DNA methylation patterns in bladder tumors of African American patients point to distinct alterations in xenobiotic metabolism.

Racial/ethnic disparities have a significant impact on bladder cancer outcomes with African American patients demonstrating inferior survival over European-American patients. We hypothesized that epigenetic difference in methylation of tumor DNA is an underlying cause of this survival health disparity. We analyzed bladder tumors from African American and European-American patients using reduced representation bisulfite sequencing (RRBS) to annotate differentially methylated DNA regions. Liquid chromatography-mass spectrometry (LC-MS/MS) based metabolomics and flux studies were performed to examine metabolic pathways that showed significant association to the discovered DNA methylation patterns. RRBS analysis showed frequent hypermethylated CpG islands in African American patients. Further analysis showed that these hypermethylated CpG islands in patients are commonly located in the promoter regions of xenobiotic enzymes that are involved in bladder cancer progression. On follow-up, LC-MS/MS revealed accumulation of glucuronic acid, S-adenosylhomocysteine, and a decrease in S-adenosylmethionine, corroborating findings from the RRBS and mRNA expression analysis indicating increased glucuronidation and methylation capacities in African American patients. Flux analysis experiments with 13C-labeled glucose in cultured African American bladder cancer cells confirmed these findings. Collectively, our studies revealed robust differences in methylation-related metabolism and expression of enzymes regulating xenobiotic metabolism in African American patients indicate that race/ethnic differences in tumor biology may exist in bladder cancer.

Macranthoidin B Modulates Key Metabolic Pathways to Enhance ROS Generation and Induce Cytotoxicity and Apoptosis in Colorectal Cancer.

BACKGROUND/AIMS: Induction of oxidative stress and reactive oxygen species (ROS) mediated-apoptosis have been utilized as effective strategies in anticancer therapy. Macranthoidin B (MB) is a potent inducer of ROS-mediated apoptosis in cancer, but its mechanism of action is poorly understood. METHOD: Superoxide production with MB exposure in colorectal cancer (CRC) cells was measured using lucigenin chemiluminescence and real-time PCR. MB's inhibitory effect on proliferation and viability of CRC cells was determined by proliferation assays. MB's effect on apoptosis of CRC cells was determined by Western blotting and annexin V-FITC/PI staining. MB's effect on the growth of CRC xenografts in mice was assessed. An established metabolomics profiling platform combining ultra-performance liquid chromatography-tandem mass spectrometry (LC-MS) with gas chromatography-mass spectrometry (GC-MS) was performed to determine MB's effect on total metabolite variation in CRC cells. RESULTS: We found that MB increases ROS generation via modulating key metabolic pathways. Using metabolomics profiling platform combining LC-MS with GC-MS, a total of 236 metabolites were identified in HCT-116 cells in which 31 metabolites were determined to be significantly regulated (p ≤ 0.05) after MB exposure. A number of key metabolites revealed by metabolomics analysis include glucose, fructose, citrate, arginine, phenylalanine, and S-adenosylhomocysteine (SAH), suggesting specific modulation of metabolism on carbohydrates, amino acids and peptides, lipids, nucleotide, cofactors and vitamins in HCT-116 CRC cells with MB treatment highly associated with apoptosis triggered by enhanced ROS and activated caspase-3. CONCLUSION: Our results demonstrate that MB represses CRC cell proliferation by inducing ROS-mediated apoptosis.

Direct Detection of Products from S-Adenosylmethionine-Dependent Enzymes Using a Competitive Fluorescence Polarization Assay.

S-Adenosylmethionine (AdoMet)-dependent methyltransferases (MTases) are an essential superfamily of enzymes that catalyze the transfer of a methyl group to several biomolecules. Alterations in the methylation of cellular components crucially impact vital biological processes, making MTases attractive drug targets for treating infectious diseases and diseases caused by overactive human-encoded MTases. Several methods have been developed for monitoring the activity of MTases, but most MTase assays have inherent limitations or are not amenable for high-throughput screening. We describe a universal, competitive fluorescence polarization (FP) assay that directly measures the production of S-adenosylhomocysteine (AdoHcy) from MTases. Our developed assay monitors the generation of AdoHcy by displacing a fluorescently labeled AdoHcy molecule complexed to a catalytically inert 5'-methylthioadenosine nucleosidase (MTAN-D198N) variant performed in a mix-and-read format. Producing the fluorescently labeled molecule involves a one-pot synthesis by combining AdoHcy with an amine-reactive rhodamine derivative, which possesses a Kd value of 11.3 ± 0.7 nM to MTAN-D198N. The developed competitive FP assay expresses a limit of detection for AdoHcy of 6 nM and exhibits a 34-fold preference to AdoHcy in comparison to AdoMet. We demonstrate the utility of the developed assay by performing a pilot screen with the NIH Clinical Collection as well as determining the kinetic parameters of l-histidine methylation for EgtD from Mycobacterium tuberculosis. Additionally, the developed assay is applicable to other AdoMet-dependent and ATP-dependent enzymes by detecting various adenosine-containing molecules including 5'-methylthioadenosine, AMP, and ADP.

High resolution mass approach to characterize refrigerated black truffles stored under different storage atmospheres.

Freshly harvested Tuber melanosporum samples were packed and stored at 4°C under reduced atmospheric pressure or modified atmosphere for four weeks. Multivariate analysis was employed to correlate the antioxidant power of the ethanolic extracts of the samples with the chemical composition determined by high resolution mass spectrometry. High performance liquid chromatography coupled with a coularray detector was applied to select the chemical species associated with the antioxidant power. Four classes of chemical compounds were investigated in more detail by a targeted approach: derivatives of glutathione, adenine (such as S-adenosyl-homocysteine), oxidized linoleic acid and ergosterol. Adducts containing glutathione and adenine with oxidized linoleic acid were observed in TM for the first time and can be considered markers of freshness of the product. S-adenosyl-homocysteine, the acetyl-carnitine adduct with cysteinyl-glycine and several oxidized linoleic acid derivatives were among the markers of degradation.

Chemiluminescence Immunoassay for S-Adenosylhomocysteine Detection and Its Application in DNA Methyltransferase Activity Evaluation and Inhibitors Screening.

Aberrant methylation by DNA transferase is associated with cancer initiation and progression. For high-throughput screening of DNA methyltransferase (MTase) activity and its inhibitors, a novel chemiluminescence immunoassay (CLIA) was established to detect S-adenosylhomocysteine (SAH), the product of S-adenosylmethionine (SAM) transmethylation reactions. We synthesized two kinds of immunogens for SAH and characterized the polyclonal antibodies in each group. The antibody with higher titer was used to develop a competitive CLIA for SAH, in which SAH in samples would compete with SAH coated on microplate in binding with SAH antibodies. Successively, horseradish peroxidase labeled goat antirabbit IgG (HRP-IgG) was conjugated with SAH antibodies on the microplate. In substrate solution containing luminol and H2O2, HRP-IgG catalyzed luminol oxidation by H2O2, generating a high chemiluminescence signal. The method could detect as low as 9.8 ng mL(-1) SAH with little cross-reaction (3.8%) to SAM. Since higher DNA MTase activity leads to more production of SAH, a correlation between the chemiluminescence intensity and DNA MTase activity was obtained in the range from 0.1 to 8.0 U/mL of DNA MTase. The inhibition study showed that, in the presence of SAM as methyl donor, Lomeguatrib, 5-Azacytidine, and 5-Aza-2'-deoxycytidine could inhibit the DNA MTase activity with IC50 values of 40.57 nM, 2.26 µM, and 0.48 µM, respectively. These results are consistent with the published studies. The proposed assay does not depend on recognizing methylated cytosines in oligonucleotides (methyl acceptor) and showed the potential as an accessible platform for sensitive detection of DNA MTase activity and screening its inhibitors.

In Vitro and In Vivo Enzyme Activity Screening via RNA-Based Fluorescent Biosensors for S-Adenosyl-l-homocysteine (SAH).

High-throughput enzyme activity screens are essential for target characterization and drug development, but few assays employ techniques or reagents that are applicable to both in vitro and live cell settings. Here, we present a class of selective and sensitive fluorescent biosensors for S-adenosyl-l-homocysteine (SAH) that provide a direct "mix and go" activity assay for methyltransferases (MTases), an enzyme class that includes several cancer therapeutic targets. Our riboswitch-based biosensors required an alternate inverted fusion design strategy, but retained full selectivity for SAH over its close structural analogue, the highly abundant methylation cofactor S-adenosyl-l-methionine (SAM). The level of ligand selectivity for these fluorescent biosensors exceeded that of commercial antibodies for SAH and proved critical to cellular applications, as we employed them to measure methylthioadenosine nucleosidase (MTAN) activity in live Escherichia coli. In particular, we were able to monitor in vivo increase of SAH levels upon chemical inhibition of MTAN using flow cytometry, which demonstrates high-throughput, single cell measurement of an enzyme activity associated with the biosynthesis of quorum sensing signal AI-2. Thus, this study presents RNA-based fluorescent biosensors as promising molecular reagents for high-throughput enzymatic assays that successfully bridge the gap between in vitro and in vivo applications.

Biochemical Assay Development for Histone Methyltransferases Using a Transcreener-Based Assay for S-Adenosylhomocysteine.

Epigenetic regulation has been implicated in diverse diseases including cancer, diabetes, and inflammation, and high-throughput screening for histone methyltransferase (HMT) inhibitors is an area of intense drug discovery effort. HMTs catalyze the transfer of methyl group from S-adenosylmethionine (SAM) to lysine or arginine on histone tails forming the methylated products and S-adenosylhomocysteine (SAH). HMTs are challenging to incorporate into biochemical assays for a number of reasons. They have slow turnovers and low Km values for SAM, which leads to low levels of product formation, and thus requires very sensitive detection methods and/or high levels of enzyme. They also have diverse acceptor substrate requirements, ranging from peptides to intact nucleosomes. Additionally, some HMTs function as complexes of three or more proteins. Developing assays for individual HMTs, including sourcing and acquiring high quality enzymes and acceptor substrates, therefore can be laborious and expensive. We recently developed the Transcreener(®) EPIGEN Methyltransferase assay, a sensitive SAH detection method with a fluorescence polarization readout, to enable universal HMT detection independent of acceptor substrate. To facilitate screening and profiling of HMTs, we describe the development of turnkey assay systems for thirteen HMTs including identification of optimal acceptor substrates and their concentrations, optimization of detection reagents, determination of initial velocity enzyme concentrations, and measurement of inhibitor potencies.

A liquid chromatography/mass spectrometry-based generic detection method for biochemical assay and hit discovery of histone methyltransferases.

Epigenetic modifications of the genome, such as DNA methylation and posttranslational modifications of histone proteins, contribute to gene regulation. Growing evidence suggests that histone methyltransferases are associated with the development of various human diseases, including cancer, and are promising drug targets. High-quality generic assays will facilitate drug discovery efforts in this area. In this article, we present a liquid chromatography/mass spectrometry (LC/MS)-based S-adenosyl homocysteine (SAH) detection assay for histone methyltransferases (HMTs) and its applications in HMT drug discovery, including analyzing the activity of newly produced enzymes, developing and optimizing assays, performing focused compound library screens and orthogonal assays for hit confirmations, selectivity profiling against a panel of HMTs, and studying mode of action of select hits. This LC/MS-based generic assay has become a critical platform for our methyltransferase drug discovery efforts.

Role of epigenetic and miR-22 and miR-29b alterations in the downregulation of Mat1a and Mthfr genes in early preneoplastic livers in rats induced by 2-acetylaminofluorene.

Carcinogenesis is a multistep sequential process of clonal expansion of initiated cells associated with the accumulation of multiple cancer-specific heritable phenotypes. The acquisition of these heritable cancer-specific alterations may be triggered by mutational and/or non-mutational changes in the genome that affect the regulation of gene expression. Currently, cancer-specific epigenetically mediated changes in gene expression are regarded as driving events in tumorigenesis. In the present study, we investigated the role of gene-specific expression changes in the mechanism of rat hepatocarcinogenesis induced by the complete hepatocarcinogen 2-acetylaminofluorene (2-AAF). The results of the present study demonstrate significant alterations in gene expression, especially of Mat1a and Mthfr genes, during early stages of rat 2-AAF-induced liver carcinogenesis. Both of these genes were downregulated in the livers of 2-AAF-treated male rats. Inhibition of Mat1a expression was associated with an increase in histone H3 lysine 27 trimethylation and a decrease in histone H3 lysine 18 acetylation at the gene promoter/first exon region. Additionally, we demonstrate for the first time a critical contribution of miR-22 and miR-29b microRNAs in the inhibition of Mat1a and Mthfr gene expression during 2-AAF-induced rat hepatocarcinogenesis. The downregulation of Mat1a and Mthfr genes was accompanied by marked functional alterations in one-carbon metabolism. The results of the present study suggest that downregulation of the Mat1a and Mthfr genes may be one of the main driver events that promote liver carcinogenesis by causing a profound accumulation of subsequent epigenetic abnormalities during progression of the carcinogenic process.

Low dietary folate and methylenetetrahydrofolate reductase deficiency may lead to pregnancy complications through modulation of ApoAI and IFN-γ in spleen and placenta, and through reduction of methylation potential.

SCOPE: Genetic or nutritional disturbances in folate metabolism lead to hyperhomocysteinemia and adverse reproductive outcomes. Folate-dependent homocysteine remethylation is required for methylation reactions and may influence choline/betaine metabolism. Hyperhomocysteinemia has been suggested to play a role in inflammation. The goal of this study was to determine whether folate-related pregnancy complications could be due to altered expression of some inflammatory mediators or due to disturbances in methylation intermediates. METHODS AND RESULTS: Pregnant mice with or without a deficiency of methylenetetrahydrofolate reductase (MTHFR) were fed control diets or folate-deficient (FD) diets; tissues were collected at embryonic day 14.5. FD decreased plasma phosphocholine and increased plasma glycerophosphocholine and lysophosphatidylcholine. Liver betaine, phosphocholine, and S-adenosylmethionine:S-adenosylhomocysteine ratios were reduced in FD. In liver, spleen, and placenta, the lowest levels of apolipoprotein AI (ApoAI) were observed in Mthfr(+/-) mice fed FD. Increased interferon-gamma (IFN-γ) was observed in spleen and placentae due to FD or Mthfr genotype. Plasma homocysteine correlated negatively with liver and spleen ApoAI, and positively with IFN-γ. CONCLUSION: Low dietary folate or Mthfr deficiency during pregnancy may result in adverse pregnancy outcomes by altering expression of the inflammatory mediators ApoAI and IFN-γ in spleen and placenta. Disturbances in choline metabolism or methylation reactions may also play a role.

Profiling of methyltransferases and other S-adenosyl-L-homocysteine-binding Proteins by Capture Compound Mass Spectrometry (CCMS).

There is a variety of approaches to reduce the complexity of the proteome on the basis of functional small molecule-protein interactions such as affinity chromatography (1) or Activity Based Protein Profiling (2). Trifunctional Capture Compounds (CCs, Figure 1A) (3) are the basis for a generic approach, in which the initial equilibrium-driven interaction between a small molecule probe (the selectivity function, here S-adenosyl-(L)-homocysteine, SAH, Figure 1A) and target proteins is irreversibly fixed upon photo-crosslinking between an independent photo-activable reactivity function (here a phenylazide) of the CC and the surface of the target proteins. The sorting function (here biotin) serves to isolate the CC - protein conjugates from complex biological mixtures with the help of a solid phase (here streptavidin magnetic beads). Two configurations of the experiments are possible: "off-bead" (4) or the presently described "on-bead" configuration (Figure 1B). The selectivity function may be virtually any small molecule of interest (substrates, inhibitors, drug molecules). S-Adenosyl-(L)-methionine (SAM, Figure 1A) is probably, second to ATP, the most widely used cofactor in nature (5, 6). It is used as the major methyl group donor in all living organisms with the chemical reaction being catalyzed by SAM-dependent methyltransferases (MTases), which methylate DNA (7), RNA (8), proteins (9), or small molecules (10). Given the crucial role of methylation reactions in diverse physiological scenarios (gene regulation, epigenetics, metabolism), the profiling of MTases can be expected to become of similar importance in functional proteomics as the profiling of kinases. Analytical tools for their profiling, however, have not been available. We recently introduced a CC with SAH as selectivity group to fill this technological gap (Figure 1A). SAH, the product of SAM after methyl transfer, is a known general MTase product inhibitor (11). For this reason and because the natural cofactor SAM is used by further enzymes transferring other parts of the cofactor or initiating radical reactions as well as because of its chemical instability (12), SAH is an ideal selectivity function for a CC to target MTases. Here, we report the utility of the SAH-CC and CCMS by profiling MTases and other SAH-binding proteins from the strain DH5α of Escherichia coli (E. coli), one of the best-characterized prokaryotes, which has served as the preferred model organism in countless biochemical, biological, and biotechnological studies. Photo-activated crosslinking enhances yield and sensitivity of the experiment, and the specificity can be readily tested for in competition experiments using an excess of free SAH.

Dietary intake of S-(alpha-carboxybutyl)-DL-homocysteine induces hyperhomocysteinemia in rats.

Betaine homocysteine S-methyltransferase (BHMT) catalyzes the transfer of a methyl group from betaine to homocysteine (Hcy), forming dimethylglycine and methionine. We previously showed that inhibiting BHMT in mice by intraperitoneal injection of S-(alpha-carboxybutyl)-DL-homocysteine (CBHcy) results in hyperhomocysteinemia. In the present study, CBHcy was fed to rats to determine whether it could be absorbed and cause hyperhomocysteinemia as observed in the intraperitoneal administration of the compound in mice. We hypothesized that dietary administered CBHcy will be absorbed and will result in the inhibition of BHMT and cause hyperhomocysteinemia. Rats were meal-fed every 8 hours an L-amino acid-defined diet either containing or devoid of CBHcy (5 mg per meal) for 3 days. The treatment decreased liver BHMT activity by 90% and had no effect on methionine synthase, methylenetetrahydrofolate reductase, phosphatidylethanolamine N-methyltransferase, and CTP:phosphocholine cytidylyltransferase activities. In contrast, cystathionine beta-synthase activity and immunodetectable protein decreased (56% and 26%, respectively) and glycine N-methyltransferase activity increased (52%) in CBHcy-treated rats. Liver S-adenosylmethionine levels decreased by 25% in CBHcy-treated rats, and S-adenosylhomocysteine levels did not change. Furthermore, plasma choline decreased (22%) and plasma betaine increased (15-fold) in CBHcy-treated rats. The treatment had no effect on global DNA and CpG island methylation, liver histology, and plasma markers of liver damage. We conclude that CBHcy-mediated BHMT inhibition causes an elevation in total plasma Hcy that is not normalized by the folate-dependent conversion of Hcy to methionine. Furthermore, metabolic changes caused by BHMT inhibition affect cystathionine beta-synthase and glycine N-methyltransferase activities, which further deteriorate plasma Hcy levels.

Neural tube defects induced by folate deficiency in mutant curly tail (Grhl3) embryos are associated with alteration in folate one-carbon metabolism but are unlikely to result from diminished methylation.

BACKGROUND: Folate one-carbon metabolism has been implicated as a determinant of susceptibility to neural tube defects (NTDs), owing to the preventive effect of maternal folic acid supplementation and the higher risk associated with markers of diminished folate status. METHODS: Folate one-carbon metabolism was compared in curly tail (ct/ct) and genetically matched congenic (+(ct)/+(ct)) mouse strains using the deoxyuridine suppression test in embryonic fibroblast cells and by quantifying s-adenosylmethionine (SAM) and s-adenosylhomocysteine (SAH) in embryos using liquid chromatography tandem mass spectrometry. A possible genetic interaction between curly tail and a null allele of 5,10-methylenetetrahydrofolate reductase (MTHFR) was investigated by generation of compound mutant embryos. RESULTS: There was no deficit in thymidylate biosynthesis in ct/ct cells, but incorporation of exogenous thymidine was lower than in +(ct)/+(ct) cells. In +(ct)/+(ct) embryos the SAM/SAH ratio was diminished by dietary folate deficiency and normalized by folic acid or myo-inositol treatment, in association with prevention of NTDs. In contrast, folate deficiency caused a significant increase in the SAM/SAH ratio in ct/ct embryos. Loss of MTHFR function in curly tail embryos significantly reduced the SAM/SAH ratio but did not cause cranial NTDs or alter the frequency of caudal NTDs. CONCLUSIONS: Curly tail fibroblasts and embryos, in which Grhl3 expression is reduced, display alterations in one-carbon metabolism, particularly in the response to folate deficiency, compared to genetically matched congenic controls in which Grhl3 is unaffected. However, unlike folate deficiency, diminished methylation potential appears to be insufficient to cause cranial NTDs in the curly tail strain, nor does it increase the frequency of caudal NTDs.

Regulation of homocysteine metabolism and methylation in human and mouse tissues.

Hyperhomocysteinemia is an independent risk factor for cardiovascular disease. Homocysteine (Hcy) metabolism involves multiple enzymes; however, tissue Hcy metabolism and its relevance to methylation remain unknown. Here, we established gene expression profiles of 8 Hcy metabolic and 12 methylation enzymes in 20 human and 19 mouse tissues through bioinformatic analysis using expression sequence tag clone counts in tissue cDNA libraries. We analyzed correlations between gene expression, Hcy, S-adenosylhomocysteine (SAH), and S-adenosylmethionine (SAM) levels, and SAM/SAH ratios in mouse tissues. Hcy metabolic and methylation enzymes were classified into two types. The expression of Type 1 enzymes positively correlated with tissue Hcy and SAH levels. These include cystathionine beta-synthase, cystathionine-gamma-lyase, paraxonase 1, 5,10-methylenetetrahydrofolate reductase, betaine:homocysteine methyltransferase, methionine adenosyltransferase, phosphatidylethanolamine N-methyltransferases and glycine N-methyltransferase. Type 2 enzyme expressions correlate with neither tissue Hcy nor SAH levels. These include SAH hydrolase, methionyl-tRNA synthase, 5-methyltetrahydrofolate:Hcy methyltransferase, S-adenosylmethionine decarboxylase, DNA methyltransferase 1/3a, isoprenylcysteine carboxyl methyltransferases, and histone-lysine N-methyltransferase. SAH is the only Hcy metabolite significantly correlated with Hcy levels and methylation enzyme expression. We established equations expressing combined effects of methylation enzymes on tissue SAH, SAM, and SAM/SAH ratios. Our study is the first to provide panoramic tissue gene expression profiles and mathematical models of tissue methylation regulation.

Sources of S-adenosyl-L-homocysteine background in measuring protein arginine N-methyltransferase activity using tandem mass spectrometry.

S-Adenosyl-L-homocysteine (AdoHcy) background signal in reactions with protein arginine N-methyltransferase 1 is investigated using an ultrahigh-performance liquid chromatography tandem mass spectrometry assay that measures AdoHcy. We identify three sources of AdoHcy background: enzymatic automethylation, AdoHcy contamination in commercial S-adenosyl-L-methionine (AdoMet), and nonenzymatic pseudo-first-order formation of AdoHcy from AdoMet. We propose a potential mechanism for the nonenzymatic production of AdoHcy and illustrate strategies for mitigating background AdoHcy that can be applied to any assay.

Simultaneous analysis of catechol-O-methyl transferase activity, S-adenosylhomocysteine and adenosine.

Novel HPLC method utilizing UV-detection was developed to analyse catechol-O-methyltransferase (COMT) products, vanillic acid and isovanillic acid, S-adenosylhomocysteine (SAH) and adenosine formed from dihydroxybenzoic acid and S-adenosyl-L-methionine (SAM) by incubation of the rat tissues. Entacapone, a COMT inhibitor, prevented the formation of SAH only partially in the striatal homogenate whereas in the kidney homogenate the increase of SAH was prevented by entacapone. In conclusion, this method was reliable, rapid and simple. COMT seemed to be partially responsible on the SAM utilizing methylations in the striatal homogenates while in the high COMT activity tissue, COMT was the main SAH producing methyltransferase.

A universal competitive fluorescence polarization activity assay for S-adenosylmethionine utilizing methyltransferases.

A high-throughput, competitive fluorescence polarization immunoassay has been developed for the detection of methyltransferase activity. The assay was designed to detect S-adenosylhomocysteine (AdoHcy), a product of all S-adenosylmethionine (AdoMet)-utilizing methyltransferase reactions. We employed commercially available anti-AdoHcy antibody and fluorescein-AdoHcy conjugate tracer to measure AdoHcy generated as a result of methyltransferase activity. AdoHcy competes with tracer in the antibody/tracer complex. The release of tracer results in a decrease in fluorescence polarization. Under optimized conditions, AdoHcy and AdoMet titrations demonstrated that the antibody had more than a 150-fold preference for binding AdoHcy relative to AdoMet. Mock methyltransferase reactions using both AdoHcy and AdoMet indicated that the assay tolerated 1 to 3 microM AdoMet. The limit of detection was approximately 5 nM (0.15 pmol) AdoHcy in the presence of 3 muM AdoMet. To validate the assay's ability to quantitate methyltransferase activity, the methyltransferase catechol-O-methyltransferase (COMT) and a known selective inhibitor of COMT activity were used in proof-of-principle experiments. A time- and enzyme concentration-dependent decrease in fluorescence polarization was observed in the COMT assay that was developed. The IC(50) value obtained using a selective COMT inhibitor was consistent with previously published data. Thus, this sensitive and homogeneous assay is amenable for screening compounds for inhibitors of methyltransferase activity.

Homocysteine affects cardiomyocyte viability: concentration-dependent effects on reversible flip-flop, apoptosis and necrosis.

BACKGROUND: Hyperhomocysteinaemia (HHC) is thought to be a risk factor for cardiovascular disease including heart failure. While numerous studies have analyzed the role of homocysteine (Hcy) in the vasculature, only a few studies investigated the role of Hcy in the heart. Therefore we have analyzed the effects of Hcy on isolated cardiomyocytes. METHODS: H9c2 cells (rat cardiomyoblast cells) and adult rat cardiomyocytes were incubated with Hcy and were analyzed for cell viability. Furthermore, we determined the effects of Hcy on intracellular mediators related to cell viability in cardiomyocytes, namely NOX2, reactive oxygen species (ROS), mitochondrial membrane potential (DeltaPsi (m)) and ATP concentrations. RESULTS: We found that incubation of H9c2 cells with 0.1 mM D,L-Hcy (= 60 microM L-Hcy) resulted in an increase of DeltaPsi (m) as well as ATP concentrations. 1.1 mM D,L-Hcy (= 460 microM L-Hcy) induced reversible flip-flop of the plasma membrane phospholipids, but not apoptosis. Incubation with 2.73 mM D,L-Hcy (= 1.18 mM L-Hcy) induced apoptosis and necrosis. This loss of cell viability was accompanied by a thread-to-grain transition of the mitochondrial reticulum, ATP depletion and nuclear NOX2 expression coinciding with ROS production as evident from the presence of nitrotyrosin residues. Notably, only at this concentration we found a significant increase in S-adenosylhomocysteine which is considered the primary culprit in HHC. CONCLUSION: We found concentration-dependent effects of Hcy in cardiomyocytes, varying from induction of reversible flip-flop of the plasma membrane phospholipids, to apoptosis and necrosis.