Mutational analyses of Plasmodium falciparum and human S-adenosylhomocysteine hydrolases

S-adenosylhomocysteine hydrolase is a prospective target for developing new anti-malarial drugs. Inhibition of the hydrolase results in an anti-cellular effect due to the suppression of adenosylmethionine-dependent transmethylations. Based on the crystal structure of Plasmodium falciparum S-adenosylhomocysteine hydrolase which we have determined recently, we performed mutational analyses on P. falciparum and human enzymes. Cys59 and Ala84 of the parasite enzyme, and the equivalent residues on the human enzyme, Thr60 and Gln85, were examined. Mutations of Cys59 and Thr60 caused dramatic impact on inhibition by 2-fluoronoraristeromycin without significant effect both on its kinetic parameters and on inhibition constant against noraristeromycin. In addition, the impact was independent from the electronegativity of the side chain of the substituting residue. These results showed that steric hindrance between a functional group at the 2-position of an adenine nucleoside inhibitor and Thr60 of the human enzyme, not an electrostatic effect, contributed to inhibitor selectivity.

Increase in S-Adenosylhomocysteine Content and Its Effect on the S-Adenosylhomocysteine Hydrolase Activity under Transient High Plasma Homocysteine Levels in Rats

The objective of this study was to examine how transient high plasma homocysteine (Hcy) levels affect the metabolism of Hcy, the activity and expression of S-adenosylhomocysteine (SAH) hydrolase which catalyzes both SAH hydrolysis and SAH synthesis. Wistar ST rats (males) were cannulated in the right jugular vein for intravenous infusion of physiological saline or DL-Hcy solutions (15 and 30 mg/mL) for 1 h at 1.1 mL/h/rat. The content of S-adenosylmethionine (SAM), SAH-synthetic activity of SAH hydrolase and the expression of SAH hydrolase mRNA in liver extracts showed no significant difference in the Hcy infused groups as compared to the Control group. On the other hand, the contents of hepatic SAH in the Hcy infused groups were dose-dependent and significantly higher than that of the Control group. Thus, this study showed that hepatic SAH increased without any increase in the SAH-synthetic activity and the expression of SAH hydrolase mRNA under transient high plasma Hcy levels after intravenous infusion of Hcy.

Catalytic mechanism of S-adenosylhomocysteine hydrolase: roles of His 54, Asp130, Glu155, Lys185, and Aspl89

S-adenosylhomocysteine hydrolase (AdoHcyase) catalyzes the hydrolysis of S-adenosylhomocysteine (AdoHcy) to form adenosine and homocysteine. The crystal structure of the K185N mutated enzyme, which has weak catalytic activity (0.1%), has been determined at 2.8 A resolution and supports the previously predicted mechanism [Takata, Y., Yamada, T., Huang, Y., Komoto, J., Gomi, T., Ogawa, H., Fujioka, M., & Takusagawa, F. (2002). Catalytic mechanism of S-adenosylhomocysteine hydrolase. Site-directed mutagenesis of Asp-130, Lys-185, Asp-189, and Asn-190. J. Biol. Chem. 277, 22670-22676]. The mutated enzyme has an intermediate structure between the open and closed conformation, observed in the substrate-free enzyme and in the inhibitor complexes, respectively. H54, H300, and H352 were mutated to asparagine, respectively, to identify the roles of the histidine residues in catalysis. The kinetic data of H54N, H300N, and H354N mutated enzymes suggest that H54 is the amino acid residue that acts as a general acid/base to cleave the C5'-S(D) bond of AdoHcy. The E155Q mutated enzyme retained a large portion of the catalytic activity (31%), while the E155D mutated enzyme lost most of it (0.3%). The NADH accumulation measurements of the mutated enzymes indicated that the C3'-oxidation and the C4'-proton abstraction are a concerted event and the C5'-S(D) bond cleavage is an independent event. The C4'-proton exchange measurements indicate that the enzyme has an open conformation when AdoHcy is converted to 3'-keto-4', 5'-dehydro-Ado in the active site. With the results of this study and those of the previous studies, a detailed catalytic mechanism of AdoHcyase is described. K185 facilitates the C3'-oxidation, D130 abstracts the C4'-proton, D189, and E155 act as a communicator between the concerted C3'-oxidation and C4'-proton abstraction, and H54 plays as a general acid to cleave the C5'-S(D) bond of AdoHcy.

Adenine and adenosine salvage pathways in erythrocytes and the role of S-adenosylhomocysteine hydrolase. A theoretical study using elementary flux modes

This article is devoted to the study of redundancy and yield of salvage pathways in human erythrocytes. These cells are not able to synthesize ATP de novo. However, the salvage (recycling) of certain nucleosides or bases to give nucleotide triphosphates is operative. As the salvage pathways use enzymes consuming ATP as well as enzymes producing ATP, it is not easy to see whether a net synthesis of ATP is possible. As for pathways using adenosine, a straightforward assumption is that these pathways start with adenosine kinase. However, a pathway bypassing this enzyme and using S-adenosylhomocysteine hydrolase instead was reported. So far, this route has not been analysed in detail. Using the concept of elementary flux modes, we investigate theoretically which salvage pathways exist in erythrocytes, which enzymes belong to each of these and what relative fluxes these enzymes carry. Here, we compute the net overall stoichiometry of ATP build-up from the recycled substrates and show that the network has considerable redundancy. For example, four different pathways of adenine salvage and 12 different pathways of adenosine salvage are obtained. They give different ATP/glucose yields, the highest being 3:10 for adenine salvage and 2:3 for adenosine salvage provided that adenosine is not used as an energy source. Implications for enzyme deficiencies are discussed.

Inhibitory effect of Epimedium extract on S-adenosyl-l-homocysteine hydrolase and biomethylation

In the present paper, the inhibitory effect of Epimedium extract on the activity of S-adenosyl-L-homocysteine (AdoHcy) Hydrolase was studied. The results showed that Epimedium extract inhibited the activity of recombinant human AdoHcy hydrolase in a dose-dependent manner. This inhibitory effect was also observed in hepatic cell line 7701 and hepatoma HepG2, however, the effect in 7701 cells was more potent than in HepG2 cells. The extract could significantly reduce AdoMet/AdoHcy ratio in 7701 cells in a dose-dependent manner, suggesting reduced biomethylation level in 7701 cells. In contrast, it resulted in elevated AdoMet/AdoHcy ratio in the HepG2 cells. The result of MALDI-MS assay indicated that epimedin A and ikarisoside F from the extract could bind to AdoHcy hydrolase. The present data suggested that Epimedium extract could inhibit the activity of AdoHcy hydrolase, thus regulating the cellular biomethylation as well as reducing cellular Hcy level. These results will provide new clues to the mechanisms of Epimedium in curing of cardiovascular disease and regulating tumor cell growth.

CD28 costimulatory signal induces protein arginine methylation in T cells

Protein phosphorylation initiates signal transduction that triggers lymphocyte activation. However, other posttranslational modifications may contribute to this process. Here, we show that CD28 engagement induced protein arginine methyltransferase activity and methylation on arginine of several proteins, including Vav1. Methylation of Vav1 and IL-2 production were reduced by inhibiting S-adenosyl-L-homocysteine hydrolase, an enzyme that regulates cellular transmethylation. Methylated Vav1 was induced in human and mouse T cells and selectively localized in the nucleus, which suggested that this form marks a nuclear function of Vav1. Our findings uncover a signaling pathway that is controlled by CD28 that is likely to be important for T cell activation.

Inhibition of transmethylation disturbs neurulation in chick embryos

Periconceptional folic acid supplementation can reduce the occurrence of neural tube defects. A low folate status will result in reduced remethylation of homocysteine (Hcy) to methionine and, subsequently, in a rise of Hcy levels. Indeed, elevated Hcy concentrations have been reported in mothers of children with neural tube defects. In our previous study, we showed that treatment of chick embryos with Hcy resulted in a delay of neural tube closure in an in vitro model. In the present study, we examined whether this effect of Hcy is due to inhibition of transmethylation via elevation of S-adenosylhomocysteine (AdoHcy). Transmethylation involves methylation of DNA, RNA and proteins by donation of a methyl group from S-adenosylmethionine (AdoMet). After application of inhibitors of S-adenosylhomocysteine hydrolase and of methionine adenosyltransferase, a delay of anterior neuropore closure, comparable to that observed after Hcy treatment, was observed. The changes in AdoMet and AdoHcy concentrations confirmed the inhibition of S-adenosylhomocysteine hydrolase or methionine adenosyltransferase, respectively, and the AdoMet/AdoHcy ratio was decreased in all cases, indicating reduced transmethylation. Moreover, the inhibition of methionine adenosyltransferase was prevented by pretreatment with methionine. This study, therefore, indicates that the Hcy-induced delay of the neural tube closure is caused by the inhibition of transmethylation via elevation of AdoHcy levels and a reduction of the AdoMet/AdoHcy ratio.

Domain motions and the open-to-closed conformational transition of an enzyme: a normal mode analysis of S-adenosyl-L-homocysteine hydrolase

The structure and fluctuations of the enzyme S-adenosyl-L-homocysteine hydrolase (SAHH) are analyzed in an effort to explain its biological function. Besides the previously identified open structure, characteristic of the substrate-free enzyme, we find two distinct structures in enzyme-inhibitor complexes, the closed and closed-twisted conformers. Both closed conformers differ from the open form by a hinge bending motion of two large domains within each subunit, which isolate the inhibitor bound in the active site from the bulk solvent. The closed-twisted form further differs from the closed form by a rigid body twist of the two-subunit dimers. The local structural fluctuations of SAHH are analyzed by performing block normal mode analysis of the tetrameric enzyme in its three forms. For the open form, we find that the four lowest-frequency normal modes, corresponding to the collective motions of the protein with the largest amplitudes, are essentially combinations of the hinge bending deformations of the individual subunits. Thus, the mechanical properties of the open structure of SAHH lead to the presence of structural fluctuations in the direction of the open-to-closed conformational transition. A candidate for such a motion has been observed in previous fluorescence depolarization studies of the enzyme. Both structural and normal mode analyses indicate that residues 180-190 and 350-356 form hinge regions, connecting large domains which tend to move as rigid bodies in response to interactions with substrate, intermediates, and the product of the enzymatic reactions. We propose that these hinge regions play a crucial role in the enzymatic mechanism of SAHH. In contrast to the open form, normal mode calculations for the closed conformations show strong coupling of the hinge bending motions of the individual subunits to each other and to other low-frequency vibrations. Thus, information about structural changes related to reaction progress in one active site may be mechanically transmitted to other subunits of the protein, explaining the cooperativity found in the enzyme kinetics.

A coupled fluorescent assay for histone methyltransferases

Histone methyltransferases (HMTs) catalyze the S-adenosylmethionine (AdoMet)-dependent methylation of lysines and arginines in the nucleosomal core histones H3 and H4 and the linker histone H1b. Methylation of these residues regulates either transcriptional activation or silencing, depending on the residue modified and its degree of methylation. Despite an intense interest in elucidating the functions of HMTs in transcriptional regulation, these enzymes have remained challenging to quantitatively assay. To characterize the substrate specificity of HMTs, we have developed a coupled-fluorescence-based assay for AdoMet-dependent methyltransferases. This assay utilizes S-adenosylhomocysteine hydrolase (SAHH) to hydrolyze the methyltransfer product S-adenosylhomocysteine (AdoHcy) to homocysteine (Hcy) and adenosine (Ado). The Hcy concentration is then determined through conjugation of its free sulfhydryl moiety to a thiol-sensitive fluorophore. Using this assay, we have determined the kinetic parameters for the methylation of a synthetic histone H3 peptide (corresponding to residues 1-15 of the native protein) by Schizosaccharomyces pombe CLR4, an H3 Lys-9-specific methyltransferase. The fluorescent SAHH-coupled assay allows rapid and facile determination of HMT kinetics and can be adapted to measure the enzymatic activity of a wide variety of AdoMet-dependent methyltransferases.

S-Adenosylhomocysteine Metabolism in Different Cell Lines: Effect of Hypoxia and Cell Density

BACKGROUND/AIMS

The methylation potential (MP) is defined as the ratio of S-adenosylmethionine (AdoMet) to S-adenosylhomocysteine (AdoHcy). It was shown recently that hypoxia increases AdoMet/AdoHcy ratio in HepG2 cells (Hermes et al., Exp Cell Res 294: 325-334, 2004). In the present study, we compared AdoMet/AdoHcy ratio and energy metabolism in HepG2, HEK-293, HeLa, MCF-7 and SK-HEP-1 cell lines under normoxia and hypoxia.

METHODS

Metabolite concentrations were measured by HPLC. In addition, AdoHcy hydrolase (AdoHcyase) activity was determined photometrically.

RESULTS

Under normoxia HepG2 cells show the highest AdoMet/AdoHcy ratio of 53.4 +/- 3.3 followed by MCF-7 and SK-HEP-1 cells with a AdoMet/AdoHcy ratio of 14.4 +/- 1.1 and 21.1 +/- 1.3, respectively. The lowest AdoMet/AdoHcy ratios are exhibited by HeLa and HEK-293 cells (6.6 +/- 0.7 and 7.1 +/- 0.3). Hypoxia does not significantly change the MP in MCF-7 and HeLa cells, but alters the MP in HepG2, HEK-293 and SK-HEP-1 cells. These alterations are dependent on the cell density. Under normoxia HepG2 cells exhibit AdoHcyase activity of 2.5 +/- 0.2 nmol min(-1) mg(-1) protein. All other cell lines show 3-5 times lower enzyme activity. Interestingly, hypoxia affects AdoHcyase activity only in HepG2 cells.

CONCLUSIONS

Our data clearly show that the cell lines are characterized by different MP and different behavior under hypoxia. That implies that a lower MP is not necessarily associated with impaired transmethylation activity and cellular function.

Binding of Cu2+ to S-adenosyl-L-homocysteine hydrolase

S-Adenosylhomocysteine (AdoHcy) hydrolase regulates biomethylation and homocysteine metabolism. It has been proposed to be a copper binding protein playing an important role in copper transport and distribution. In the present work, the kinetics of binding and releasing of copper ions was studied using fluorescence method. The dissociation constant for copper ions with AdoHcy hydrolase was determined by fluorescence quenching titration and activity titration methods using ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), and glycine as competitive chelators. The experimental results showed that copper ions bind to AdoHcy hydrolase with a K(d) of approximately 10(-11) M. The association rate constant was determined to be 7 x 10(6) M(-1)s(-1). The releasing of copper ions from the enzyme was found to be biphasic with a k(1) of 2.8 x 10(-3) s(-1) and k(2) of 1.7x10(-5) s(-1). It is suggested that copper ions do not bind to the substrate binding sites because the addition of adenine substrate did not compete with the binding of copper to AdoHcy hydrolase. Interestingly, it was observed that EDTA could bind to AdoHcy hydrolase with a dissociation constant of K(1) = 8.0 x 10(-5) M and result in an increased affinity (K(d) = approximately 10(-17) M) of binding of copper ions to the enzyme.

The Arabidopsis HOMOLOGY-DEPENDENT GENE SILENCING1 gene codes for an S-adenosyl-L-homocysteine hydrolase required for DNA methylation-dependent gene silencing

Genes introduced into higher plant genomes can become silent (gene silencing) and/or cause silencing of homologous genes at unlinked sites (homology-dependent gene silencing or HDG silencing). Mutations of the HOMOLOGY-DEPENDENT GENE SILENCING1 (HOG1) locus relieve transcriptional gene silencing and methylation-dependent HDG silencing and result in genome-wide demethylation. The hog1 mutant plants also grow slowly and have low fertility and reduced seed germination. Three independent mutants of HOG1 were each found to have point mutations at the 3' end of a gene coding for S-adenosyl-l-homocysteine (SAH) hydrolase, and hog1-1 plants show reduced SAH hydrolase activity. A transposon (hog1-4) and a T-DNA tag (hog1-5) in the HOG1 gene each behaved as zygotic embryo lethal mutants and could not be made homozygous. The results suggest that the homozygous hog1 point mutants are leaky and result in genome demethylation and poor growth and that homozygous insertion mutations result in zygotic lethality. Complementation of the hog1-1 point mutation with a T-DNA containing the gene coding for SAH hydrolase restored gene silencing, HDG silencing, DNA methylation, fast growth, and normal seed viability. The same T-DNA also complemented the zygotic embryo lethal phenotype of the hog1-4 tagged mutant. A model relating the HOG1 gene, DNA methylation, and methylation-dependent HDG silencing is presented.

Synthesis of 5′-methylenearisteromycin and its 2-fluoro derivative with potent antimalarial activity due to inhibition of the parasite S-adenosylhomocysteine hydrolase1

5'-methylenearisteromycin 5 and its 2-fluoro derivative 6, which were designed as antimalarial agents because of their AdoHcy hydrolase inhibition, were synthesized from D-ribose, using a stereoselective intramolecular radical cyclization as the key step to construct the carbocyclic structure. These compounds were evaluated as AdoHcy hydrolase inhibitors with the recombinant human and malarial parasite enzymes. Although 5 and 6 were both potent inhibitors of the malarial parasite AdoHcy hydrolase, the 2-fluoro derivative 6 proved to be superior due to its lower inhibitory effect on the human enzyme. In addition, 6 was identified as a potent antimalarial agent using an in vitro assay system with Plasmodium falciparum.

Inactivation of S-adenosylhomocysteine hydrolase with haloethyl and dihalocyclopropyl esters derived from homoadenosine-6′-carboxylic acid

In a search for new inhibitors that exploit 5'-6' 'hydrolytic activity' of AdoHcy hydrolase, a new series of haloethyl and dihalocyclopropyl esters 2-3 were designed and their interaction with the enzyme studied. Incubation of the enzyme with 2-3 resulted in time- and concentration-dependent inactivation of AdoHcy hydrolase as well as almost total depletion of its NAD(+) content. Further results indicated that the 'oxidative' but not the 'hydrolytic' activity was involved in the inactivation process.

Inactivation of human S-adenosylhomocysteine hydrolase by covalent labeling of cysteine 195 with thionucleoside derivatives

A new series of 5'-thioadenosine derivatives 1-4 were synthesized for selectively targeting (195)Cys of human AdoHcy hydrolase. Their incubation with the enzyme resulted in time- and concentration-dependent inactivation, without major modifications of the NAD(+)/NADH ratio. The electrospray mass analysis of the inactivated enzyme with 1, 2, 3, and 4b showed that inhibition was accompanied by the formation of a specific and covalent labeling of each AdoHcy hydrolase subunit. Proteolytic cleavage (endo-Lys-C) and subsequent peptide characterization of the labeled enzyme revealed that (195)Cys was the residue modified during the inactivation process.

L-deaza-5'-noraisteromycin

(+/-)-1-Deazaaristeromycin (4) has been reported to be an inactivator of S-adenosylhomocysteine (AdoHcy) hydrolase and, as a consequence, to affect S-adenosylmethionine (AdoMet) mediated macromolecular biomethylations. To extend this to our program focused on 5'-noraristeromycin derivatives as inhibitors of the same hydrolase enzyme as potential antiviral agents, both enantiomers of 1-deaza-5'-noraristeromycin (5 and 20) have been prepared. Compounds 5 and 20 were evaluated against the following viruses: vaccinia, cowpox, monkeypox, Ebola, herpes simplex type 1 and 2, human cytomegalovirus, Epstein Barr, varicella zoster, hepatitis B, hepatitis C, HIV-1 and HIV-2, adenovirus type 1, measles, Pichinde, parainfluenza type 3, influenza A (H1N1 and H3N2), influenza B, Venezuelan equine encephalitis, rhinovirus type 2, respiratory syncytial, yellow fever, and West Nile. No activity was found nor was there any cytotoxicity to the viral host cells.

Cell shape regulation and co-translocation of actin and adenosyl homocysteinase in response to intermediate hypertonicity

Hypertonic stimulation induced association of S-adenosyl-L-homocysteine hydrolase (SAHH) with the F-actin-rich cell cortex in Dictyostelium. At intermediate, but not higher, levels of hypertonicity, SAHH further translocated from the cortex to the cytosol in company with a fraction of actin and cofilin. At the same time the cells rounded up. Acidification of the cells stimulated both the cell rounding and the translocation of actin and SAHH, whereas alkalinization retarded these responses, suggesting that cellular pH is involved in their control. On the other hand, mutant analysis suggested that neither cGMP signaling nor conventional myosin is required.

Association of diamine oxidase and S-adenosylhomocysteine hydrolase in Nicotiana tabacum extracts

The oxidative deamination of methylated putrescine by a diamine oxidase activity (DAO) is an important step in the biosynthesis of nicotine in tobacco and tropane alkaloids in several Solanaceous plants. A polyclonal rabbit antiserum was previously developed to a purported purified DAO enzyme from Nicotiana tabacum. The antiserum bound to a single 53 kDa protein and immunoprecipitated 80% of DAO activity from tobacco root extracts. In an effort to obtain DAO cDNAs, this antiserum was used to screen a tobacco cDNA expression library and three distinct immunoreactive cDNA clones were isolated. These cDNAs encoded predicted proteins that were either identical or nearly identical to predicted S-adenosylhomocysteine hydrolase (SAHH) from two Nicotiana species. Thus, the rabbit antiserum was not specific to DAO, even though it immunodepleted the majority of DAO activity from root extracts. Alternative hypotheses to explain the DAO immunodepletion results (such as poisoning of DAO activity or that SAHH is a bifunctional enzyme) were tested and ruled out. Therefore, we hypothesize that SAHH associates with DAO as part of a larger multienzyme complex that may function in planta as a nicotine metabolic channel.

The S-adenosyl homocysteine hydrolase inhibitor 3-deaza-adenosine prevents oxidative damage and cognitive impairment following folate and vitamin E deprivation in a murine model of age-related, oxidative stress-induced neurodegeneration

Deficiencies in folate promote neurodegeneration and potentiate the influence of other risk factors for neurodegeneration. This is accomplished at least in part by increasing levels of the neurotoxin homocysteine (HC). The S-adenosyl homocysteine (SAH) hydrolase inhibitor 3-deaza-adenosine (DZA) prevents HC accumulation following folate deprivation. We tested the ability of dietary supplementation with DZA to counteract the deleterious influence of folate deprivation. Folate deficiency has previously been shown to potentiate the impact of apolipoprotein E (ApoE); ApoE-/- mice deprived of folate demonstrated increased oxidative damage in brain tissue and impaired cognitive performance as compared to normal mice or to ApoE-/- mice receiving folate. Herein, we demonstrate that dietary supplementation with DZA prevented both the increase in oxidative damage and impaired cognition characteristic of ApoE-/- mice following folate deprivation. These findings suggest that manipulation of the methionine cycle by DZA can counteract folate deficiency. Because folate deprivation, increased HC, and apolipoprotein E deficiency are all risk factors for Alzheimer's disease, these findings also underscore that DZA might be useful in a therapeutic approach to delay neurodegeneration in Alzheimer's disease.

Hepatic transmethylation reactions in micropigs with alcoholic liver disease

Alcoholic liver disease is associated with abnormal hepatic methionine metabolism, including increased levels of homocysteine and S-adenosylhomocysteine (SAH) and reduced levels of S-adenosylmethionine (SAM) and glutathione (GSH). The concept that abnormal methionine metabolism is involved in the pathogenesis of alcoholic liver disease was strengthened by our previous findings in a micropig model where combining dietary folate deficiency with chronic ethanol feeding produced maximal changes in these metabolites together with early onset of microscopic steatohepatitis and an eightfold increase in plasma aspartate aminotransferase. The goal of the present study was to determine potential mechanisms for abnormal levels of these methionine metabolites by analyzing the transcripts and activities of transmethylation enzymes in the livers of the same micropigs. Ethanol feeding or folate deficiency, separately or in combination, decreased transcript levels of methylenetetrahydrofolate reductase (MTHFR), methionine adenosyltransferase (MAT1A), glycine-N-methyltransferase (GNMT) and S-adenosylhomocysteine hydrolase (SAHH). Ethanol feeding alone reduced the activities of methionine synthase (MS) and MATIII and increased the activity of GNMT. Each diet, separately or in combination, decreased the activities of MTHFR and SAHH. In conclusion, the observed abnormal levels of methionine metabolites in this animal model of accelerated alcoholic liver injury can be ascribed to specific effects of ethanol with or without folate deficiency on the expressions and activities of hepatic enzymes that regulate transmethylation reactions. These novel effects on transmethylation reactions may be implicated in the pathogenesis of alcoholic liver disease.

Influence of an altered methylation potential on mRNA methylation and gene expression in HepG2 cells

S-adenosylhomocysteine (AdoHcy), a by-product and inhibitor of S-adenosylmethionine (AdoMet)-dependent methylation reactions, is removed by AdoHcy hydrolase. The ratio of AdoMet and AdoHcy, also termed methylation potential (MP), is a metabolic indicator for cellular methylation status. In the present study, we have investigated the influence of hypoxia and inhibition of AdoHcy hydrolase on MP in HepG2 cells. Furthermore, we studied the impact of deviations in MP on mRNA and DNA methylation and the expression of selected genes: erythropoietin, VEGF-A, AdoHcy hydrolase, cyclophilin, and HIF-1alpha. Under hypoxic conditions, the MP raised from 53.4 +/- 3.3 to 239.4 +/- 24.8, which is the result of increased AdoMet and decreased AdoHcy levels. Inhibition of AdoHcy hydrolase by adenosine-2',3'-dialdehyde leads to a 40-fold reduction of the MP under both normoxic and hypoxic conditions. Hypoxia increases erythropoietin (2.7-fold) and VEGF-A (5-fold) mRNA expression. During a reduced MP erythropoietin mRNA expression is lowered under normoxia and hypoxia by 70%, whereas VEGF-A mRNA expression is only reduced under hypoxic conditions by 60%. The mRNA expression of AdoHcy hydrolase, HIF-1alpha, and cyclophilin is insensitive to an altered MP. Furthermore, decreased MP leads to a highly significant decrease in overall mRNA methylation. Our results show that the mRNA levels of the studied genes respond differentially to changes in MP. This implies that genes with a slower transcription rate and mRNAs with a slower turnover are insensitive to short-term changes in MP.

Trypanosoma cruzi: molecular cloning and characterization of the S-adenosylhomocysteine hydrolase

S-Adenosylhomocysteine (AdoHcy) hydrolase has emerged as an attractive target for antiparasitic drug design because of its role in the regulation of all S-adenosylmethionine-dependent transmethylation reactions, including those reactions crucial for parasite replication. From a genomic DNA library of Trypanosoma cruzi, we have isolated a gene that encodes a polypeptide containing a highly conserved AdoHcy hydrolase consensus sequence. The recombinant T. cruzi enzyme was overexpressed in Escherichia coli and purified as a homotetramer. At pH 7.2 and 37 degrees C, the purified enzyme hydrolyzes AdoHcy to adenosine and homocysteine with a first-order rate constant of 1 s(-1) and synthesizes AdoHcy from adenosine and homocysteine with a pseudo-first-order rate constant of 3 s(-1) in the presence of 1 mM homocysteine. The reversible catalysis depends on the binding of NAD(+) to the enzyme. In spite of the significant structural homology between the parasitic and human AdoHcy hydrolase, the K(d) of 1.3 microM for NAD(+) binding to the T. cruzi enzyme is approximately 11-fold higher than the K(d) (0.12 microM) for NAD(+) binding to the human enzyme.

Adenosine binding sites at S-adenosylhomocysteine hydrolase are controlled by the NAD+/NADH ratio of the enzyme

S-Adenosylhomocysteine hydrolase (AdoHcy hydrolase) catalyzes the reversible hydrolysis of S-adenosylhomocysteine (AdoHcy) to adenosine (Ado) and homocysteine. On the basis of the kinetics of Ado binding to AdoHcy hydrolase we have shown that AdoHcy hydrolase binds Ado with different affinities [Kidney Blood Press. Res. 19 (1996) 100]. Since AdoHcy hydrolase in its totally reduced form binds Ado with high affinity we determined in the present study the Ado binding characteristics of purified AdoHcy hydrolase from bovine kidney (native form) and of reconstituted forms with defined NAD(+)/NADH ratios. AdoHcy hydrolase in its native form and at a ratio of 50% NAD(+) and 50% NADH exhibits two binding sites for Ado with a K(D1) of 9.2+/-0.6 nmol/L and a K(D2) of 1.4+/-0.1 micromol/L, respectively. Binding of Ado to AdoHcy hydrolase in its NADH form and in its NAD(+) form exhibits only one binding site with high affinity 48.3+/-2.7 nmol/L for the NADH form and with a low affinity of 4.9+/-0.3 micromol/L for the NAD(+) form. To identify these two Ado binding sites, AdoHcy hydrolase was covalently modified with [2-3H]-8-azido-Ado. After irradiation of the native AdoHcy hydrolase two different photolabeled peptides were isolated and identified as Asp(307)-Val(325) and Tyr(379)-Thr(410). When the reconstituted AdoHcy hydrolase in its NADH and in its NAD(+) form was irradiated with [2-3H]-8-azido-Ado only one peptide was identified as Asn(312)-Lys(318) from the NADH form and as Asp(391)-Ala(396) from the NAD(+) form. Based on the crystallographic data, the labeled peptide Asp(391)-Ala(396) (low affinity binding site), appears to belong to the catalytic domain of AdoHcy hydrolase, whereas the labeled peptide, identified as Asn(312)-Lys(318) (high affinity binding site), is located in the NAD domain. In conclusion, our data show that AdoHcy hydrolase has two different Ado binding sites which are dependent upon the enzyme-bound NAD(+)/NADH ratios.

S-adenosylhomocysteine hydrolase, S-adenosylmethionine, S-adenosylhomocysteine: correlations with ribavirin induced anemia

Objective is to speculate on the ribavirin induced anemia by inhibiting S-adenosylhomocysteine (SAH)-hydrolase activity. The major toxicity associated with the use of ribavirin is hemolytic anemia. This adverse effect has been ascribed to the accumulation of ribavirin triphosphate in the erythrocyte, which interferes with erythrocyte function. Ribavirin has been found to inhibit SAH-hydrolase activity in erythrocyte. SAH is further hydrolyzed to adenosine and homocysteine by SAH-hydrolase. The formation of S-adenosylmethionine (SAM) is then demethylated to SAH. SAH, the metabolite of SAM, on the other hand is a powerful inhibitor of methyltransferase enzymes, competing for the SAM binding site. A concurrent decrease in SAM and an increase in SAH levels would inhibit methylation of many tissue components including proteins, DNA, RNA, phospholipids and other small molecules. The enzyme protein carboxyl methyltransferase type II has been recently shown to play a crucial role in the repair of damaged proteins. SAM is the methyl donor of the reaction, and its demethylated product, SAH is the natural inhibitor of this reaction, as well as of most SAM-dependent methylations. The biological function of this transmethylation reaction is related to the repair or degradation of age-damaged proteins. Methyl ester formation in erythrocyte membrane proteins has been used as a marker reaction to tag these abnormal residues and to monitor their increase associated with erythrocyte ageing diseases. Liver disease is complicated by cholesterol deposition in hepatic and extrahepatic membranes. Erythrocyte membrane fluidity has been improved with the administration of SAM and correlated with the cholesterol/phospholipid ratio of the membranes. The levels of SAH-hydrolase activity were also found to undergo a sharp decrease with red cell ageing. The similarity of these alterations with certain morphofunctional characteristics of erythrocyte in some conditions as chronic renal failure, liver disease and hereditary spherocytosis makes it possible to hypothesize that the inhibition of SAH-hydrolase could constitute at least a part of ribavirin induced hemolytic anemia.

Stereocontrolled synthesis of diene and enyne sugar-modified nucleosides and their interaction with S-adenosyl-L-homocysteine hydrolase

Conjugated diene 5-7 and enyne 8 analogs derived from adenosine and uridine were synthesized employing Pd-catalyzed cross-coupling reactions.