64-2.52) NS P, tasting vs Analyte nonfasting donors (c) Plasma Serum Choline, [micro] mol/L <0.0001 <0.0001 Betaine, [micro] mol/L <0.0001 <0.0001 DMG, [micro] mol/L 0.003 NS (a) Concentrations are given as the median (25th-75th percentiles). (b) Concentrations in plasma and serum compared by unpaired t-test. (c) Concentrations in fasting and nonfasting donors compared by unpaired t-test. (d) NS, not significant.