Generation and initial characterization of a novel polyclonal antibody directed against homocysteine thiolactone-modified low density lipoprotein

Mechanism of arterial injury exacerbated by hyperhomocysteinemia in spontaneously hypertensive rats

Hypertension associated with hyperhomocysteinemia (HHcy) accounts for 75% of hypertension in China. HHcy plays a synergistic role with hypertension in vascular injury and significantly increases the incidence of cardiovascular and cerebrovascular diseases. The present study aimed to explore the molecular mechanism of HHcy-induced arterial injury in hypertension. Spontaneously hypertensive rats (SHR) were injected intraperitoneally with DL-homocysteine (Hcy) to construct the model of hypertension associated with HHcy (HHcy + SHR). Biological network was employed to identify the material basis of arterial injury in hypertension associated with HHcy. The prediction molecules in oxidative stress and inflammation pathways were experimentally verified by quantitative real-time polymerase chain reaction (qRT-PCR) and western blot (WB) analysis. The HHcy + SHR group significantly increased oxidative stress pathway molecules: nicotinamide adenine dinucleotide phosphate oxidase (Nox); inflammatory pathway molecules: vascular adhesion protein-1 (VAP-1), interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-a); as well as inflammatory pathway regulatory factors: nuclear factor-κ-gene binding (NF-κB) p65 and protein kinase B (Akt1). Among them, IL-6 was also significantly increased in the HHcy group. Both oxidative stress and inflammation contributed to the arterial injury of hypertension associated with HHcy, and inflammation mechanism might play a leading role in HHcy aggravating arterial injury, at least partially through the Akt1/NF-κB p65/IL-6 signaling pathway.

Current progress on the mechanisms of hyperhomocysteinemia-induced vascular injury and use of natural polyphenol compounds

Cardiovascular disease is one of the most common diseases in the elderly population, and its incidence has rapidly increased with the prolongation of life expectancy. Hyperhomocysteinemia is an independent risk factor for various cardiovascular diseases, including atherosclerosis, and damage to vascular function plays an initial role in its pathogenesis. This review presents the latest knowledge on the mechanisms of vascular injury caused by hyperhomocysteinemia, including oxidative stress, endoplasmic reticulum stress, protein N-homocysteinization, and epigenetic modification, and discusses the therapeutic targets of natural polyphenols. Studies have shown that natural polyphenols in plants can reduce homocysteine levels and regulate DNA methylation by acting on oxidative stress and endoplasmic reticulum stress-related signaling pathways, thus improving hyperhomocysteinemia-induced vascular injury. Natural polyphenols obtained via daily diet are safer and have more practical significance in the prevention and treatment of chronic diseases than traditional drugs.

Homocysteine Modification in Protein Structure/Function and Human Disease

Epidemiological studies established that elevated homocysteine, an important intermediate in folate, vitamin B₁₂, and one carbon metabolism, is associated with poor health, including heart and brain diseases. Earlier studies show that patients with severe hyperhomocysteinemia, first identified in the 1960s, exhibit neurological and cardiovascular abnormalities and premature death due to vascular complications. Although homocysteine is considered to be a nonprotein amino acid, studies over the past 2 decades have led to discoveries of protein-related homocysteine metabolism and mechanisms by which homocysteine can become a component of proteins. Homocysteine-containing proteins lose their biological function and acquire cytotoxic, proinflammatory, proatherothrombotic, and proneuropathic properties, which can account for the various disease phenotypes associated with hyperhomocysteinemia. This review describes mechanisms by which hyperhomocysteinemia affects cellular proteostasis, provides a comprehensive account of the biological chemistry of homocysteine-containing proteins, and discusses pathophysiological consequences and clinical implications of their formation.

Hyperhomocysteinemia, Suppressed Immunity, and Altered Oxidative Metabolism Caused by Pathogenic Microbes in Atherosclerosis and Dementia

Many pathogenic microorganisms have been demonstrated in atherosclerotic plaques and in cerebral plaques in dementia. Hyperhomocysteinemia, which is a risk factor for atherosclerosis and dementia, is caused by dysregulation of methionine metabolism secondary to deficiency of the allosteric regulator, adenosyl methionine. Deficiency of adenosyl methionine results from increased polyamine biosynthesis by infected host cells, causing increased activity of ornithine decarboxylase, decreased nitric oxide and peroxynitrate formation and impaired immune reactions. The down-regulation of oxidative phosphorylation that is observed in aging and dementia is attributed to deficiency of thioretinaco ozonide oxygen complexed with nicotinamide adenine dinucleotide and phosphate, which catalyzes oxidative phosphorylation. Adenosyl methionine biosynthesis is dependent upon thioretinaco ozonide and adenosine triphosphate (ATP), and the deficiency of adenosyl methionine and impaired immune function in aging are attributed to depletion of thioretinaco ozonide from mitochondrial membranes. Allyl sulfides and furanonaphthoquinones protect against oxidative stress and apoptosis by increasing the endogenous production of hydrogen sulfide and by inhibiting electron transfer to the active site of oxidative phosphorylation. Diallyl trisulfide and napabucasin inhibit the signaling by the signal transducer and activator of transcription 3 (Stat3), potentially enhancing immune function by effects on T helper lymphocytes and promotion of apoptosis. Homocysteine promotes endothelial dysfunction and apoptosis by the unfolded protein response and endoplasmic reticulum stress through activation of the N-methyl D-aspartate (NMDA) receptor, causing oxidative stress, calcium influx, apoptosis and endothelial dysfunction. The prevention of atherosclerosis and dementia may be accomplished by a proposed nutritional metabolic homocysteine-lowering protocol which enhances immunity and corrects the altered oxidative metabolism in atherosclerosis and dementia.

Comparison of Protein N-Homocysteinylation in Rat Plasma under Elevated Homocysteine Using a Specific Chemical Labeling Method

Elevated blood concentrations of homocysteine have been well established as a risk factor for cardiovascular diseases and neuropsychiatric diseases, yet the etiologic relationship of homocysteine to these disorders remains poorly understood. Protein N-homocysteinylation has been hypothesized as a contributing factor; however, it has not been examined globally owing to the lack of suitable detection methods. We recently developed a selective chemical method to label N-homocysteinylated proteins with a biotin-aldehyde tag followed by Western blotting analysis, which was further optimized in this study. We then investigated the variation of protein N-homocysteinylation in plasma from rats on a vitamin B₁₂ deficient diet. Elevated “total homocysteine” concentrations were determined in rats with a vitamin B₁₂ deficient diet. Correspondingly, overall levels of plasma protein N-homocysteinylation displayed an increased trend, and furthermore, more pronounced and statistically significant changes (e.g., 1.8-fold, p-value: 0.03) were observed for some individual protein bands. Our results suggest that, as expected, a general metabolic correlation exists between “total homocysteine” and N-homocysteinylation, although other factors are involved in homocysteine/homocysteine thiolactone metabolism, such as the transsulfuration of homocysteine by cystathionine β-synthase or the hydrolysis of homocysteine thiolactone by paraoxonase 1 (PON1), may play more significant or direct roles in determining the level of N-homocysteinylation.

Hyperhomocysteinemia Relates to the Subtype of Antiphospholipid Antibodies in Non-SLE Patients

Abnormal increases of antiphospholipid antibody and plasma homocysteine levels are recently emerging as nonlipidic risk factors for cerebral atherogenesis and thrombosis. Both antiphospholipid antibody and homocysteine share many similar bioeffects in hemostasis, but their interaction is still inconsistent. In this study, we examined the relation between the plasma homocysteine level and lupus anticoagulant, anticardiolipin antibody, and anti-β2-glycoprotein I antibody in patients with noncardiac cerebral ischemia. Systemic lupus erythrematosus patients were excluded. The results showed a higher frequency of moderate hyperhomocysteinemia in patients with an abnormal increase of lupus anticoagulant only. Neither the serum folate and cobalamin levels nor methylenetetrahydrofolate reductase allele mutation contributes to this result. Accordingly, homocysteine interacts with lupus anticoagulant to promote cerebral atherosclerosis and ischemia. The role of vasculopathic or prothrombotic autoantibody generation in response to specific pathological change such as hyperhomocysteinemia warrants further investigation.

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.

Gold nanoparticle sensor for homocysteine thiolactone-induced protein modification

Homocysteine thiolactone-induced protein modification (HTPM) is a unique post-translational protein modification that is recognized as an emergent biomarker for cardiovascular disease. HTPM involves the site-specific acylation of proteins at lysine residues by homocysteine thiolactone (HTL) to produce protein homocystamide, which has been found at elevated levels in patients with coronary heart disease. Herein, we report the development of a novel gold nanoparticle (GNP) biochemical sensor for detection of protein homocystamide in an in vitro serum protein-based model system. Human serum albumin (HSA) and human sera were subjected to HTPM in vitro to produce HSA-homocystamide or serum protein homocystamide, respectively, which was subsequently treated with citrate-capped GNPs. This GNP sensor typically provided instantaneous visual confirmation of HTPM in the protein model systems. Transmission electron microscopy images of the GNPs in the presence of HSA-homocystamide suggest that modification-directed nanoparticle assembly is the mechanism by which the biochemical sensor produces a colorimetric signal. The resultant nanoparticle-protein assembly exhibited excellent thermal and dilutional stability, which is expected for a system stabilized by chemisorption and intermolecular disulfide bonding. The sensor typically provided a linear response for modified human sera concentrations greater than approximately 5 mg/mL. The calculated limit of detection and calibration sensitivity for the method in human sera were 5.2 mg/mL and 13.6 AU . (microg/mL)-1, respectively.

The molecular basis of homocysteine thiolactone-mediated vascular disease

Accumulating evidence suggests that a metabolite of homocysteine (Hcy), the thioester Hcy-thiolactone, plays an important role in atherogenesis and thrombosis. Hcy-thiolactone levels are elevated in hyperhomocysteinemic humans and mice. The thioester chemistry of Hcy-thiolactone underlies its ability to form isopeptide bonds with protein lysine residues, which impairs or alters the protein's function. Protein targets for the modification by Hcy-thiolactone in human blood include fibrinogen, low-density lipoprotein, and high-density lipoprotein. Protein N-homocysteinylation leads to pathophysiological responses, including increased susceptibility to thrombogenesis caused by N-Hcy-fibrinogen, and an autoimmune response elicited by N-Hcy-proteins. Chronic activation of these responses in hyperhomocysteinemia over many years could lead to vascular disease. This article reviews recent evidence supporting the hypothesis that Hcy-thiolactone contributes to pathophysiological effects of Hcy on the vascular system.

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.

Plasma homocysteine thiolactone adducts associated with risk of coronary heart disease

BACKGROUND

Homocysteine thiolactone adducts have been proposed as the culprit of homocysteine related cardiovascular diseases. We studied the association of these adducts in plasma, and the gene polymorphism of paraoxonase-2 with coronary heart disease.

METHODS

254 patients and 308 controls were recruited for the study. Homocysteine thiolactone adducts were determined with ELISA. The codon 311 polymorphism of paraoxonase-2 gene was genotyped by using polymerase chain reaction and restrictive digestion.

RESULTS

The plasma level of homocysteine thiolactone adducts were significantly higher in patients than in controls (40.65 +/- 10.87 u/ml vs. 30.58 +/- 10.20 u/ml, P <0.01), with odds ratio of 7.34 (95% confidence interval 4.020-13.406, P <0.01), and increased according to the number of atherosclerotic coronary arteries: 35.59 +/- 10.34 units/ml (n = 76); 41.88 +/- 8.83 (n = 70) and 43.13 +/- 11.47 (n = 108) in subjects with 1, 2 and 3 affected arteries, respectively (r =0.174, P < 0.01). The frequency of CC genotype was significantly higher in patients with coronary heart disease (7.48%) than in controls (1.62%, P < 0.01), with adjusted odds ratio of 4.367 (95% confidence interval: 1.178 to 16.191, P < 0.01), so was the C allele (23.2% vs. 14.9%, P < 0.05).

CONCLUSIONS

High plasma homocysteine thiolactone adducts and the CC 311 genotype of paraoxonase-2 gene may be the emerging risk factor for coronary heart disease.

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.

Preferential inhibition of paraoxonase activity of human paraoxonase 1 by negatively charged lipids

To determine the causes responsible for a preferential decrease of paraoxonase activity, which has been observed in the serum of patients with cardiovascular diseases, the inactivation or inhibition of paraoxonase 1 (PON1) by various endogenous factors was examined using paraoxon or phenyl acetate as a substrate. When purified PON1 was incubated with various endogenous oxidants or aldehydes, they failed to cause a preferential reduction of paraoxonase activity, suggesting no participation of the inactivation mechanism in the preferential loss of paraoxonase activity. Next, when we examined the inhibition of PON1 activity by endogenous lipids, monoenoic acids such as palmitoleic acid or oleic acid inhibited paraoxonase activity preferentially, in contrast to a parallel inhibition of both activities by polyunsaturated or saturated acids. Noteworthy, oleoylglycine inhibited paraoxonase activity, but not arylesterase activity, complying with the selective inhibition of paraoxonase activity. Moreover, such a selective inhibition of paraoxonase activity was also expressed by lysophosphatidylglycerol or lysophosphatidylinositol, but not by lysophosphatidylserine or lysophosphatidylcholine, indicating the importance of the type of head group. Furthermore, such a preferential or selective inhibition of paraoxonase activity was also observed with PON1 associated with HDL or plasma. These data suggest that some negatively charged lipids may correspond to factors causing the preferential inhibition of paraoxonase activity of PON1.

Purification of antibodies against N-homocysteinylated proteins by affinity chromatography on Nω-homocysteinyl-aminohexyl-Agarose

Modification with homocysteine (Hcy)-thiolactone leads to the formation of N-Hcy-Lys-protein. Although N-Hcy-Lys-proteins are immunogenic, pure antibodies have not yet been obtained. Here we describe synthesis and application of Nomega-homocysteinyl-aminohexyl-Agarose for affinity purification of anti-N-Hcy-Lys-protein antibodies. Nomega-homocysteinyl-aminohexyl-Agarose was prepared by N-homocysteinylation of omega-aminohexyl-Agarose with Hcy-thiolactone. Immune serum was obtained from rabbits inoculated with N-Hcy-Lys-keyhole limpet hemocyanine and IgG fraction prepared by chromatography on protein A-Agarose. Anti-N-Hcy-Lys-protein IgG was adsorbed on Nomega-homocysteinyl-aminohexyl-Agarose column at pH 8.6 and eluted with a pH 2.3 buffer. Enzyme-linked immunosorbent assays demonstrate that the antibody recognizes specifically N-homocysteinylated variants of hemoglobin, albumin, transferrin, and antitrypsin.

Autoantibodies AgainstN-Homocysteinylated Proteins in Humans

BACKGROUND AND PURPOSE

Homocysteine (Hcy)-thiolactone mediates protein N-homocysteinylation in humans. Protein N-linked Hcy comprises a major pool of Hcy in human blood, greater that the "total" Hcy pool. N-homocysteinylated proteins are structurally different, compared with native proteins, and are thus likely to be recognized as neoself antigens and induce an autoimmune response. This study was undertaken to provide evidence for anti-Nepsilon-Hcy-Lys-protein antibody and to examine associations between the antibody level, Hcy, and stroke in humans.

METHODS

ELISA was used to quantify anti-Nepsilon-Hcy-Lys-protein antibodies in human serum.

RESULTS

We found that autoantibodies that specifically recognize Nepsilon-Hcy-Lys epitope on Hcy-containing proteins occur in humans. Serum levels of anti-Nepsilon-Hcy-Lys-protein autoantibodies positively correlate with plasma total Hcy levels, but not with plasma cysteine or methionine levels. In a group of exclusively male patients with stroke, mean level of anti-Nepsilon-Hcy-Lys-protein autoantibodies was approximately 50% higher than in a group of healthy subjects.

CONCLUSIONS

These findings support a hypothesis that Nepsilon-Hcy-Lys-protein is a neoself antigen, which may contribute to immune activation, an important modulator of atherogenesis.

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.

S-Homocysteinylation of transthyretin is detected in plasma and serum of humans with different types of hyperhomocysteinemia

While the association of homocystinuria with disease is known for more than four decades, mild hyperhomocysteinemia has been detected more recently as a risk factor for a number of diseases. However, the mechanism which apparently renders (even mild) hyperhomocystenemia harmful is not known. Following reports on N-homocysteinylation of proteins by the homocysteine derivative homocysteine thiolactone, it has been suggested that homocysteinylation of proteins may contribute to the induction of biological effects by homocysteine. This has prompted us to study by electrospray ionization mass spectrometry homocysteinylation of transthyretin (TTR) in plasma and serum of humans with different types of hyperhomocysteinemia. We did not detect any N-homocysteinylation, but found pronounced S-homocysteinylation of TTR, if the concentration of total homocysteine was high. Our findings support a possible role of S-homocysteinylation of proteins in the mediation of detrimental effects of hyperhomocysteinemia. Careful study of posttranslational modifications of individual proteins may contribute to a better understanding of diseases associated with hyperhomocysteinemia.

Homocysteine is a protein amino acid in humans. Implications for homocysteine-linked disease

Homocysteine is thought to be a non-protein amino acid. However, in vitro studies suggest that homocysteine is likely to be incorporated by indirect mechanisms into proteins in living organisms. Here I show that homocysteine is a protein amino acid in humans. Homocysteine bound by amide or peptide linkages (Hcy-N-protein) is present in human hemoglobin, serum albumin, and gamma-globulins. 1 molecule of homocysteine per 1000 or 1670 molecules of methionine was present in hemoglobin or albumin, respectively. Other proteins, such as low density lipoprotein, high density lipoprotein, transferrin, antitrypsin, and fibrinogen, contained lower amounts of Hcy-N-protein. In human plasma, levels of Hcy-N-protein represented from 0.3 to 23% of total homocysteine. Thus, Hcy-N-protein is a significant component of homocysteine metabolism in humans, possibly contributing to adverse effects of homocysteine on human cells.

Homocysteine thiolactone and protein homocysteinylation: mechanistic studies with model peptides and proteins

In vitro incubations were performed to show that homocysteine thiolactone could generate covalent adducts with model peptides and proteins. MS and MS/MS data suggest that the thiolactone reacts with the side-chain amino group of lysine residues as well as with the N-terminal amino group or C-terminal carboxy group. For larger peptides and proteins, the contribution from the in-amino groups of lysine residues should be predominant. These data could help explain the detrimental effects of elevated levels of homocysteine and homocysteine thiolactone.

Protein N-homocysteinylation: implications for atherosclerosis

Elevated levels of homocysteine (Hcy) are associated with various human pathologies, including cardiovascular disease. However, it is not exactly known why Hcy is harmful. A plausible hypothesis is that the indirect incorporation of Hcy into protein, referred to as protein N-homocysteinylation, leads to cell damage. A translational pathway involves: 1) reversible S-nitrosylation of Hcy with nitric oxide produced by nitric oxide synthase; 2) aminoacylation of tRNAMet with S-nitroso-Hcy catalyzed by MetRS; and 3) transfer of S-nitroso-Hcy from S-nitroso-Hcy-tRNAMet into growing polypeptide chains at positions normally occupied by methionine. Subsequent transnitrosylation leaves Hcy in the protein chain. A post-translational pathway involves: 1) metabolic conversion of Hcy to thiolactone by methionyl-tRNAsynthetase (MetRS), and 2) acylation of protein lysine residues by Hcy thiolactone. The levels of Hcy thiolactone and N-homocysteinylated protein in human vascular endothelial cells depend on the ratio of Hcy/Met, levels of folic acid, and HDL, factors linked to cardiovascular disease. HDL-associated human serum Hcy thiolactonase/paraoxonase hydrolyzes thiolactone to Hcy, thereby minimizing protein N-homocysteinylation. Variations in Hcy thiolactonase may play an important role in Hcy-associated human cardiovascular disease.

Homocysteine thiolactone and protein homocysteinylation in human endothelial cells: implications for atherosclerosis

Editing of the nonprotein amino acid homocysteine by certain aminoacyl-tRNA synthetases results in the formation of the thioester homocysteine thiolactone. Here we show that in the presence of physiological concentrations of homocysteine, methionine, and folic acid, human umbilical vein endothelial cells efficiently convert homocysteine to thiolactone. The extent of this conversion is directly proportional to homocysteine concentration and inversely proportional to methionine concentration, suggesting involvement of methionyl-tRNA synthetase. Folic acid inhibits the synthesis of thiolactone by lowering homocysteine and increasing methionine concentrations in endothelial cells. We also show that the extent of post-translational protein homocysteinylation increases with increasing homocysteine levels but decreases with increasing folic acid and HDL levels in endothelial cell cultures. These data support a hypothesis that metabolic conversion of homocysteine to thiolactone and protein homocysteinylation by thiolactone may play a role in homocysteine-induced vascular damage.

Calcium-dependent human serum homocysteine thiolactone hydrolase. A protective mechanism against protein N-homocysteinylation

Homocysteine thiolactone is formed in all cell types studied thus far as a result of editing reactions of some aminoacyl-tRNA synthetases. Because inadvertent reactions of thiolactone with proteins are potentially harmful, the ability to detoxify homocysteine thiolactone is essential for biological integrity. This work shows that a single specific enzyme, present in mammalian but not in avian sera, hydrolyzes thiolactone to homocysteine. Human serum thiolactonase, a 45-kDa protein component of high density lipoprotein, requires calcium for activity and stability and is inhibited by isoleucine and penicillamine. Substrate specificity studies suggest that homocysteine thiolactone is a likely natural substrate of this enzyme. However, thiolactonase also hydrolyzes non-natural substrates, such as phenyl acetate, p-nitrophenyl acetate, and the organophospate paraoxon. N-terminal amino acid sequence of pure thiolactonase is identical with that of human paraoxonase. These and other data indicate that paraoxonase, an organophosphate-detoxifying enzyme whose natural substrate and function remained unknown up to now, is in fact homocysteine thiolactonase. By detoxifying homocysteine thiolactone, the thiolactonase/paraoxonase would protect proteins against homocysteinylation, a potential contributing factor to atherosclerosis.

Characterization of the adduct formed from the reaction between homocysteine thiolactone and low-density lipoprotein: antioxidant implications

Homocysteine thiolactone is a cyclic thioester that is implicated in the development of atherosclerosis. This molecule will readily acylate primary amines, forming a homocystamide adduct, which contains a primary amine and a thiol. Here, we have characterized and evaluated the antioxidant potential of the homocystamide-low-density lipoprotein (LDL) adduct, a product of the reaction between homocysteine thiolactone and LDL. Treatment of LDL with homocysteine thiolactone resulted in a time-dependent increase in LDL-bound thiols that reached approximately 250 nmol thiol/mg LDL protein. The thiol groups of the homocystamide-LDL adduct were labeled with the thiol-reactive nitroxide, methanethiosulfonate spin label. Using paramagnetic relaxing agents and the electron spin resonance spin labeling technique, we determined that the homocystamide adducts were predominately exposed to the aqueous phase. The homocystamide-LDL adduct was resistant to myoglobin- and Cu2(+)-mediated oxidation (with respect to native LDL), as measured by the formation of conjugated dienes and thiobarbituric acid reactive substances, and the depletion of vitamin E. This antioxidant effect was due to increased thiol content, as the effect was abolished with N-ethylmaleamide pre-treatment. We conclude that the reaction between homocysteine thiolactone and LDL generates an LDL molecule that is more resistant to oxidative modification than native LDL. The potential relationship between the homocystamide-LDL adduct and the development of atherosclerosis is discussed.