S-adenosylhomocysteine Hydrolase Activity Research Articles

Nitric oxide modulates folate-mediated one-carbon metabolism and mitochondrial energy levels of peaches during cold storage.

Folate-mediated one-carbon metabolism (FOCM) is closely associated with postharvest preservation. This study investigated the effects of exogenous nitric oxide (NO) on FOCM, storage quality, energy metabolism, and mitochondrial membrane integrity in cold-storage peach fruit. In this experiment, peaches were soaked with 1.5 mmol L-1S-nitrosoglutathione (GSNO) as NO donor, and the negative treatment (NT) solution containing 5 μmol L-1 carboxy-PTIO (c-PTIO, NO scavenger), 200 μmol L-1 NG-Nitro-L-arginine methyl ester (L-NAME, NO synthase-like enzyme inhibitor), and 200 μmol L-1 sodium tungstate dihydrate (nitrate reductase inhibitor) and stored at 0°C. The results showed that NO decreased the activity of S-adenosylmethionine synthase and S-adenosylhomocysteine hydrolase and increased the activity of methionine sulfoxide reductase A, as well as the content of N5-methyl-THF, the ratio of tetrahydrofolate (THF), homocysteine, methionine, S-adenosylmethionine (SAM), and SAM to S-adenosylhomocysteine compared with the control, indicating that NO effectively increased FOCM flux by affecting the activity of FOCM enzymes. Meanwhile, NO increased the activities of H+-ATPase, Ca2+-ATPase, cytochrome c oxidase, succinate dehydrogenase, and the contents of adenosine triphosphate and adenosine diphosphate, and maintained high energy charge in peaches during storage. NO retarded the increase in mitochondrial permeability transition, reactive oxygen species content, and the decrease in mitochondrial membrane fluidity, membrane potential, and swelling. NT treatment exhibited the opposite results. In conclusion, these results suggested that NO could induce the accumulation of folate and FOCM flux and maintain mitochondrial energy levels, which might be responsible for maintaining the quality of peaches during cold storage.

Plant homocysteine, a methionine precursor and plant's hallmark of metabolic disorders.

Homocysteine (Hcy) is a sulfur-containing non-proteinogenic amino acid, which arises from redox-sensitive methionine metabolism. In plants, Hcy synthesis involves both cystathionine β-lyase and S-adenosylhomocysteine hydrolase activities. Thus, Hcy itself is crucial for de novo methionine synthesis and S-adenosylmethionine recycling, influencing the formation of ethylene, polyamines, and nicotianamine. Research on mammalian cells has shown biotoxicity of this amino acid, as Hcy accumulation triggers oxidative stress and the associated lipid peroxidation process. In addition, the presence of highly reactive groups induces Hcy and Hcy derivatives to modify proteins by changing their structure and function. Currently, Hcy is recognized as a critical, independent hallmark of many degenerative metabolic diseases. Research results indicate that an enhanced Hcy level is also toxic to yeast and bacteria cells. In contrast, in the case of plants the metabolic status of Hcy remains poorly examined and understood. However, the presence of the toxic Hcy metabolites and Hcy over-accumulation during the development of an infectious disease seem to suggest harmful effects of this amino acid also in plant cells. The review highlights potential implications of Hcy metabolism in plant physiological disorders caused by environmental stresses. Moreover, recent research advances emphasize that recognizing the Hcy mode of action in various plant systems facilitates verification of the potential status of Hcy metabolites as bioindicators of metabolism disorders and thus may constitute an element of broadly understood biomonitoring.

Association among urinary polycyclic aromatic hydrocarbons metabolites, SAHH activity and H19 expression in coke oven workers

Objective: To investigate the relationship of polycyclic aromatic hydrocarbons (PAHs) exposure, S-adenosylhomocysteine hydrolase (SAHH) activity and long noncoding RNA H19 gene expression in the urine of coke oven workers. Methods: In September 2019, in a coking plant in Taiyuan City, 146 male workers who had worked in coke oven operations for one year were selected through a completely random sampling method, and their basic personal information was collected by questionnaire survey, and blood and urine samples were collected. The levels of 4 PAHs metabolites 2-hydroxfluorene (2-FLU), 2- hydroxynaphthalene (2-NAP), 9-hydroxyphenanthren (9-PHE), and 1-hydroxypyrene (1-OHP) in urine were detected by high performance liquid chromatography (HPLC) -fluorescence detection method. HPLC-UV detection method was used to detect the content of S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH) in plasma, and the SAHH activity value was obtained by calculating the ratio. Reverse transcription PCR method was used to determine the H19 gene expression level. Urine levels of 2-FLU, 2-NAP, 9-PHE, and 1-OHP were divided into Q(1), Q(2), Q(3), and Q(4) groups according to quartiles (P(25), P(50), P(75)). Regression, trend test and restricted cubic splines were used to analyze the relationship among PAHs metabolites, SAHH activity, H19 gene expression and their dose-response. Results: The median age of coke oven workers was 39.60 years old, the median length of service was 20.38 years, and the urinary levels of 2-FLU, 2-NAP, 9- PHE, and 1-OHP were 0.29, 0.74, 0.09, and 0.06 μg/mmol Cr, respectively. The levels of 2-FLU, 2-NAP and 9-PHE in the urine of workers were significantly different between groups with different 1-OHP levels (P<0.05). After adjusting for age, length of service, smoking, drinking, and levels of 2-FLU, 2-NAP and 9-PHE, SAHH activity decreased with the increase of urinary 1-OHP level (OR=0.63, 95%CI: 0.41-0.98, P=0.038), showing a nonlinear relationship (P(nonlinear)= 0.030). H19 gene expression increased with the increase of urinary 1- OHP level (OR=1.51, 95%CI: 1.03-2.19, P=0.033), there was a linear relationship (P(trend)= 0.058). The relationship between the other three metabolites in urine and SAHH activity and H19 gene expression was not statistically significant (P>0.05) . Conclusion: Urinary 1-OHP level may be a risk factor for decreased SAHH activity and increased H19 gene expression in coke oven workers.

Effects of H19/SAHH/DNMT1 on the oxidative DNA damage related to benzo[a]pyrene exposure.

The mechanisms that long noncoding RNA (lncRNA) H19 binding to S-adenosylhomocysteine hydrolase (SAHH) interacted with DNA methyltransferase 1 (DNMT1) and then regulated DNA damage caused by polycyclic aromatic hydrocarbons (PAHs) remain unclear. A total of 146 occupational workers in a Chinese coke-oven plant in 2014 were included in the final analyses. We used high-performance liquid chromatography mass spectrometry (HPLC-MS) equipped to detect urine biomarkers of PAHs exposure, including 2-hydroxynaphthalene (2-NAP), 2-hydroxyfluorene (2-FLU), 9-hydroxyphenanthrene (9-PHE) and 1-hydroxypyrene (1-OHP). The levels of SAM and SAH in plasma were detected by HPLC-ultraviolet. By constructing various BEAS-2B cell models exposed to 16μM benzo[a]pyrene (BaP) for 24h, toxicological parameters reflecting distinct mechanisms were evaluated. We documented that urinary 1-hydroxypyrene (1-OHP) levels were positively associated with blood H19 RNA expression (OR: 1.51, 95% CI: 1.03-2.19), but opposite to plasma SAHH activity (OR: 0.63, 95% CI: 0.41-0.98) in coke oven workers. Moreover, by constructing various BEAS-2B cell models exposed to benzo[a]pyrene (BaP), we investigated that H19 binding to SAHH exaggerated DNMT1 expressions and activity. Suppression of H19 enhanced the interaction of SAHH and DNMT1 in BaP-treated cells, decreased eight-oxoguanine DNA glycosylase 1 (OGG1) methylation, reduced oxidative DNA damage and lessened S phase arrest. However, SAHH or DNMT1 single knockdown and SAHH/DNMT1 double knockdown showed the opposite trend. A H19/SAHH/DNMT1 axis was involved in OGG1 methylation, oxidative DNA damage and cell cycle arrest by carcinogen BaP.

Epigenetic Upregulation of H19 and AMPK Inhibition Concurrently Contribute to S-Adenosylhomocysteine Hydrolase Deficiency-Promoted Atherosclerotic Calcification.

S-adenosylhomocysteine (SAH) is a risk factor of cardiovascular disease; inhibition of SAH hydrolase (SAHH) results in SAH accumulation and induces endothelial dysfunction and atherosclerosis. However, the effect and mechanism of SAHH in atherosclerotic calcification is still unclear. We aimed to explore the role and mechanism of SAHH in atherosclerotic calcification. The relationship between SAHH and atherosclerotic calcification was investigated in patients with coronary atherosclerotic calcification. Different in vivo genetic models were used to examine the effect of SAHH deficiency on atherosclerotic calcification. Human aortic and murine vascular smooth muscle cells (VSMCs) were cultured to explore the underlying mechanism of SAHH on osteoblastic differentiation of VSMCs. The expression and activity of SAHH were decreased in calcified human coronary arteries and inversely associated with coronary atherosclerotic calcification severity, whereas plasma SAH and total homocysteine levels were positively associated with coronary atherosclerotic calcification severity. Heterozygote knockout of SAHH promoted atherosclerotic calcification. Specifically, VSMC-deficient but not endothelial cell-deficient or macrophage-deficient SAHH promoted atherosclerotic calcification. Mechanistically, SAHH deficiency accumulated SAH levels and induced H19-mediated Runx2 (runt-related transcription factor 2)-dependent osteoblastic differentiation of VSMCs by inhibiting DNMT3b (DNA methyltransferase 3b) and leading to hypomethylation of the H19 promoter. On the contrary, SAHH deficiency resulted in lower intracellular levels of adenosine and reduced AMPK (AMP-activated protein kinase) activation. Adenosine supplementation activated AMPK and abolished SAHH deficiency-induced expression of H19 and Runx2 and osteoblastic differentiation of VSMCs. Finally, AMPK activation by adenosine inhibited H19 expression by inducing Sirt1 (sirtuin-1)-mediated histone H3 hypoacetylation and DNMT3b-mediated hypermethylation of the H19 promoter in SAHH deficiency VSMCs. We have confirmed a novel correlation between SAHH deficiency and atherosclerotic calcification and clarified a new mechanism that epigenetic upregulation of H19 and AMPK inhibition concurrently contribute to SAHH deficiency-promoted Runx2-dependent atherosclerotic calcification.

DJ-1 suppresses ferroptosis through preserving the activity of S-adenosyl homocysteine hydrolase

Ferroptosis is a newly characterized form of regulated cell death mediated by iron-dependent accumulation of lipid reactive oxygen species and holds great potential for cancer therapy. However, the molecular mechanisms underlying ferroptosis remain largely elusive. In this study, we define an integrative role of DJ-1 in ferroptosis. Inhibition of DJ-1 potently enhances the sensitivity of tumor cells to ferroptosis inducers both in vitro and in vivo. Metabolic analysis and metabolite rescue assay reveal that DJ-1 depletion inhibits the transsulfuration pathway by disrupting the formation of the S-adenosyl homocysteine hydrolase tetramer and impairing its activity. Consequently, more ferroptosis is induced when homocysteine generation is decreased, which might be the only source of glutathione biosynthesis when cystine uptake is blocked. Thus, our findings show that DJ-1 determines the response of cancer cells to ferroptosis, and highlight a candidate therapeutic target to potentially improve the effect of ferroptosis-based antitumor therapy.

Combined Single-Cell Profiling of lncRNAs and Functional Screening Reveals that H19 Is Pivotal for Embryonic Hematopoietic Stem Cell Development.

The generation of hematopoietic stem cells (HSCs) from embryonic endothelial precursors and pre-HSCs is precisely regulated by signaling pathwaysand transcription factors. Nevertheless, regulatory roles of non-coding RNAs remain unknown. Taking advantage of our ability to capture rare pre-HSCs and HSCs invivo, we generated a single-cell landscape of long non-coding RNAs (lncRNAs) during HSC development. Combining bioinformatics and functional screening, we identified 6 lncRNAs influencing hematopoiesis invitro. We further revealed that H19 lncRNA is pivotal forinvivo HSC emergence in aorta-gonads-mesonephros region. Early H19 lncRNA deficiency blocked endothelial-to-hematopoietic transition, which was independent of the H19-derived miR, miR-675. Moreover, H19-deficient pre-HSCs displayed promoter hypermethylation and concomitant downregulation of several master hematopoietic transcription factors, including Runx1 and Spi1. H19 deficiency increased the activity of S-adenosylhomocysteine hydrolase, a regulator of DNA methylation, which partially contributed to the observed hematopoietic defect. Our findings provide a resource for further analysis of lncRNAs in embryonic HSC development.

Knock-down of AHCY and depletion of adenosine induces DNA damage and cell cycle arrest

Recently, functional connections between S-adenosylhomocysteine hydrolase (AHCY) activity and cancer have been reported. As the properties of AHCY include the hydrolysis of S-adenosylhomocysteine and maintenance of the cellular methylation potential, the connection between AHCY and cancer is not obvious. The mechanisms by which AHCY influences the cell cycle or cell proliferation have not yet been confirmed. To elucidate AHCY-driven cancer-specific mechanisms, we pursued a multi-omics approach to investigate the effect of AHCY-knockdown on hepatocellular carcinoma cells. Here, we show that reduced AHCY activity causes adenosine depletion with activation of the DNA damage response (DDR), leading to cell cycle arrest, a decreased proliferation rate and DNA damage. The underlying mechanism behind these effects might be applicable to cancer types that have either significant levels of endogenous AHCY and/or are dependent on high concentrations of adenosine in their microenvironments. Thus, adenosine monitoring might be used as a preventive measure in liver disease, whereas induced adenosine depletion might be the desired approach for provoking the DDR in diagnosed cancer, thus opening new avenues for targeted therapy. Additionally, including AHCY in mutational screens as a potential risk factor may be a beneficial preventive measure.

Effect of lyophilized prune extract on hyperhomocysteinemia in mice

Altered homocysteine metabolism defined as hyperhomocysteinemia is implicated as pathogenic factor in several cardiovascular diseases and atherosclerosis. The purpose of this study was to investigate the efficacy of prune extract, a good source of phenolic antioxidants, on lowering plasma homocysteine level in male hyperhomocysteinemic mice from average weight of 28 g. The administration of lyophilized prune extract was carried out by intraperitoneal injection one day preceding and one hour before sacrifice of mice. Prune extract decreased significantly plasma homocysteine level, correlated with an increased activity of S-adenosylhomocysteine (SAH) hydrolase and NAD(P)H: quinone oxydoreductase-1 activities. Our results suggest a beneficial effect of prune extract on hyperhomocysteinemia with reduction of homocysteine level by its conversion on to SAH by S-adenosylhomocysteine hydrolase, which is activated by NAD+, a by-product of NAD(P)H: quinone oxydo reductase-1.

Metformin alters DNA methylation genome-wide via the H19/SAHH axis

The molecular mechanisms underlying the antineoplastic properties of metformin, a first-line drug for type 2 diabetes, remain elusive. Here we report that metformin induces genome-wide alterations in DNA methylation by modulating the activity of S-adenosylhomocysteine hydrolase (SAHH). Exposing cancer cells to metformin leads to hypermethylation of tumor-promoting pathway genes and concomitant inhibition of cell proliferation. Metformin acts by upregulating microRNA let-7 through AMPK activation, leading to degradation of H19 long noncoding RNA, which normally binds to and inactivates SAHH. H19 knockdown activates SAHH, enabling DNA methyltransferase 3B to methylate a subset of genes. This metformin-induced H19 repression and alteration of gene methylation are recapitulated in endometrial cancer tissue samples obtained from patients treated with antidiabetic doses of metformin. Our findings unveil a novel mechanism of action for the drug metformin with implications for the molecular basis of epigenetic dysregulation in cancer. This novel mechanism of action also may be occurring in normal cells.

Toxicological assays for testing effects of an epigenetic drug on development, fecundity and survivorship of malaria mosquitoes.

Insecticidal resistance poses a major problem for malaria control programs. Mosquitoes adapt to a wide range of changes in the environment quickly, making malaria control an omnipresent problem in tropical countries. The emergence of insecticide resistant populations warrants the exploration of novel drug target pathways and compounds for vector mosquito control. Epigenetic drugs are well established in cancer research, however not much is known about their effects on insects. This study provides a simple protocol for examining the toxicological effects of 3-Deazaneplanocin A (DZNep), an experimental epigenetic drug for cancer therapy, on the malaria vector, Anopheles gambiae. A concentration-dependent increase in mortality and decrease in size was observed in immature mosquitoes exposed to DZNep, whereas the compound reduced the fecundity of adult mosquitoes relative to control treatments. In addition, there was a drug-dependent decrease in S-adenosylhomocysteine (SAH) hydrolase activity in mosquitoes following exposure to DZNep relative to control treatments. These protocols provide the researcher with a simple, step-by-step procedure to assess multiple toxicological endpoints for an experimental drug and, in turn, demonstrate a unique multi-prong approach for exploring the toxicological effects of water-soluble epigenetic drugs or compounds of interest against vector mosquitoes and other insects.

Regulation of S-Adenosylhomocysteine Hydrolase by Lysine Acetylation

S-Adenosylhomocysteine hydrolase (SAHH) is an NAD(+)-dependent tetrameric enzyme that catalyzes the breakdown of S-adenosylhomocysteine to adenosine and homocysteine and is important in cell growth and the regulation of gene expression. Loss of SAHH function can result in global inhibition of cellular methyltransferase enzymes because of high levels of S-adenosylhomocysteine. Prior proteomics studies have identified two SAHH acetylation sites at Lys(401) and Lys(408) but the impact of these post-translational modifications has not yet been determined. Here we use expressed protein ligation to produce semisynthetic SAHH acetylated at Lys(401) and Lys(408) and show that modification of either position negatively impacts the catalytic activity of SAHH. X-ray crystal structures of 408-acetylated SAHH and dually acetylated SAHH have been determined and reveal perturbations in the C-terminal hydrogen bonding patterns, a region of the protein important for NAD(+) binding. These crystal structures along with mutagenesis data suggest that such hydrogen bond perturbations are responsible for SAHH catalytic inhibition by acetylation. These results suggest how increased acetylation of SAHH may globally influence cellular methylation patterns.

Thymine-based molecular beacon for sensing adenosine based on the inhibition of S-adenosylhomocysteine hydrolase activity

This study presents a thymine (T)-based molecular beacon (MB) used for probing S-adenosylhomocysteine hydrolase (SAHH)-catalyzed hydrolysis of S-adenosylhomocysteine (SAH) and for sensing adenosine based on the inhibition of SAHH activity. The designed MB (T8-MB-T8) contained a 15-mer loop and a stem that consisted of a pair of 8-mer T bases, a fluorophore unit at the 5′-end, and a quencher unit at the 3′-end. In the presence of Hg2+, a change in the conformation of T8-MB-T8 placed the fluorophore unit and the quencher in proximity to each other and caused collisional quenching of fluorescence between them. The Hg2+-induced fluorescence quenching of T8-MB-T8 occurred because the Hg2+ induced T–T mismatches to form stable T–Hg2+–T coordination in the MB stem. SAHH catalyzed the hydrolysis of SAH to produce homocysteine. The generated homocysteine enabled the Hg2+ to be removed from a hairpin-shaped T8-MB-T8 through the formation of a strong Hg2+–S bond, leading to the restoration of its fluorescence. The T8-MB-T8·Hg2+ probe showed a limit of detection for SAHH of 4unitsL−1 (approximately 0.24nM) and was reusable for detecting the SAHH/SAH system. Because adenosine was an effective SAHH activity inhibitor, the T8-MB-T8·Hg2+ probe combining the SAHH and SAH systems was used for sensitive and selective detection of adenosine in urine without the interference of other adenosine analogs.

A fluorescence-based assay for the measurement of S-adenosylhomocysteine hydrolase activity in biological samples

The methylation of DNA, RNA, and proteins plays crucial roles in numerous biological processes, including epigenetic control, virus replication, and cell differentiation. In mammals, the rate-limiting step of the S-adenosylmethionine-dependent methylation process is exclusively controlled by S-adenosylhomocysteine (S-AdoHcy) hydrolase (SAHH). SAHH hydrolyzes S-AdoHcy to adenosine and homocysteine (Hcy) and is therefore a potential therapeutic target for various diseases, including cancer, malaria, and viral diseases. However, a simple and highly sensitive assay for the evaluation of SAHH activity, particularly for drug discovery, had not yet been developed. Here we present the development of a fluorescence-based assay for the measurement of SAHH activity in biological samples. We combined the advantages of the detection of fluorescent thiol groups in Hcy by ThioGlo1 with the S-AdoHcy-driven enzyme-coupled reaction. Our results confirmed the reliability of the proposed assay for the measurement of the SAHH activity of purified SAHH and showed the potential of this assay for the measurement of the SAHH activity of biological samples. Therefore, the proposed SAHH activity assay may be utilized in clinical laboratories and in high-throughput screenings for the identification of new SAHH inhibitors with potentially beneficial effects on numerous pathologies.

A method for fluorescence sensing of adenosine and alkaline phosphatase based on the inhibition of S-adenosylhomocysteine hydrolase activity

This study presents a simple fluorescent method for the sensitive and selective detection of adenosine, based on adenosine inhibiting S-adenosylhomocysteine hydrolase (SAHH)-catalyzed hydrolysis of S-adenosylhomocysteine (SAH). Because of homocysteine (HCys) belonging to the thiol and amino groups, 2,3-naphthalenedicarboxaldehyde (NDA) can selectively react with HCys to form a 6-membered ring without the addition of nucleophiles. Electrospray ionization mass spectrometry was used to obtain the molecular mass of the resulting products, which is helpful in proposing the possible reaction mechanism between NDA and HCys. When SAHH catalyzed the cleavage of SAH, the generated HCys reacted with NDA to form highly fluorescent products with a quantum yield of 34%. The addition of adenosine to an SAH solution resulted in the inhibition of SAHH activity. Consequently, HCys production decreased with an increase in adenosine concentration. Under optimal NDA derivatization conditions, the SAHH-based probe showed a limit of detection (at a signal-to-noise ratio of 3) for adenosine of 0.3μM. Selectivity of the SAHH-based probe is more than 100-fold for adenosine over any adenosine analog. We validated the applicability of this probe by determining adenosine concentration in urine samples. The SAHH-based probe was also used to evaluate the activity and inhibition of alkaline phosphatase, which can convert adenosine monophosphate to adenosine.

Clinical picture of S-adenosylhomocysteine hydrolase deficiency resembles phosphomannomutase 2 deficiency

We report on the seventh known patient with S-adenosylhomocysteine hydrolase (SAHH) deficiency presenting at birth with features resembling phosphomannomutase 2 (PMM2-CDG Ia) deficiency. Plasma methionine and total homocysteine levels were normal at 2months and increased only after the 8th month of age. SAHH deficiency was confirmed at 4.5years of age by showing decreased SAHH activity (11% in both erythrocytes and fibroblasts), and compound heterozygosity for a known mutation c.145C>T (p.R49C) and a novel variant c.211G>A (p.G71S) in the AHCY gene. Retrospective analysis of clinical features revealed striking similarities between SAHH deficiency and the PMM2-CDG Ia.

Reduced adenosine release from the aged mammalian heart

Adenosine (ADO) released in the heart results in enhanced coronary blood flow and reduced catecholamine release and myocardial responsiveness to adrenergic stimulation (anti-adrenergic action). ADO release from the adrenergic-stimulated aged heart is less than that from the young adult heart. Because adrenergic signaling in the aged heart is impaired, this study was conducted to determine if reduced ADO release from the aged heart results from this reduced adrenergic responsiveness. Hearts of 3-4 months (young adult) and 21-22 months (aged) Fischer-344 rats were perfused with ADO deamination and re-phosphorylation inhibited. Coronary effluent ADO levels were determined. Cellular-free ADO levels with and without sodium acetate (NaAc)-induced mitochondrial AMP synthesis were assessed using formed S-adenosylhomocysteine (SAH) in L-homocysteine thiolactone (L-HC)-treated hearts. The activities of SAH-hydrolase were determined. Aged heart ADO release was 61% less than from young hearts. NaAc augmented young heart ADO release by 104%, while that of aged hearts remained unchanged. SAH synthesis was 51% and 56% lower in the aged heart in the absence and presence of NaAc, respectively, despite an 89% greater SAH hydrolase activity found in the aged hearts. Since synthesized AMP may be diverted to IMP and ultimately inosine by AMP deaminase, inosine release was determined. Aged heart inosine levels in the absence and presence of NaAc were 74% and 59% less than for the young hearts. It is concluded that a reduced mitochondrial AMP synthesis is in part responsible for the attenuation in ADO release from the adrenergic-stimulated aged heart.

Mice Deficient in Cystathionine Beta Synthase Display Increased Dyrk1A and SAHH Activities in Brain

Hyperhomocysteinemia is associated with brain disease. However, biological actions linking hyperhomocysteinemia to neuronal abnormalities are not well understood. We recently found a relationship between Dyrk1A protein expression, a serine/threonine kinase that might be responsible for cognitive functions in Down's syndrome, and hepatic S-adenosylhomocysteine hydrolase (SAHH) activity, which plays a key role in S-adenosylmethionine-dependent methylation reactions. Considering the role of methylation and Dyrk1A in cognitive functions, the aim of this study was to investigate the relationship between Dyrk1A and SAHH activity in brain of hyperhomocysteinemic mice. We found an increase in Dyrk1A protein expression and activity in brain of hyperhomocysteinemic mice, concomitant with an increased SAHH activity. The effect of overexpression of protein Dyrk1A on SAHH activity was confirmed in brain of Dyrk1A transgenic mice, and additionally we found a positive correlation between Dyrk1A and SAHH activity. These observations suggest a potential effect of Dyrk1A on brain phenotypes linked to hyperhomocysteinemia.

Suppression of Methylation-Mediated Transcriptional Gene Silencing by βC1-SAHH Protein Interaction during Geminivirus-Betasatellite Infection

DNA methylation is a fundamental epigenetic modification that regulates gene expression and represses endogenous transposons and invading DNA viruses. As a counter-defense, the geminiviruses encode proteins that inhibit methylation and transcriptional gene silencing (TGS). Some geminiviruses have acquired a betasatellite called DNA β. This study presents evidence that suppression of methylation-mediated TGS by the sole betasatellite-encoded protein, βC1, is crucial to the association of Tomato yellow leaf curl China virus (TYLCCNV) with its betasatellite (TYLCCNB). We show that TYLCCNB complements Beet curly top virus (BCTV) L2- mutants deficient for methylation inhibition and TGS suppression, and that cytosine methylation levels in BCTV and TYLCCNV genomes, as well as the host genome, are substantially reduced by TYLCCNB or βC1 expression. We also demonstrate that while TYLCCNB or βC1 expression can reverse TGS, TYLCCNV by itself is ineffective. Thus its AC2/AL2 protein, known to have suppression activity in other geminiviruses, is likely a natural mutant in this respect. A yeast two-hybrid screen of candidate proteins, followed by bimolecular fluorescence complementation analysis, revealed that βC1 interacts with S-adenosyl homocysteine hydrolase (SAHH), a methyl cycle enzyme required for TGS. We further demonstrate that βC1 protein inhibits SAHH activity in vitro. That βC1 and other geminivirus proteins target the methyl cycle suggests that limiting its product, S-adenosyl methionine, may be a common viral strategy for methylation interference. We propose that inhibition of methylation and TGS by βC1 stabilizes geminivirus/betasatellite complexes.

Colorimetric Assay for S-Adenosylhomocysteine Hydrolase Activity and Inhibition Using Fluorosurfactant-Capped Gold Nanoparticles

This study reports a simple colorimetric method for the sensitive detection of S-adenosylhomocysteine hydrolase (SAHH) activity and inhibition using fluorosurfactant-capped gold nanoparticles (FSN-AuNPs). FSN stabilizes the AuNPs against conditions of high ionic strength, and FSN-AuNPs are merely aggregated in the presence of homocysteine (HCys) and cysteine. Because of this feature, FSN-AuNPs were found to be dispersed in the presence of S-adenosylhomocysteine (SAH) that lacks a free thiol group. After SAHH catalyzed the hydrolysis of SAH, the produced HCys molecules were bound to the surface of AuNPs through the formation of Au-S bonds. As a result, the nanoparticle (NP) aggregation occurred through electrostatic attraction between each HCys-attached AuNP. This approach had a minimum detectable concentration of 100 units/L (~6 nM). Additionally, because adenosine analogs are capable of inhibiting SAHH activity, the addition of adenosine analogs to a solution containing SAH and SAHH resulted in the suppression of hydrolyzed SAH-induced NP aggregation. Adenosine analogs exhibited the following trend in the half-maximal inhibitory concentrations: adenosine > adenosine monophosphate > adenosine diphosphate ~ adenosine triphosphate. We have demonstrated that the combination of SAHH inhibition and FSN-AuNPs can be utilized for the selective detection of adenosine.