Assays for total homocysteine and other thiols by capillary electrophoresis-laser-induced fluorescence detection. I. Preanalytical condition studies

In recent papers, we presented a new analytical method for thiol quantification in serum. It is based on the use of capillary electrophoresis and laser-induced fluorescence to analyze thiol 6-iodoacetamidofluoresceine (IAF) derivatives. Quantitative results of homocysteine, glutathione, cysteine-glycin, and cysteine were shown (Clin. Chem. 45 (1999) 412). A comprehensive comparison of the quantitation of homocysteine in serum, using high-performance liquid chromatography/conventional fluorescence detection and fluorescence polarization immunoassay was also used (E. Caussé et al., Electrophoresis 21 (2000) 2074). Sample preparation prior to derivatization with IAF had never been investigated. In this work we present the results of quantitation of thiols in serum and plasma with three different anticoagulants widely used: ethylenediaminetetraacetic acid (EDTA), heparin, and sodium citrate. We show that serum and EDTA plasma gave the same results. Then serum protein precipitations by acetonitrile, acetone, sulfosalicylic acid, perchloric acid and trichloracetic acid, prior to derivatization by IAF, were also investigated. Their influence on the concentrations of the thiols were determined. Sulfosalicylic acid and acetonitrile precipitations are well adapted, whereas acetone cannot be used.     

N-methyl-D-glucamine improves the laser-induced fluorescence capillary electrophoresis performance in the total plasma thiols measurement

We describe an ultrarapid capillary electrophoresis with laser-induced fluorescence (CE-LIF) method for total plasma thiols measurement. Reduced thiols by 10% tri-n-butylphosphine (TBP) were derivatized in 10 min at room temperature with 5-iodoacetamidofluorescein (5-IAF) as fluorescent reagent. We show that CE-LIF allows a baseline separation of total plasma cysteinylglycine, homocysteine, cysteine, and glutathione in less than 5 min when N-methyl-D-glucamine in run buffer was added. CE was compared with high-performance liquid chromatography (HPLC) with fluorescence detection. The Bland-Altman test and Passing-Bablok regression demonstrates that the results obtained by CE-LIF and by HPLC are highly comparable. The simplified procedure of sample preparation, the short incubation and fast separation times, the high specificity, sensitivity and reproducibility, and the lower cost of analysis suggest that our proposed method can be considered valuable for the automation analysis in a routine laboratory.     

Determination of homocysteine and other low-molecular-weight amino thiols in blood plasma

A current approach to determining low-molecular-weight amino thiols (cysteine, homocysteine, and glutathione) in model samples and blood plasma is considered. Procedures for determining 2–100 μM homocysteine in blood plasma with the use of microcolumn chromatography and capillary electrophoresis were developed. Photometric and fluorimetric detection techniques were used to identify amino thiols. Monobromobimane and 5-iodoacetamidofluorescein were used as labels. Mass spectrometry was used to confirm the structures of test amino thiol derivatives.     

Measurement of total plasma and cerebrospinal fluid homocysteine by fluorescence following high-performance liquid chromatography and precolumn derivatization with o-phthaldialdehyde.

Precolumn derivatization of amino acids with o-phthaldialdehyde followed by high-performance liquid chromatographic separation and fluorescence detection is used in many clinical and experimental laboratories for the measurement of primary amino acids. This technique was adapted for the measurement of homocysteine in plasma and cerebrospinal fluid (CSF) following alkylation of the free sulphydryl group with iodoacetate. The minimum detection limits are less than 1 microM in plasma and 80 nM in CSF. Within-day and between-day coefficients of variation for plasma and for CSF are less than 10%. Values for normal plasma homocysteine range from 6.04 to 16.2 microM and for CSF from 0.28 to 0.66 microM.     

Fluorescent chemodosimeter for Cys/Hcy with a large absorption shift and imaging in living cells

A novel molecule T1 with efficient intramolecular charge transfer was designed as a fluorescent chemodosimeter for cysteine (Cys) and homocysteine (Hcy). Upon addition of Cys/Hcy, T1 exhibited greatly enhanced fluorescence intensity as well as a large absorption peak shift (70 nm), and can be used for bioimaging of Cys/Hcy in living cells and detection in human plasma by visual color change. The detection mechanism was proved by (1)H NMR, mass spectrometry analysis and Gaussian calculations.     

A one-step selective fluorescence turn-on detection of cysteine and homocysteine based on a facile CdTe/CdS quantum dots–phenanthroline system

In this paper, we report a simple, selective, sensitive and low-cost turn-on photoluminescent sensor for cysteine and homocysteine based on the fluorescence recovery of the CdTe/CdS quantum dots (QDs)–phenanthroline (Phen) system. In the presence of Phen, the fluorescence of QDs could be quenched effectively due to the formation of the non-fluorescent complexes between water-soluble thioglycolic acid (TGA)-capped QDs and Phen. Subsequently, upon addition of cysteine and homocysteine, the strong affinity of cysteine and homocysteine to QDs enables Phen to be dissociated from the surface of QDs and to form stable and luminescent complexes with cysteine and homocysteine in solution. Thus, the fluorescence of CdTe/CdS QDs was recovered gradually. A good linear relationship was obtained from 1.0 to 70.0 μM for cysteine and from 1.0 to 90.0 μM for homocysteine, respectively. The detection limits of cysteine and homocysteine were 0.78 and 0.67 μM, respectively. In addition, the method exhibited a high selectivity for cysteine and homocysteine over the other substances, such as amino acids, thiols, proteins, carbohydrates, etc. More importantly, the sensing system can not only achieve quantitative detection of cysteine and homocysteine but also could be applied in semiquantitative cysteine and homocysteine determination by digital visualization. Therefore, as a proof-of-concept, the proposed method has potential application for the selective detection of cysteine and homocysteine in biological fluids.     

Determination of different species of homocysteine in human plasma by high-performance liquid chromatography with ultraviolet detection

This assay measures reduced, free oxidized, protein-bound, and total homocysteine in human plasma. Oxidized species of homocysteine are converted to reduced form by sodium borohydride, and, after precolumn derivatization with 2-chloro-1-methylquinolinium tetrafluoroborate, homocysteine 2-S-quinolinium derivative is separated from those of other plasma thiol derivatives, and quantitated by ion-paired reversed-phase high-performance liquid chromatography with ultraviolet detection. The reduced homocysteine sulfhydryl groups are trapped with minimal oxidation by derivatizing blood samples at the time of collection. With the use of this precise and sensitive HPLC method utilizing popular ultraviolet detection, homocysteine in plasma can be detected and quantitated at the level of 0.1 and 0.2 for reduced fraction, and 0.3 and 0.5 nmol/ml for total homocysteine, respectively. The method is applied for determination of different fractions of homocysteine in plasma of apparently healthy men and women.     

高效液相色谱法测定人血浆中总高半胱氨酸含量

建立了快速检测人血浆中总高半胱氨酸含量的高效液相色谱（HPLC）方法。采用反相HPLC分析前,以巯基特异性荧光试剂ABDF对血浆中巯基进行衍生．方法简便、灵敏、准确,无干扰峰影响。平均回收率为97．75％,日内和日间精密度分别为4．49％和9．79％。     

Rapid and Sensitive Determination of Five Amine Biomarkers in Plasma Samples from Stroke Patients by MEKC with Precolumn Derivatization

The current diagnosis of stroke remains hampered and delayed due to lack of a suitable mechanism for the rapid, accurate, and analytically sensitive biomarker-based testing methods. In the present study, a MEKC method coupled with highly sensitive laser-induced fluorescence detection and precolumn derivatization was originally described for the rapid, sensitive determination of five amine biomarkers including homocysteine (Hcy), glutamic acid (Glu), putrescine (Put), spermine (Spm) and spermidine (Spd) of stroke in plasma samples from stroke patients. 3-(2-Furoyl)quinoline-2-carboxaldehyde (FQ) was selected as the fluorescence reagent and optimized for the advantageous high-throughput derivatization with total 30 μL reaction system in 384-well microplates for the rapid determination of the targeted analytes. Results showed that the proposed derivatization reaction approach coupled with MEKC-LIF was an efficient method for the rapid determination of five amine biomarkers of stroke in plasma samples.

     

A fluorescence turn-on probe for the detection of thiol-containing amino acids in aqueous solution and bioimaging in cells

A turn-on fluorescent probe, based on a water-soluble terphenyl derivative, for the detection of cysteine and homocysteine is reported. The aldehyde groups in the probe play crucial roles in providing reaction with thiol groups in the amino acids, leading to a formation of thiazolidine (from cysteine) or thiazinane ring (from homocysteine). As a result, the new formation of such rings alters the electronic property of the conjugated system in the probe and results in emission enhancement. The probe in aqueous solution exhibits a remarkable increase in its quantum yield upon exposure to cysteine (up to 20-fold) and to homocysteine (up to 700-fold), while slight quenching is observed in the presence of glutathione. Moreover, an investigation on time-resolved fluorescence spectra of the probe in the presence of cysteine and homocysteine reveals potential discriminatory detection of cysteine and homocysteine. Bioimaging of the thiols in live HeLa cells was successfully applied.     

Determining sulfur-containing amino acids by capillary electrophoresis: a fast novel method for total homocyst(e)ine human plasma

A high-performance capillary electrophoresis (HPCE) method based on laser-induced fluorescence detection is presented here. It enables the determination of sulfur-containing amino acids within 15 min. Fluorescence of sulfur-containing amino acids in plasma is linear over a range of 50-150 micromol/L for L-methionine, 5-100 micromol/L for L-homocysteine, and 50-200 micromol/L for L-cysteine. For homocysteine, we were able to detect 1 fmol injected, equivalent to a plasma concentration of 10 nmol/L. A similar sensitivity is present for cysteine, an even lower one being found for methionine. The intra- and interassay relative standard deviations are < 1%. High-performance liquid chromatography (HPLC) methods are commonly employed for quantifying blood concentrations of sulfur-containing amino acids. A comparative analysis of HPCE and HPLC quantitation of homocysteine has been carried out in 61 blood samples. Plasma concentrations measured by HPCE were in good agreement with those obtained employing an HPLC-based method, a satisfactory correlation being observed between the concentrations obtained by the two methods (r= 0.9972). Thus, the HPCE-based procedure presented here for the measurement of sulfur-containing amino acids in plasma is a simple, fast, accurate, and very sensitive method, suitable for routine determinations in clinical studies.     

Measurement of thiols in human plasma using liquid chromatography with precolumn derivatization and fluorescence detection

A liquid chromatography (LC) method for the simultaneous measurement of the main low molecular mass thiols (i.e., cysteine, cysteinylglycine, homocysteine, and glutathione) in human plasma is described. The sample treatment consists of the reduction of disulfide bounds with tri-n-butylphosphine and protein precipitation with trichloroacetic acid followed by precolumn derivatization with a thiol-selective fluorogenic reagent (7-fluoro-2,1,3-benzoxadiazole-4-sulfonamide). The structure of thiol derivatives is assessed using electrospray ionization-mass spectrometry (MS). The stability of resulting adducts in acidic medium (24 h at 10 degrees C) allows the automation of the technique and a high throughput of samples (approximately 50 per day). Separation is complete within 12 min using isocratic reversed-phase mode, and detection is operated by spectrofluorimetry (lambda ex = 385 nm and lambda em = 515 nm). Quantitation is performed by an internal standardization mode using thioglycolic acid. The LC method is fully validated, and homocysteine concentrations obtained in plasma samples are compared with values measured using either fluorescence polarization immunoassay or capillary gas chromatography-MS; a good correlation is observed between LC and both methods. The method has been applied in daily use to a large-scale study in a human healthy population, and some resulting data are discussed.     

柱前衍生高效液相色谱-荧光检测法测定血浆中同型半胱氨酸

建立了一种柱前衍生高效液相色谱-荧光检测法用于测定血浆中同型半胱氨酸(Hcy)。使用三(2-羧乙基)膦盐酸盐(TCEP)为还原剂,N-(1-芘)马来酰亚胺(NPM)为衍生剂进行样品预处理,Agilent Hypersil C-18柱(250 mm×4.0 mm, 5 μm)进行分离,流动相为15 mmol/L醋酸钠-乙腈-混合酸(300 mL水中含1 mL醋酸和1 mL磷酸)混合溶液,采用梯度洗脱,荧光检测激发波长为330 nm,发射波长为380 nm。Hcy的回收率为(102.08±4.94)%。线性范围为0.500～100 μmol/L,检出限（以信噪比为3计）为0.016 μmol/L。日内与日间相对标准偏差均小于5%。利用该方法对7例高血压患者和7例健康志愿者的血浆进行了测定,结果表明两组间的Hcy含量存在显著的差异(p＜0.05)。本方法简单、快速、灵敏、特异,适用于血浆Hcy的临床定量测定。     

Microchannel-assisted thermal-lens spectrometry for microchip analysis

We present a new method for homocysteine quantitation in human plasma based on in-capillary reaction of homocysteine with 2,2′-dipyridyl disulfide. Homocysteine is in this so-called thiol-exchange reaction quantitatively transformed in mixed disulfide concomitantly with formation of an equimolar amount of 2-thiopyridone that is further separated by micellar electrokinetic chromatography and determined specifically at 343 nm. The concentration of homocysteine is thus estimated indirectly from the result of 2-thiopyridone determination. The linear detection range for concentration versus peak area for the assay was from 0.03–3 mM (correlation coefficient 0.994) with a detection limit of 6 μM and a limit of quantitation 20 μM. The inter-day reproducibility of the peak area and the migration time were 1.37% and 0.05%, respectively. The method is simple, relatively rapid and can be easily automated. Moreover the common capillary electrophoresis apparatus with a UV detector can be used to distinguish between normal and pathological hyperhomocysteinemia plasma samples.     

High-throughput capillary electrophoretic method for determination of total aminothiols in plasma and urine.

Increased interest in the analysis of aminothiols in body fluids during the last years results in a request for high-throughput analytical methods for their determination. We report here a novel, high-throughput method for the determination of total concentrations of biogenous aminothiols - homocysteine, cysteine, glutathione, cysteinylglycine, gamma-glutamylcysteine, and of penicilamine, mercaptopropionylglycine, and cysteamine, three compounds used to treat disorders of aminothiol metabolism in plasma and urine. Samples were reduced with tris(carboxyethyl)phosphine and labeled with 5-(bromomethyl)fluorescein. Capillary electrophoretic separations were performed in 60 mmol/L borate - 15 mmol/L sodium dodecyl sulfate - 2-amino-2-methyl-1-propanol, pH 10.0, with laser-induced fluorescence detection. Analysis time was less than 2 min. The assay is linear (r > 0.999) up to 500 micromol/L. Reproducibilities of migration times (coefficient of variation, CV) were < 0.5%. Interassay repeatabilities (CV, n = 10) were 5.08% and 6.09% for 5 micromol/L addition of homocysteine and 0.60% and 3.78% for 100 micromol/L addition of cysteine in plasma and urine, respectively. Recovery values were within 94-106% and sensitivity was better than 0.19 micromol/L for all analyzed compounds. Results agreed well with a standard high-performance liquid chromatography (HPLC) method. The diagnostic usefulness of the method has been proven on 79 samples of cystinuric patients and 12 samples of homocystinuric patients. We report here a novel method for the determination of aminothiols in body fluids by capillary electrophoresis (CE). Determination is fast and sensitive enough for diagnostic purposes.     

Plasma thiols redox status by laser-induced fluorescence capillary electrophoresis

High concentrations of total plasma thiols such as cysteine and homocysteine are important risk factors for atherosclerosis and cardiovascular diseases. We have recently described a new laser-induced fluorescence capillary electrophoresis (CE-LIF) method to measure total plasma thiols, in which the baseline separation of cysteinylglycine, homocysteine, cysteine, and glutathione was achieved by adding the organic base N-methyl-D-glucamine to the run buffer. However, because the active fractions of homocysteine and cysteine responsible for vascular injuries are still unknown, research calls for a set up of methods able to analyze different forms of plasma thiols. In this paper, we present an improvement of our previous method that allows the measurement of different thiol forms. Total, reduced, and free thiols were measured by varying the order of disulfide reduction with tributylphosphine and proteins precipitation with 5-sulfosalicylic acid. After derivatization with 5-iodoacetamidofluorescein, samples were separated and measured by CE-LIF using a phosphate/borate buffer in the presence of 75 mmol/L N-methyl-D-glucamine. Oxidized thiols and protein bound thiols were calculated by difference, free minus reduced and total minus free form, respectively. Linearity, reproducibility, analytical recovery, and sensitivity were evaluated. The assay was used to measure the thiols redox status in 15 plasma samples from healthy volunteers.     

Detection of homocysteine by conventional and microchip capillary electrophoresis/electrochemistry

A method based on capillary electrophoresis (CE) with electrochemical (EC) detection for the determination of both total homocysteine (tHcy) and protein-bound homocysteine (pbHcy) in plasma is described. Both end-column and off-column amperometric detection were investigated. Off-column detection resulted in a more sensitive assay for the determination of homocysteine (Hcy). The detection limit for homocysteine was 500 nM using off-column EC detection and the response was linear over the range 1-100 microM. Therefore, this assay is appropriate for the quantification of Hcy over the physiological concentration ranges found in all disease states. Methodologies for the determination of tHcy and pbHcy in human plasma were investigated and optimized and the concentrations of both pbHcy and tHcy in plasma obtained from a healthy individual were determined to be 2.79+/-0.31 nuM (n = 4) and 3.37+/-0.15 microM (n = 3), respectively. The methodology was then transferred to a microchip CE-EC format and Hcy and reduced glutathione (GSH) were detected. Future work will focus on the development of ancillary methodologies to identify the other forms of Hcy in vivo.     

Interaction of CdTe nanocrystals with thiol-containing amino acids at different pH: a fluorimetric study

3-Mercaptopropionic acid (MPA)-capped CdTe nanocrystals (NCs) were synthesized in aqueous medium, and their interaction with cysteine (Cys) and homocysteine (Hcy) was studied by steady-state and time-resolved fluorescence spectra at different pH. At 6.4?<?pH?<?8.0, the fluorescence of CdTe NCs can be effectively enhanced by Cys and Hcy. While pH?>?9.6, only Cys quenches the fluorescence of the CdTe NCs, no fluorescence changes are observed for Hcy. Mechanism study shows that these pH manipulating fluorescence responses can be attributed to the following two reasons: first, both the thiol–thiolate equilibrium of Cys (Hcy) and the number of undercoordinated NCs surface sites capped with dual coordinated ligands are strong pH-dependent; second, different thiol-containing amino acids, with different redox energy level, can lead to distinguishable fluorescence responses of NCs. Based on these unique fluorescence responses, the possibilities of developing a sensitive detecting technique for Cys/Hcy and Cys through pH modulation can be explored.     

The development of a fluorescence turn-on sensor for cysteine, glutathione and other biothiols. A kinetic study

Two fluorescence probes for the detection of cysteine (Cys), glutathione (GSH) and other biothiols, such as homocysteine (Hcy) and cysteinyl-glycine (Cys-Gly), were developed. These molecular probes are coumarin-based derivatives containing a chalcone-like moiety that reacts with biothiols through a Michael addition reaction, leading to strong fluorescence enhancements. The reactivity of the tested biothiols toward both probes (ChC1 and ChC2) follows the order Cys > GSH > Hcy > Cys-Gly, ChC1 being less reactive than ChC2. Possible interference with other amino acids was assessed. ChC1 and ChC2 display a highly selective fluorescence enhancement with thiols, allowing these probes to be used for fluorimetric thiol determination in SH-SY5Y cells.     

基于碳点荧光恢复法测定生物巯基化合物

碳点(CDots)是一种新型荧光纳米材料,Cu2+可以有效猝灭其荧光;而当有生物巯基化合物存在时,碳点-Cu2+体系的荧光可以恢复.基于此原理,我们成功地构建了检测生物体内总巯基化合物的新方法.该方法具有很好的选择性,常见氨基酸和金属离子对谷胱甘肽(GSH)、半胱氨酸(Cys)和高半胱氨酸(Hcy)的检测无影响.最佳实验条件下,谷胱甘肽、半胱氨酸、高半胱氨酸的浓度在6.0×10-6mol/L～1.0×10-4mol/L与相对荧光强度呈线性,R>0.996,检出限为2.0×10-6mol/L.该体系成功用于血清样品中总巯基化合物的检测.