Quantitative assay of plasma homocysteine thiolactone by gas chromatography/mass spectrometry

Comprehensive studies on the development of HPLC-MS/MS and HPLC-FL based methods for routine determination of homocysteine thiolactone in human urine

The report presents a new, robust, and reproducible liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) and HPLC-fluorescence (FL) based methods for the determination of urinary homocysteine thiolactone (HTL). In particular, a versatile sample preparation procedure was designed to purify urine samples, involving chloroform liquid-liquid extraction (LLE) of HTL and its re-extraction (re-LLE) with formic acid, prior to chromatographic analysis. In relation to HPLC-FL assay, the quantification of HTL additionally uses o-phthaldialdehyde (OPA) as the on-column derivatization agent, while HPLC-MS/MS assay employs homoserine lactone (HSL) as an internal standard (IS). The baseline separation of the analyte and IS (if applicable) is accomplished under hydrophilic interactions chromatography (HILIC) and reverse phase (RP)-HPLC conditions in the case of HPLC-MS/MS and HPLC-FL based method, respectively. The assays linearity was observed within 20-400 nmol/L for HTL in urine, covering the expected unknown analyte's concentration in study samples. The value of 20 nmol/L in urine was recognized as the limit of quantification (LOQ) for both methods. The assays were successfully applied to urine samples delivered by fifteen apparently healthy volunteers showing that they are suitable for screening of human urine.

The Use of Single Drop Microextraction and Field Amplified Sample Injection for CZE Determination of Homocysteine Thiolactone in Urine

Two cheap, simple and reproducible methods for the electrophoretic determination of homocysteine thiolactone (HTL) in human urine have been developed and validated. The first method utilizes off-line single drop microextraction (SDME), whereas the second one uses off-line SDME in combination with field amplified sample injection (FASI). The off-line SDME protocol consists of the following steps: urine dilution with 0.2 mol/L, pH 8.2 phosphate buffer (1:2, v/v), chloroform addition, drop formation and extraction of HTL. The pre-concentration of HTL inside a separation capillary was performed by FASI. For sample separation, the 0.1 mol/L pH 4.75 phosphate buffer served as the background electrolyte, and HTL was detected at 240 nm. A standard fused-silica capillary (effective length 55.5 cm, 75 μm id) and a separation voltage of 21 kV (~99 μA) were used. Electrophoretic separation was completed within 7 min, whereas the LOD and LOQ for HTL were 0.04 and 0.1 μmol/L urine, respectively. The calibration curve in urine was linear in the range of 0.1-0.5 μmol/L, with R2 = 0.9991. The relative standard deviation of the points of the calibration curve varied from 2.4% to 14.9%. The intra- and inter-day precision and recovery were 6.4-10.2% (average 6.0% and 6.7%) and 94.9-102.7% (average 99.7% and 99.5%), respectively. The analytical procedure was successfully applied to the analysis of spiked urine samples obtained from apparently healthy volunteers.

Determination of homocysteine thiolactone in human urine by capillary zone electrophoresis and single drop microextraction

A simple, fast, sensitive and reproducible capillary zone electrophoresis (CZE) method with single drop microextraction (SDME) for determination of homocysteine thiolactone (HTL) in human urine has been developed and validated. The method is characterized by good precision, high accuracy, short analysis time and low consumption of reagents. The procedure consists only of few steps: urine sample centrifugation, dilution with phosphate buffer and methanol, chloroform addition onto the top of donor phase, on-line SDME in CE system, sample separation by CZE and ultraviolet detection of HTL at 240 nm. The background electrolyte was 0.1 M pH 4.75 phosphate buffer. Effective separation was achieved within 6.04 min under the separation voltage of 24 kV (~110 μA). The LOQ and LOD for HTL were 50 and 25 nM urine, respectively. The calibration curve in urine showed linearity in the range of 50-200 nM, with R² 0.9995. The intra- and inter-day precision and recovery were 4.0-14.5% (average 8.7% and 9.3%) and 92.7-115.5% (average 103.6% and 104.8%), respectively. The procedure was successfully applied to analysis of urine samples.

Application of GC-MS technique for the determination of homocysteine thiolactone in human urine

It is well established that homocysteine thiolactone (HTL) is associated with some health disorders, including cardiovascular diseases. HTL is a by-product of sulfur metabolic cycle. So far, its presence has been confirmed in human plasma and urine. It has been also shown that a vast majority of HTL is removed from human body through kidney. Thus, the aim of the current investigations has been the identification, separation and quantification of HTL in urine samples. For the first time a cheap, reliable and robust GC-MS method was developed for the determination of HTL in human urine in the form of its volatile isobutyl chloroformate derivative. Separation of the analyte and internal standard (homoserine lactone (HSL)) was achieved in 15 min followed by mass spectrometry detection (MS). Isocratic elution was accomplished with helium at a flow rate of 1 mL min^-1 and a gradient of the column temperature was concomitant with the analysis. The mass spectrometer was set to the electron impact mode at 70 eV. The ion source, quadrupole and MS interface temperatures were set to 230 °C, 150 °C and 250 °C, respectively. Elaborated analytical procedure allows quantification of analyte in a linear range of 0.01-0.20 nmol mL^-1 urine. The LOQ and LOD values were 0.01 and 0.005 nmol mL^-1, respectively. The method accuracy ranged from 98.0% to 103.2%, while precision varied from 6.4% to 9.5% and from 10.7% to 16.9% for intra- and inter-day measurements, respectively. Finally, the method has been successfully implemented in the analysis of 12 urine samples donated by apparently healthy volunteers. Concentration of HTL ranged from <LOQ to 163 pmol mL^-1 urine (0.51 to 13.1 μmol mol^-1 Crn).

Development and validation of a LC-MS/MS method for homocysteine thiolactone in plasma and evaluation of its stability in plasma samples

The present study demonstrates the development and validation of a sensitive method for the quantification of homocysteine thiolactone (HCTL) in human plasma using the technique of LC-MS/MS. The gradient elution of HCTL was achieved within 5min using ZIC HILIC column having acetonitrile with 0.1% formic acid and water with 0.1% formic acid. The method was validated for the linearity, sensitivity, accuracy, precision, recovery, matrix effect and stability. A good linearity was found within a range of 0.5-32.5nmol/ml. Quantification was performed using multiple reaction monitoring (MRM) mode based on the molecular/fragment ion transitions for HCTL (118/56) and homatropine (276.1/142.2) as internal standard. Generally, HCTL levels in plasma were found to be highly unstable. In order to verify the stability of the HCTL levels in plasma for a longer period, the samples were extracted immediately and stored at -86°C. Using the above method it was found to be stable for a period of 1 month. The method was well applied for quantification of HCTL in plasma of healthy human volunteers.

Chemical biology of homocysteine thiolactone and related metabolites

Protein-related homocysteine (Hcy) metabolism produces Hcy-thiolactone, N-Hcy-protein, and N epsilon-homocysteinyl-lysine (N epsilon-Hcy-Lys). Hcy-thiolactone is generated in an error-editing reaction in protein biosynthesis when Hcy is erroneously selected in place of methionine by methionyl-tRNA synthetase. Hcy-thiolactone, an intramolecular thioester, is chemically reactive and forms isopeptide bonds with protein lysine residues in a process called N-homocysteinylation, which impairs or alters the protein's biological function. The resulting protein damage is exacerbated by a thiyl radical-mediated oxidation. N-Hcy-proteins undergo structural changes leading to aggregation and amyloid formation. These structural changes generate proteins, which are toxic and which induce an autoimmune response. Proteolytic degradation of N-Hcy-proteins generates N epsilon-Hcy-Lys. Levels of Hcy-thiolactone, N-Hcy-protein, and N epsilon-Hcy-Lys increase under pathological conditions in humans and mice and have been linked to cardiovascular and brain disorders. This chapter reviews fundamental biological chemistry of Hcy-thiolactone, N-Hcy-protein, and N epsilon-Hcy-Lys and discusses their clinical significance.

Plasma homocysteine thiolactone associated with risk of macrovasculopathy in Chinese patients with type 2 diabetes mellitus

INTRODUCTION

This study investigated the role of homocysteine thiolactone (HcyT) in the development of macrovascular complications in Chinese patients with type 2 diabetes. HcyT has been proposed as a possible molecular basis for homocysteine (Hcy)-induced vascular damage.

METHODS

One hundred and sixty subjects were recruited into this study: 40 healthy controls and 120 patients with type 2 diabetes. Plasma Hcy levels were measured by polarization immunoassay and HcyT concentrations were monitored using high-performance liquid chromatography on a reversephase C18 column with ultraviolet detection. Plasma folic acid and vitamin B(12) levels were measured using radioimmunoassay methods.

RESULTS

Plasma Hcy and HcyT concentrations in patients with type 2 diabetes were significantly higher than in healthy controls (Hcy [25th and 75th quartiles]: 9.28 [7.51-11.82] vs. 5.64 [5.17-8.00] micromol/L, P=0.01; HcyT: 3.38 [2.94-4.73] vs. 2.91 [2.77-3.08] nmol/L, P<0.05). Plasma Hcy and HcyT levels in patients with macrovasculopathy (MAVP) were significantly higher compared with patients without MAVP (Hcy: 10.36 [7.67-12.45] vs. 7.85 [6.76-10.52] micromol/L, P<0.05; HcyT: 4.27 [3.02-5.11] vs. 3.12 [2.63-3.77] nmol/L, P<0.05). Plasma HcyT concentrations were positively correlated with urinary excretion of albumin/creatinine (Alb/Cr; r=0.285, P=0.007), duration of diabetes (r=0.249, P=0.019), age (r=0.233, P=0.028), and fibrinogen levels (r=0.289, P=0.034). Plasma HcyT concentrations were negatively correlated with high-density lipoprotein levels (r=-0.223, P=0.037). Binary logistic regression showed that HcyT, Hcy, smoking, serum triglyceride, and urine Alb/Cr were significantly associated with the risk of diabetic MAVP (P<0.05).

CONCLUSION

Hcy and HcyT levels were associated with the development and progression of diabetic MAVP. HcyT may provide a plausible chemical mechanism for explaining Hcy toxicity in the human vascular endothelium.

Mechanisms of homocysteine toxicity in humans

Homocysteine, a non-protein amino acid, is an important risk factor for ischemic heart disease and stroke in humans. This review provides an overview of homocysteine influence on endothelium function as well as on protein metabolism with a special respect to posttranslational modification of protein with homocysteine thiolactone. Homocysteine is a pro-thrombotic factor, vasodilation impairing agent, pro-inflammatory factor and endoplasmatic reticulum-stress inducer. Incorporation of Hcy into protein via disulfide or amide linkages (S-homocysteinylation or N-homocysteinylation) affects protein structure and function. Protein N-homocysteinylation causes cellular toxicity and elicits autoimmune response, which may contribute to atherogenesis.

Pathophysiological consequences of homocysteine excess

Elevated level of the nonprotein amino acid homocysteine (Hcy) is a risk factor for cardiovascular diseases, neurodegenerative diseases, and neural tube defects. However, it is not clear why excess Hcy is harmful. To explain Hcy toxicity, the "Hcy-thiolactone hypothesis" has been proposed. According to this hypothesis, metabolic conversion of Hcy to a chemically reactive metabolite, Hcy-thiolactone, catalyzed by methionyl-tRNA synthetase is the first step in a pathway that contributes to Hcy toxicity in humans. Plasma Hcy-thiolactone levels are elevated in human subjects with hyperhomocysteinemia caused by mutations in CBS or MTHFR genes. Plasma and urinary Hcy-thiolactone levels are also elevated in mice fed a high-methionine diet. Hcy-thiolactone can be detrimental because of its intrinsic ability to form N-Hcy-protein adducts, in which a carboxyl group of Hcy is N-linked to epsilon-amino group of a protein lysine residue. This article reviews recent studies of Hcy-thiolactone and N-Hcy-protein in the human body, including their roles in autoimmune response, cellular toxicity, and atherosclerosis. Potential utility of Hcy-thiolactone, N-Hcy-protein, or anti-N-Hcy-protein autoantibodies as markers of Hcy excess is discussed.

The determination of homocysteine-thiolactone in human plasma

The thioester homocysteine-thiolactone, a reactive metabolite of homocysteine, has been implicated in human cardiovascular disease. However, data on the levels of homocysteine-thiolactone in humans are limited, mostly due to a lack of facile and reliable assays. Here we describe a sensitive assay for the determination of plasma homocysteine-thiolactone and demonstrate its utility with a cohort of 60 healthy human subjects. Plasma homocysteine-thiolactone is first separated from macromolecules by ultrafiltration and then selectively extracted with chloroform/methanol. Further purification of plasma homocysteine-thiolactone is achieved by high-performance liquid chromatography on a cation exchange microbore column. The detection and quantification is by monitoring fluorescence after postcolumn derivatization with o-phthaldialdehyde. The limit of detection is 0.36 nM. Using this assay, homocysteine-thiolactone concentrations in plasma from normal healthy human subjects (n=60) were found to vary from zero to 34.8 nM, with an average of 2.82+/-6.13 nM. In 29 of the 60 human plasma samples analyzed, homocysteine-thiolactone levels were below the detection limit. Homocysteine-thiolactone represented from 0 to 0.28%, on average 0.023+/-0.05%, of plasma total homocysteine.

Urinary Excretion of Homocysteine-Thiolactone in Humans

Background: A metabolite of homocysteine (Hcy), the thioester Hcy-thiolactone, has been implicated in coronary heart disease in humans. Because inadvertent reactions of Hcy-thiolactone with proteins can lead to cell and tissue damage, the ability to detoxify or eliminate Hcy-thiolactone is essential for biological integrity. We examined the hypothesis that the human body eliminates Hcy-thiolactone by urinary excretion.

Methods: We used a sensitive HPLC method with postcolumn derivatization and fluorescence detection to examine Hcy-thiolactone concentrations in human urine and plasma.

Results: We discovered a previously unknown pool of Hcy-thiolactone in human urine. Urinary concentrations of Hcy-thiolactone (11–485 nmol/L; n = 19) were ∼100-fold higher than those in plasma (&lt;0.1–22.6 nmol/L; n = 20). Urinary Hcy-thiolactone accounted for 2.5–28.3% of urinary total Hcy, whereas plasma Hcy-thiolactone accounted for &lt;0.002–0.29% of plasma total Hcy. Urinary concentrations of Hcy-thiolactone, but not of total Hcy, were negatively correlated with urinary pH. Clearance of Hcy-thiolactone, relative to creatinine, was 0.21–6.96. In contrast, relative clearance of Hcy was 0.001–0.003.

Conclusions: The analytical methods described here can be used to quantify Hcy-thiolactone in biological fluids. Using these methods we showed that the human body eliminates Hcy-thiolactone by urinary excretion. Our data also suggest that the protonation status of its amino group affects Hcy-thiolactone excretion.

Cross-talk between Cys34 and Lysine Residues in Human Serum Albumin Revealed by N-Homocysteinylation

Protein N-homocysteinylation involves a post-translational modification by homocysteine (Hcy)-thiolactone. In humans, about 70% of circulating Hcy is N-linked to blood proteins, mostly to hemoglobin and albumin. It was unclear what protein site(s) were prone to Hcy attachment and how N-linked Hcy affected protein function. Here we show that Lys(525) is a predominant site of N-homocysteinylation in human serum albumin in vitro and in vivo. We also show that the reactivity of albumin lysine residues, including Lys(525), is affected by the status of Cys(34). The disulfide forms of circulating albumin, albumin-Cys(34)-S-S-Cys and albumin-Cys(34)-S-S-Hcy, are N-homocysteinylated faster than albumin-Cys(34)-SH. Although N-homocysteinylations of albumin-Cys(34)-SH and albumin-Cys(34)-S-S-Cys yield different primary products, subsequent thiol-disulfide exchange reactions result in the formation of a single product, N-(Hcy-S-S-Cys)-albumin-Cys(34)-SH. We also show that N-homocysteinylation affects the susceptibility of albumin to oxidation and proteolysis. The data suggest that a disulfide at Cys(34) of albumin promotes conversion of N-(Hcy-SH)-albumin-Cys(34)-SH to a proteolytically sensitive form N-(Hcy-S-S-Cys)-albumin-Cys(34)-SH, which would facilitate clearance of the N-homocysteinylated form of mercaptoalbumin.

Homocysteine-thiolactone and S-nitroso-homocysteine mediate incorporation of homocysteine into protein in humans

Indirect pathways, involving homocysteine (Hcy)-thiolactone and S-nitroso-Hcy, allow incorporation of Hcy into protein. Hcy-thiolactone, synthesized by methionyl-tRNA synthetase in all organisms investigated, including human, modifies proteins post-translationally by forming adducts in which Hcy is linked by amide bonds to epsilon-amino group of protein lysine residues. S-Nitroso-Hcy, synthesized in human vascular endothelial cells, is incorporated translationally into peptide bonds in protein at positions normally occupied by methionine. Hcy-N-hemoglobin and Hcy-N-albumin constitute a major pool of Hcy in human blood. Hcy-thiolactone is present in human plasma. Modification with Hcy-thiolactone leads to protein damage. Hcy-thiolactone is detoxified by Hcy-thiolactonase/paraoxonase present in a subset of high-density lipoprotein particles in humans.