S-Adenosyl-l-ethionine is a Catalytically Competent Analog of S-Adenosyl-l-methione (SAM) in the Radical SAM Enzyme HydG

Radical S-adenosyl-l-methionine (SAM) enzymes initiate biological radical reactions with the 5'-deoxyadenosyl radical (5'-dAdo•). A [4Fe-4S]+ cluster reductively cleaves SAM to form the Ω organometallic intermediate in which the 5'-deoxyadenosyl moiety is directly bound to the unique iron of the [4Fe-4S] cluster, with subsequent liberation of 5'-dAdo•. Here we present synthesis of the SAM analog S-adenosyl-l-ethionine (SAE) and show SAE is a mechanistically-equivalent SAM-alternative for HydG, both supporting enzymatic turnover of substrate tyrosine and forming the organometallic intermediate Ω. Photolysis of SAE bound to HydG forms an ethyl radical trapped in the active site. The ethyl radical withstands prolonged storage at 77 K and its EPR signal is only partially lost upon annealing at 100 K, making it significantly less reactive than the methyl radical formed by SAM photolysis. Upon annealing above 77K, the ethyl radical adds to the [4Fe-4S]2+ cluster, generating an ethyl-[4Fe-4S]3+ organometallic species termed ΩE.

Identification of an Important Odorant Precursor in Durian: First Evidence of Ethionine in Plants

On the basis of the following data from the literature, we hypothesized the presence of ethionine in durian pulp: (1) the major odorants in terms of quantity as well as odor potency in durian pulp are ethanethiol and its derivatives; (2) genome analysis of durian assigned methionine γ-lyase (MGL), the enzyme that converts methionine to methanethiol, a key role for durian odor formation; and (3) MGL accepts not only methionine but also ethionine as a substrate. A targeted search by liquid chromatography-tandem mass spectrometry allowed us to confirm the presence of ethionine in durian pulp. Quantitation of ethionine in samples of different varieties (Monthong, Krathum, Chanee, and Kanyao) showed concentrations (621-9600 μg/kg) in the same range but below the methionine concentrations (16100-30200 μg/kg). During fruit ripening, the ethionine concentration increased as well as the ethanethiol concentration. Final evidence for the role of ethionine as an ethanethiol precursor was provided by demonstrating the formation of (²H₅)ethanethiol after adding (²H₅)ethionine to durian pulp.

Programmed Effects in Neurobehavior and Antioxidative Physiology in Zebrafish Embryonically Exposed to Cadmium: Observations and Hypothesized Adverse Outcome Pathway Framework

Non-communicable diseases (NCDs) are a major cause of premature mortality. Recent studies show that predispositions for NCDs may arise from early-life exposure to low concentrations of environmental contaminants. This developmental origins of health and disease (DOHaD) paradigm suggests that programming of an embryo can be disrupted, changing the homeostatic set point of biological functions. Epigenetic alterations are a possible underlying mechanism. Here, we investigated the DOHaD paradigm by exposing zebrafish to subtoxic concentrations of the ubiquitous contaminant cadmium during embryogenesis, followed by growth under normal conditions. Prolonged behavioral responses to physical stress and altered antioxidative physiology were observed approximately ten weeks after termination of embryonal exposure, at concentrations that were 50–3200-fold below the direct embryotoxic concentration, and interpreted as altered developmental programming. Literature was explored for possible mechanistic pathways that link embryonic subtoxic cadmium to the observed apical phenotypes, more specifically, the probability of molecular mechanisms induced by cadmium exposure leading to altered DNA methylation and subsequently to the observed apical phenotypes. This was done using the adverse outcome pathway model framework, and assessing key event relationship plausibility by tailored Bradford-Hill analysis. Thus, cadmium interaction with thiols appeared to be the major contributor to late-life effects. Cadmium-thiol interactions may lead to depletion of the methyl donor S-adenosyl-methionine, resulting in methylome alterations, and may, additionally, result in oxidative stress, which may lead to DNA oxidation, and subsequently altered DNA methyltransferase activity. In this way, DNA methylation may be affected at a critical developmental stage, causing the observed apical phenotypes.

The association between progression of atherosclerosis and the methylated amino acids asymmetric dimethylarginine and trimethyllysine

Objective

We previously showed that treatment with folic acid (FA)/B₁₂ was associated with more rapid progression of coronary artery disease (CAD). High doses of FA may induce methylation by increasing the availability of S-adenosyl-methionine (SAM). Asymmetric dimethylarginine (ADMA) and trimethyllysine (TML) are both produced through proteolytic release following post-translational SAM–dependent methylation of precursor amino acid. ADMA has previously been associated with CAD. We investigated if plasma levels of ADMA and TML were associated with progression of CAD as measured by quantitative coronary angiography (QCA).

Methods

183 patients from the Western Norway B Vitamin Intervention Trial (WENBIT) undergoing percutaneous coronary intervention (PCI) were randomized to daily treatment with 0.8 mg FA/0.4 mg B₁₂ with and without 40 mg B₆, B₆ alone or placebo. Coronary angiograms and plasma samples of ADMA and TML were obtained at both baseline and follow-up (median 10.5 months). The primary end-point was progression of CAD as measured by diameter stenosis (DS) evaluated by linear quantile mixed models.

Results

A total of 309 coronary lesions not treated with PCI were identified. At follow-up median (95% CI) DS increased by 18.35 (5.22–31.49) percentage points per µmol/L ADMA increase (p-value 0.006) and 2.47 (0.37–4.58) percentage points per µmol/L TML increase (p-value 0.021) in multivariate modeling. Treatment with FA/B₁₂ (±B₆) was not associated with ADMA or TML levels.

Conclusion

In patients with established CAD, baseline ADMA and TML was associated with angiographic progression of CAD. However, neither ADMA nor TML levels were altered by treatment with FA/B₁₂ (±B₆).

Trial Registration

Controlled-Trials.com NCT00354081

Characterization of isolated yeast growth response to methionine analogs

Methionine is one of the first limiting amino acids in poultry nutrition. The use of methionine-rich natural feed ingredients, such as soybean meal or rapeseed meal may lead to negative environmental consequences. Amino acid supplementation leads to reduced use of protein-rich ingredients. The objectives of this study were isolation of potentially high content methionine-containing yeasts, quantification of methionine content in yeasts and their respective growth response to methionine analogs. Minimal medium was used as the selection medium and the isolation medium of methionine-producing yeasts from yeast collection and environmental samples, respectively. Two yeasts previously collected along with six additional strains isolated from Caucasian kefir grains, air-trapped, cantaloupe, and three soil samples could grow on minimal medium. Only two of the newly isolated strains, K1 and C1, grew in minimal medium supplied with either methionine analogs ethionine or norleucine at 0.5% (w/v). Based on large subunit rRNA sequences, these isolated strains were identified as Pichia udriavzevii/Issatchenkia orientalis. P. kudriavzevii/I. orentalis is a generally recognized as a safe organism. In addition, methionine produced by K1 and C1 yeast hydrolysate yielded 1.3 ± 0.01 and 1.1 ± 0.01 mg g(-1) dry cell. Yeast strain K1 may be suitable as a potential source of methionine for dietary supplements in organic poultry feed but may require growth conditions to further increase their methionine content.

Small-molecule inhibitors of the protein methyltransferase SET7/9 identified in a high-throughput screen

Aberrant expression of chromatin-modifying enzymes (CMEs) is associated with a range of human diseases, including cancer. CMEs are now an important target area in drug discovery. Although the role that histone and protein (lysine) methyltransferases (PMTs) play in the regulation of transcription and cell growth is increasingly recognized, few small-molecule inhibitors of this class of enzyme have been reported. Here we describe an assay suitable for primary compound screening for the identification of PMT inhibitors. The assay followed the methylation of histones in the presence of the PMT SET7/9 and the radioactive cofactor S-adenosyl-methionine using scintillating microplates (FlashPlate) and was used to screen approximately 65 000 compounds (% coefficient of variation = 10%; Z' = 0.6). The hits identified from a library of more than 63 000 diverse small molecules included a series of rhodanine compounds with micromolar activity. A screen of the National Cancer Institute Diversity Set (2000 compounds) identified an orsein derivative that inhibited SET7/9 (~20 µM) and showed modest growth inhibition associated with the expected cellular phenotype of reduced histone methylation in a human tumor cell line. The assay represents a useful tool for the identification of inhibitors of PMT activity.

Automated annotation and quantification of metabolites in 1H NMR data of biological origin

In (1)H NMR metabolomic datasets, there are often over a thousand peaks per spectrum, many of which change position drastically between samples. Automatic alignment, annotation, and quantification of all the metabolites of interest in such datasets have not been feasible. In this work we propose a fully automated annotation and quantification procedure which requires annotation of metabolites only in a single spectrum. The reference database built from that single spectrum can be used for any number of (1)H NMR datasets with a similar matrix. The procedure is based on the generalized fuzzy Hough transform (GFHT) for alignment and on Principal-components analysis (PCA) for peak selection and quantification. We show that we can establish quantities of 21 metabolites in several (1)H NMR datasets and that the procedure is extendable to include any number of metabolites that can be identified in a single spectrum. The procedure speeds up the quantification of previously known metabolites and also returns a table containing the intensities and locations of all the peaks that were found and aligned but not assigned to a known metabolite. This enables both biopattern analysis of known metabolites and data mining for new potential biomarkers among the unknowns.

Single-gene knockout of a novel regulatory element confers ethionine resistance and elevates methionine production in Corynebacterium glutamicum

Despite the availability of genome data and recent advances in methionine regulation in Corynebacterium glutamicum, sulfur metabolism and its underlying molecular mechanisms are still poorly characterized in this organism. Here, we describe the identification of an ORF coding for a putative regulatory protein that controls the expression of genes involved in sulfur reduction dependent on extracellular methionine levels. C. glutamicum was randomly mutagenized by transposon mutagenesis and 7,000 mutants were screened for rapid growth on agar plates containing the methionine antimetabolite D,L-ethionine. In all obtained mutants, the site of insertion was located in the ORF NCgl2640 of unknown function that has several homologues in other bacteria. All mutants exhibited similar ethionine resistance and this phenotype could be transferred to another strain by the defined deletion of the NCgl2640 gene. Moreover, inactivation of NCgl2640 resulted in significantly increased methionine production. Using promoter lacZ-fusions of genes involved in sulfur metabolism, we demonstrated the relief of L-methionine repression in the NCgl2640 mutant for cysteine synthase, o-acetylhomoserine sulfhydrolase (metY) and sulfite reductase. Complementation of the mutant strain with plasmid-borne NCgl2640 restored the wild-type phenotype for metY and sulfite reductase.

Isolation, cloning and expression of a multifunctional O-methyltransferase capable of forming 2,5-dimethyl-4-methoxy-3(2H)-furanone, one of the key aroma compounds in strawberry fruits

Strawberry fruits contain an uncommon group of key aroma compounds with a 2,5-dimethyl-3(2H)-furanone structure. Here, we report on the methylation of 2,5-dimethyl-4-hydroxy-3(2H)-furanone (DMHF) to 2,5-dimethyl-4-methoxy-3(2H)-furanone (DMMF) by a S-adenosyl-L-methionine dependent O-methyltransferase, the cloning of the corresponding cDNA and characterization of the encoded protein. Northern-hybridization indicated that the Strawberry-OMT specific transcripts accumulated during ripening in strawberry fruits and were absent in root, petiole, leaf and flower. The protein was functionally expressed in E. coli and exhibited a substrate specificity for catechol, caffeic acid, protocatechuic aldehyde, caffeoyl CoA and DMHF. A common structural feature of the accepted substrates was a o-diphenolic structure also present in DMHF in its dienolic tautomer. FaOMT is active as a homodimer and the native enzyme shows optimum activity at pH 8.5 and 37 degrees C. It does not require a cofactor for enzymatic activity. Due to the expression pattern of FaOMT and the enzymatic activity in the different stages of fruit ripening we suppose that FaOMT is involved in lignification of the achenes and the vascular bundles in the expanding fruit. In addition, it is concluded that the Strawberry-OMT plays an important role in the biosynthesis of strawberry volatiles such as vanillin and DMMF.

Protein repair methyltransferase from the hyperthermophilic archaeon Pyrococcus furiosus. Unusual methyl-accepting affinity for D-aspartyl and N-succinyl-containing peptides

Protein l-isoaspartate-(d-aspartate) O-methyltransferases (EC ), present in a wide variety of prokaryotic and eukaryotic organisms, can initiate the conversion of abnormal l-isoaspartyl residues that arise spontaneously with age to normal l-aspartyl residues. In addition, the mammalian enzyme can recognize spontaneously racemized d-aspartyl residues for conversion to l-aspartyl residues, although no such activity has been seen to date for enzymes from lower animals or prokaryotes. In this work, we characterize the enzyme from the hyperthermophilic archaebacterium Pyrococcus furiosus. Remarkably, this methyltransferase catalyzes both l-isoaspartyl and d-aspartyl methylation reactions in synthetic peptides with affinities that can be significantly higher than those of the human enzyme, previously the most catalytically efficient species known. Analysis of the common features of l-isoaspartyl and d-aspartyl residues suggested that the basic substrate recognition element for this enzyme may be mimicked by an N-terminal succinyl peptide. We tested this hypothesis with a number of synthetic peptides using both the P. furiosus and the human enzyme. We found that peptides devoid of aspartyl residues but containing the N-succinyl group were in fact methyl esterified by both enzymes. The recent structure determined for the l-isoaspartyl methyltransferase from P. furiosus complexed with an l-isoaspartyl peptide supports this mode of methyl-acceptor recognition. The combination of the thermophilicity and the high affinity binding of methyl-accepting substrates makes the P. furiosus enzyme useful both as a reagent for detecting isomerized and racemized residues in damaged proteins and for possible human therapeutic use in repairing damaged proteins in extracellular environments where the cytosolic enzyme is not normally found.

Equilibration of methionine into the Met-tRNA pool of rabbit reticulocyte lysates

We have measured the rate of equilibration of [35S]methionine into the Met-tRNA pool of rabbit reticulocyte lysate as in [FEBS Lett. (1982) 143, 301-305]. Our results indicate that hemin-deficiency inhibits the equilibration of methionine into the tRNA pool much less than protein synthesis or the equilibration of alanine into the tRNA pool, whereas cycloheximide inhibits these processes similarly. This finding is consistent with our previous data and supports the hypothesis that with hemin-deficiency much of the Met-tRNAf that becomes bound to 40 S subunits subsequently undergoes enzymatic deacylation.

The natural occurrence of ethionine in bactdria

Two unknown radioactive areas appeared after radioautography and two dimensional paper chromatography of culture medium in which Escherichia coli was grown. These materials were studied by paper chromatography and paper electrophoresis of several derivatives and identified as ethionine and ethionine sulfone, the latter an artifact. Chromatographic coincidence of the unknowns and their derivatives with authentic materials establishes the identification. Ethionine was found in cellular extracts and in the growth media of Escherichia coli, Bacillus megaterium, Pseudomonas aeruginosa, and Aerobacter aerogenes but not in Scenedesmus, Saccharomyces cerevisiae, or bovine lymphosarcoma cells. Ethionine was synthesized by resting E. coli cultures from radioactive sulfate and from radioactive methionine. Growing cells labeled ethionine within 1 minute after addition of radioactive sulfate to cultures. Levels of radioactivity in ethionine increased with time. No incorporation of this amino acid could be detected in the cellular proteins formed under the conditions of this study.

Suppression by phenobarbital of ethionine-induced hepatocellular carcinoma formation and hepatic S-adenosylethionine levels

An 18-month carcinogenicity study was conducted in male weanling F344 rats (28/group) to examine the effects of the simultaneous feeding of selected concentrations of ethionine and 0.05% phenobarbital in a normal chow diet. The effects of a 1-6-week feeding of phenobarbital and ethionine on the hepatic levels of the related metabolites S-adenosylmethionine, S-adenosylhomocysteine and S-adenosylethionine were also examined. Ethionine at 0.3% or 0.1% induced hepatocellular carcinoma (HCCa) at incidences of 90% (19/21) and 89% (24/27), respectively. Adding phenobarbital to the 0.1% ethionine diet reduced the incidence of HCCa to 36% (10/28) and reduced the number of liver tumor-associated deaths occurring prior to terminal sacrifice from 10/27 to 1/28. No hepatic tumors were observed in rats fed 0, 0.003, 0.01, or 0.03% ethionine. Phenobarbital alone or combined with 0.03% ethionine produced no hepatic tumors. Dietary ethionine at 0.1% reduced the intracellular hepatic level of S-adenosylmethionine to <50% of that seen in control rats. Phenobarbital alone had little effect on either S-adenosylmethionine or S-adenosylhomocysteine levels. The combination of phenobarbital and 0.1% ethionine led to increases in the hepatic levels of S-adenosylmethionine of 40-60% after 3 and 6 weeks of feeding, compared to those seen in rats receiving 0.1% ethionine alone. Ethionine feeding resulted in high levels of S-adenosylethionine in the livers. Combining phenobarbital with ethionine in the diet led to 30-50% reductions in hepatic S-adenosylethionine content. The results indicate that phenobarbital inhibits hepatocarcinogenesis by ethionine, that ethionine may cause HCCa via methyl group insufficiency, and that at levels of < or =0.03% ethionine did not show evidence of tumorigenicity.

Persistence of increased levels of ribosomal gene activity in CHO-K1 cells treated in vitro with demethylating agents

The rate of ribosomal gene activity was evaluated by silver staining of the Nucleolus Organisers (NOs) in cultured CHO-K1 cells after a 12 h pulse with two demethylating agents (L-ethionine and 5-azacytidine). Silver staining of the NOs was measured every 24 h, from 24 up to 110 h after seeding. The purpose was to test the hypothesis that drug-induced demethylation is associated to heritable modifications of rDNA activity. Ribosomal gene activity was shown to be significantly increased by both agents. The increase persisted throughout the experiments, thereby suggesting the heritability of this epigenetic modification. The analysis of heritable DNA damage or modification is an important task in studying the risk of cancer onset and the mechanisms of cancer induction. In these studies two main results were obtained: (i) heritable DNA variations can be induced by both mutational and epigenetic changes; (ii) the modified end-point was not negatively selected.

A specific mechanism for ethionine-induced embryonic gene activity

A specific mechanism was given for ethionine-induced alpha-fetoprotein gene activity and is as follows: 1. Ethionine acts on competent cell types (e.g. stem cells) having one alpha-fetoprotein-enhancer-albumin gene region that is active and possesses embryonic-like low levels of S-adenosyl-L-methionine synthesis: DNA methylase genes for the enhancer regions are in the heterochromatic state. 2. ATP: L-methionine-S-adenosyltransferase acts upon ethionine and ATP to form S-adenosyl-L-ethionine; this lowers the amount of S-adenosyl-L-methionine synthesized and in turn also the synthesis of methyl-nicotinamide; the concentration of nicotinamide increases; there is an inhibition of polyADP ribosylation; hyporibosylation of histone 1 of nucleosomes; deblocking of embryonic type heterochromatin; and finally the second alpha-fetoprotein gene becomes activated. 3. Reversal occurs with the introduction of methionine; increase of S-adenosyl-L-methionine synthesis; increased methylnicotinamide synthesis; increased polyADP-ribose synthesis; ribosylation of H-1 protein to normal levels; and then the packing configuration of chromatin causes rerepression of alpha-fetoprotein genes. It is suggested that ethionine has the ability to perturb a methyl-sensitive heterochromatin that is peculiar to chromatin synthesized during embryogenesis. Therefore such repressed embryonic genes as alpha-fetoprotein are differentially susceptible to low concentrations of active methyl groups. Ethionine causes this hypomethylated heterochromatin by interference with S-adenosyl-L-methionine synthesis.

The use of a spectrophotometric assay to study the interaction of S-adenosylmethionine synthetase with methionine analogues

We have developed a continuous spectrophotometric assay for S-adenosylmethionine synthetase and, using this assay, have examined the interaction of five potential inhibitors with the E. coli enzyme. S-Vinylhomocysteine and S-allylhomocysteine were found to be substrates, while S-(methanethio)cysteine and S-(methanethio)homocysteine were found to be competitive inhibitors. S-Cyanohomocysteine is neither a substrate nor an inhibitor.

Regulation of biosynthesis of thiolutin and aureothricin in Streptomyces kasugaensis

L-Methionine and DL-ethionine decreased production of thiolutin and aureothricin in Streptomyces kasugaensis. In the presence of L-methionine the culture also produced 3-methylthioacrylic acid, 3-methylthiopropionic acid and 3,6-bis-(2-methylthioethyl)-2,5-dioxopiperazine. Production of the metabolites depended on the concentration of L-methionine in the medium.

Formation of S-adenosylethionine in liver of rats chronically fed with DL-ethionine

The early changes in the metabolism of L-ethionine were examined in rats preexposed to chronic administration of DL-ethionine. The capacity of liver to accumulate S-adenosylethionine after a single injection of L-ethionine decreases rapidly from the onset of the carcinogenic regimen. This drop is caused by diminished S-adenosylethionine synthesis, a consequence of lower activity of the ATP-L-methionine adenosyltransferase. This change is accompanied by the rapid increase of the concentration of free ethionine and ethionine sulfoxide. The concentration of hepatic ATP depends in the control animals on the L-ethionine dose and is inversely related to the S-adenosylethionine concentration, but in DL-ethionine-pretreated rats it becomes gradually independent of the L-ethionine dose. The alterations in L-ethionine metabolism observed are not attributed to the change in the ratio of hepatocytes to oval cells but rather to the functional alterations of hepatocytes.

Two subtypes of reversible cell cycle restriction points exist in cultured normal human keratinocyte progenitor cells

Three methods can induce reversible arrest of the growth of cultured human keratinocytes in the G1 phase of the cell cycle. These include the incubation of cells in medium containing transforming growth factor (TGF)-beta or ethionine, or in isoleucine-deficient medium. The current studies were performed to determine if the growth arrest induced by these methods occurs at a common or at a distinct G1 state(s). We first evaluated the relative time interval required for arrested cells to initiate DNA synthesis after growth restimulation with mitogenic medium. The results show that ethionine arrested cells require 22 to 28 hours to initiate DNA synthesis and that a maximum rate of DNA synthesis occurs at 46 hours. Cells arrested by isoleucine deficiency required 10 to 12 hours to initiate DNA synthesis with peak DNA synthesis occurring at 24 hours. Finally, TGF-beta arrested cells require only 6 to 8 hours to initiate DNA synthesis and show a maximum rate of DNA synthesis at 18 hours. We next evaluated if cells that were growth arrested at these states were differentially capable of initiating DNA synthesis in different types of potentially mitogenic medium. The results show that if TGF-beta arrested cells were refed TGB-beta free serum containing medium with ethionine or similar medium with isoleucine deficiency, no DNA synthesis occurred. In contrast, if cells whose growth was arrested in ethionine-containing medium or in isoleucine-deficient medium were refed mitogenic medium containing TGF-beta, significant DNA synthesis was detected. These results suggest that least two different types of reversible growth arrest states exist in cultured human keratinocytes. One appears to be mediated by receptor-dependent processes, such as that induced by TGF-beta, and the other appears to be mediated by other types of metabolic events, such as those induced by ethionine treatment or by isoleucine deficiency.

The sulfate activation locus of Escherichia coli K12: cloning, genetic, and enzymatic characterization

The sulfate activation locus of Escherichia coli K12 has been cloned by complementation. The genes and gene products of this locus have been characterized by correlating the enzyme activity, complementation patterns, and polypeptides associated with subclones of the cloned DNA. The enzymes of the sulfate activation pathway, ATP sulfurylase (ATP:sulfate adenylyltransferase, EC 2.7.7.4) and APS kinase (ATP:adenosine-5'-phosphosulfate 3'-phosphotransferase, EC 2.7.1.25) have been overproduced approximately 100-fold. Overproduction of ATP sulfurylase requires the expression of both the cysD gene, encoding a 27-kDa polypeptide, and a previously unidentified gene, denoted cysN, which encodes a 62-kDa polypeptide. Purification of ATP sulfurylase to homogeneity reveals that the enzyme is composed of two types of subunits which are encoded by cysD and cysN. Insertion of a kanamycin resistance gene into plasmid or chromosomal cysN prevents sulfate activation and decreases expression of the downstream cysC gene. cysC appears to be the APS kinase structural gene and encodes a 21-kDa polypeptide. The genes are adjacent and are transcribed counterclockwise on the E. coli chromosome in the order cysDNC. cysN and cysC are within the same operon and cysDNC are not in an operon containing cysHIJ.

Differential metabolic response of rat liver, kidney and spleen to ethionine exposure. S-adenosylamino acids, homocysteine and reduced glutathione in tissues

Intraperitoneal injection of ethionine to male rats for up to 12 days caused a pronounced fall in S-adenosylmethionine (AdoMet) in liver, but did not or only slightly affect AdoMet in kidney and spleen. Liver and to a lesser degree kidney showed a dose-dependent, massive accumulation of the metabolic product, S-adenosylethionine (AdoEth), and this metabolic response was most pronounced within the first days of exposure. Trace amounts of AdoEth was demonstrated in the spleen. Both S-adenosylhomocysteine (AdoHcy) and homocysteine (Hcy) in the liver were markedly increased in a dose- and time-dependent manner. There was a moderate increase in Hcy content in spleen and kidney, whereas the AdoHcy levels in these tissues were not affected. The amount of reduced glutathione (GSH) was significantly increased in liver and kidney. This response in liver was evident within 2 days of ethionine exposure and then leveled off whereas there was a gradual increase in GSH in kidney. The GSH content in spleen was unaltered. In addition to a massive build-up of AdoEth, the unique features of the metabolic response of the liver are a pronounced decrease in the AdoMet/AdoHcy ratio (from 15 to 2) associated with an elevated Hcy content and a rapid increase in the amount of GSH. The possibility that the metabolic response of the liver could be assigned to the existence of isozymes or metabolic pathways unique to hepatic cells is discussed.

Early alteration in liver of rats fed 2-fluorenylacetamide investigated by L-ethionine probe

The metabolic activity of liver of rats fed a diet containing 0.03% 2-N-fluorenylacetamide (FAA) was investigated using the probe of L-[ethyl-14C]ethionine (613 mumol L-E/100 g body wt.). Shortly after the onset of the carcinogenic regimen, the capacity of liver to accumulate S-adenosylethionine (SAE) began to decline, reaching its minimum (30% of the concentration in control rats) within 3 weeks. This decreased capacity to accumulate SAE results from the FAA-induced decrease in activity of ATP-L-methionine adenosyltransferase. The concentration of hepatic ATP assayed without L-ethionine (L-E) probe also declined during the first 2 weeks of the carcinogenic regimen, but then increased, attaining the normal values within 2 more weeks. Administering the L-E probe to the FAA-fed rats produced an even greater drop in hepatic ATP concentration during the first 2 weeks; however from the third week on, the L-E dose produced no depressing effect, despite the SAE accumulation remaining at its same depressed levels and, therefore, trapping the same amount of ATP as in the previous weeks. The results show that the modification of L-E metabolism and of ATP turnover, observed previously in DL-E fed rats, need not be specific for the carcinogen fed and can occur even when the carcinogens are metabolized by different enzymatic systems.

The role of formate and S-adenosylmethionine in the reversal of nitrous oxide inhibition of formate oxidation in the rat

Studies have been performed in rats in order to test whether methionine reverses the inhibition of formate oxidation produced by nitrous oxide by virtue of the conversion of methionine to formate. At a dose of methionine (100 mg/kg, 671 mumol/kg) that completely reverses the nitrous oxide inhibition of formate oxidation no significant conversion of the methyl group, carboxyl, or backbone of methionine to formate was apparent. No increases in hepatic formate levels were seen after the administration of 671 mumol/kg methionine or ethionine, and formate treatment did not alter the rate of 14CO2 formed after methionine was administered labeled in the methyl, carboxyl, or backbone position. The reversal of nitrous oxide inhibition of formate oxidation was found to correlate temporally with either S-adenosylmethionine levels after methionine administration or S-adenosylethionine levels following ethionine treatment. After methionine or ethionine administration, elevated hepatic steady state levels of tetrahydrofolate were observed and were coincident with elevated S-adenosylmethionine or S-adenosylethionine. Since formate oxidation rates are dependent on the hepatic tetrahydrofolate level, the mechanism of methionine reversal of nitrous oxide inhibition appears to be related to effects of hepatic S-adenosylmethionine which are important in maintaining and regulating tetrahydrofolate, rather than formate generation from methionine.

The synthesis of the adenosyl-moiety of S-adenosylethionine in liver of rats fed DL-ethionine

The synthesis of the adenosyl-moiety of S-adenosylethionine from glycine was studied in normal rats and rats fed a diet containing 0.30% DL-ethionine for three weeks, using a L-ethionine probe. The liver of rats, pretreated by DL-ethionine feeding, incorporated--after injection of 20 mg L-ethionine/100 g body wt.--six times more C-14 from glycine-2-14C into the adenosyl-moiety of SAE than normal rats. Further increasing of doses up to 100 mg/100 g body wt. did not substantially change the incorporation in pre-exposed rats while in normal rats an almost complete inhibition of the incorporation was observed. The liver cells exposed to a chronic effect of DL-ethionine are able, after higher demands for ATP, to increase the ATP generation more effectively than normal cells. This adaptation to the toxic effect of ethionine represents one of the first alterations of the metabolic functions of liver parenchyme in the course of hepatic DL-ethionine cancerogenesis.

An ethA mutation in Bacillus subtilis 168 permits induction of sporulation by ethionine and increases DNA modification of bacteriophage phi 105

In contrast to Escherichia coli and Salmonella typhimurium, Bacillus subtilis could convert ethionine to S-adenosylethionine (SAE), as can Saccharomyces cerevisiae. This conversion was essential for growth inhibition by ethionine because metE mutants which were deficient in S-adenosylmethionine synthetase activity, were resistant to 10 mM ethionine and converted only a small amount of ethionine to SAE. Another mutation (ethA1) produced partial resistance to ethionine (2 mM) and enabled continual sporulation in glucose medium containing 4 mM DL-ethionine. This sporulation induction probably resulted from the effect of SAE, since it was abolished by the addition of a metE1 mutation. The induction of sporulation was not simply controlled by the ratio of SAE to S-adenosylmethionine, but apparently depended on another effect of the ethA1 mutation, which could be demonstrated by comparing the restriction of clear plaque mutants of bacteriophage phi 105 grown in an ethA1 strain with the restriction of those grown in the standard strain. The phages grown in the ethA1 strain showed increased protection against BsuR restriction. We propose that SAE induces sporulation through the inhibition of a key methylation reaction.

Studies on DNA methyltransferase and alteration of the enzyme activity by chemical carcinogens

DNA in mammalian cells is enzymatically methylated at the 5-position of cytosine via S-adenosylmethionine and DNA methyltransferase. Several chemical carcinogens have been shown to inhibit this reaction, altering DNA methylation. We have been studying the mechanism by which carcinogens alter the methylation of DNA in order to better understand the cellular regulation of DNA methylase activity and to understand the role, if any, of DNA methylation in the carcinogenic process. We have utilized an in vitro assay for DNA methylase isolated from purified rat-liver nuclei. Ethionine, a liver carcinogen, given to rats 17 hr after partial hepatectomy inhibited the incorporation of [methyl-3H]-methionine into 5-methylcytosine residues of DNA. DNA isolated from these ethionine-treated rats was able to accept methyl groups from S-adenosylmethionine 8 times more than control DNA. It was further demonstrated that S-adenosylethionine competitively inhibited the DNA methylase resulting in hypomethylated DNA. N-Methyl-N-nitro-N-nitrosoguanidine reacted with the DNA methylase at the sulfhydryl sites inactivating the enzyme. Methylnitrosourea did not react directly with the methylase enzyme, but when reacted with DNA, the DNA methylase activity was inhibited by the carcinogen alkylated DNA. Sodium selenite also inhibited the enzyme non-competitively with a Ki of 6.7 microM. 5-Azacytidine prevented the 2 to 3 fold increase in DNA methylase seen 2 days following partial hepatectomy. All of these data with various carcinogens, altering DNA methylation by different mechanisms, support the hypothesis that DNA methylation plays a role in the initiation of carcinogenesis.

Ethionine in the analysis of the possible separate roles of methionine and choline deficiencies in carcinogenesis

The importance of ethionine, the ethyl analogue of methionine, as a metabolic probe to study the possible roles of methionine and choline in liver carcinogenesis has been briefly reviewed. Ethionine-induced liver carcinogenesis is similar in many aspects, including initiation, promotion, and progression, to carcinogenesis with other agents. However, the special role of methionine in preventing virtually all metabolic and pathologic effects of ethionine, including liver cancer, places ethionine in a special position. On the basis of these observations and our current knowledge about choline deficiency in the genesis of liver cancer, we proposed that choline and methionine play separate but overlapping roles in the initiation and promotion of liver carcinogenesis.

Hypomethylation of DNA in ethionine-fed rats

Ethionine, the hepatocarcinogenic antimetabolite of methionine, was fed to rats in carcinogenic doses for 1-10 weeks. Levels of 5-methyldeoxycytidine (5-MC) in nuclear DNA and total cellular levels of S-adenosylmethionine (AdoMet) and S-adenosylethionine (AdoEt) were determined at 1, 5 and 10 weeks in livers of control and ethionine-treated animals. The percentage of deoxycytidine residues modified to 5-MC in hepatic DNA of ethionine-fed animals was the same as that in the control animals at 1 week but was 3.6% and 7.6% lower than that observed in control animals at 5 and 10 weeks, respectively. Significant levels of AdoEt, a DNA methylase inhibitor, as well as decreases in the levels of AdoMet were also observed in the livers of ethionine-fed animals. In a second study, the levels of 5-MC, AdoMet and AdoEt were determined in the pancreas, kidneys, testes and thymus of control rats and rats fed ethionine for 10 weeks. Only the testes, an organ known to be susceptible to the toxic effects of ethionine, showed a significant (p less than 0.02) decrease in 5-MC in response to ethionine feeding. AdoEt was present in all tissues studied, except thymus, but at lower levels than those observed in the liver. These results demonstrate that ethionine administration alone under conditions which cause tumors is sufficient for the production of hypomethylated DNA in the target organ and one extrahepatic tissue studied. Hypomethylation of hepatic DNA would appear to result from the accumulation of AdoEt coupled with the decreased levels of AdoMet.

Denatured proteins are degraded more rapidly than abnormal proteins in cell-free extracts of Saccharomyces cerevisiae

Abnormal proteins synthesized in the presence of ethionine were degraded more rapidly than the normal ones in cell-free extracts of ethanol-grown yeast. The denatured proteins, however, were degraded in preference to their native counterparts which were either normal or abnormal.

Chemical carcinogenesis. II. Imidazole-ring-opened derivatives from 7-ethyl- and 1,7-diethylguanosine

The UV-spectral and chromatographic analyses of the derivatives of the two synthetic standards 7-ethylguanosine and 1,7-diethylguanosine are here reported. The derivatives obtained from the dialkyl compound exhibit a striking similarity to those found in the "pyrimidine-nucleotide-like" fraction of rat liver tRNA ethylated in vivo by ethionine. The finding of imidazole-ring-opened products in tRNA ethylation by ethionine could be significant from the point of view of chemical carcinogenesis: in fact, imidazole-ring-opening of 1,7-dialkylguanosines directly at level of RNA with consequent formation of substituted pyrimidines is a transversion, i.e. a mutagenic event which would cause a change in the expression of genetic information since a purine has been transformed into a pyrimidine.

The conversion of sterigmatocystin to O-methylsterigmatocystin and aflatoxin B1 by a cell-free preparation

A cell-free system derived from a versicolorin A-accumulating mutant of Aspergillus parasiticus was found to convert sterigmatocystin to both O-methylsterigmatocystin and aflatoxin B1. It is suggested that the similarity in the chromatographic properties of these two metabolites has caused erroneous conclusions to be made with regards to the biosynthesis of aflatoxin B1.

Altered methionyl-tRNA synthetase in a Spirulina platensis mutant resistant to ethionine

Compared with the parental strain, a Spirulina platensis mutant that is resistant to ethionine incorporated methionine into protein at a reduced rate, whereas ethionine incorporation was practically nil. The methionyl-tRNA synthetase present in crude extracts from the resistant strain showed a reduced affinity for methionine and ethionine.

Metabolism of N-acetyl-L-ethionine sulfoxide in rats

N-Acetyl-L-ethionine sulfoxide, the main urinary metabolic product of L-ethionine in rats, is produced and excreted by hepatocytes, as shown by its identification in liver extracts of rats administered L-ethionine and in the cultivation medium from primary culture of rat hepatocytes incubated with L-[ethyl-1-14C] ethionine. The metabolic fate of N-acetyl-L[ethyl-1-14C] ethionine isolated from the urine of rats injected with L-ethionine was examined in rats in vivo. This compound can be metabolized, to a small extent, into the usual metabolic products of L-ethionine: S-adenosylethionine and, to an increased extent to CO2.

Hepatic levels of S-adenosylethionine and S-adenosylmethionine in rats and hamsters during subchronic feeding of DL-ethionine

The levels of S-adenosylethionine (AdoEt) and of S-adenosylmethionine (AdoMet) in the livers of weanling male rats and male and female hamsters fed ethionine for 1-6 weeks were determined. Ethionine was fed at levels of 0, 0.1, and 0.3% in the diet, and the animals were sacrificed after 0, 1, 3 and 6 weeks of treatment. In both species the hepatic contents of AdoEt were dependent upon the level of ethionine in the diet. For the 6-week experimental period the hepatic levels of AdoEt average 81 microgram/g liver in male hamsters fed 0.1% ethionine in the diet and 160 microgram/g in those fed 0.3% ethionine; the corresponding AdoEt levels in female hamsters were 104 and 191 microgram/g liver, respectively. No marked shifts in hepatic AdoEt levels were seen in either male or female hamsters although a gradual rise in hepatic AdoEt from 145 to 233 microgram/g was noted in the female hamsters receiving 0.3% ethionine in the diet for 1-6 weeks. AdoEt levels in the livers of rats fed 0.3% ethionine were quite variable with values of 123, 305 and 127 microgram/g liver noted at weeks 1, 3 and 6 respectively. In rats fed the 0.1% ethionine diet the liver AdoEt levels dropped from 103 to 61 microgram/g from weeks 1 to 6, In animals fed the ethionine-free diet, the hepatic contents of AdoMet were relatively constant throughout the 6-week experimental period, with average values of 25, 17, and 29 microgram/g liver respectively in the male rats, male hamsters and female hamsters. Chronic ethionine administration always suppressed hepatic AdoMet levels. This suppression was generally greater in animals fed the 0.1% ethionine than in those fed the 0.3% ethionine diet. Thus, the average hepatic AdoMet level in rats, male hamsters and female hamsters receiving the 0.1% ethionine diet for 3-6 weeks were 32, 18, and 45% respectively, of the corresponding AdoMet levels in control animals: however, the corresponding AdoMet levels in animals receiving the 0.3% ethionine diet were 66, 42, and 62% of the respective control values. Feeding 0.1% ethionine to male hamsters led to exceedingly low levels of liver AdoMet (1.4-2.9 microgram/g). No direct correlations could be made between the effects of ethionine feeding on the hepatic AdoEt and AdoMet levels in rats and hamsters and the previously reported differences in carcinogenicity by ethionine in these species.

Methionine-defective mutants in Candida utilis

Ethionine-resistant mutants of Candida utilis CCY-158 overproducing methionine have been isolated. In these mutants the intracellular methionine concentration decreased significantly during the stationary phase. The wild-type strain CCY-158 and the ethionine-resistant mutants isolated were able to use methionine as the nitrogen source but not as the carbon source. From these ethionine-resistant mutants we isolated mutants unable to use methionine as nitrogen source (Mec- mutants), the principal alteration being at the level of methionine uptake. Some of the Mec mutants lost also the ability to use other amino acids as nitrogen source.

Properties of ribosomes and ribosomal RNAs synthesized by Escherichia coli grown in the presence of ethionine. Normal maturation of ribosomal RNA in the absence of methylation

Analysis of 16-S rRNA synthesized in Escherichia coli D10 (met-) incubated in a medium containing ethionine in place of methionine shows that it lacks most and probably all of the methyl groups present in normal 16-SrRNA but possesses the same 3'-OH, and 5'-phosphate terminal sequences as the latter. 23-S rRNA formed in ethionine-treated cells also contains normal terminal sequences. 5-S rRNAs of normal and ethionine-treated E. coli D10 are identical. These results lead to the conclusion that methylation of ribosomal precursor RNAs is not necessary for their maturation to products with normal chain lengths and does not influence the conformation of 16-S rRNA.

The dose-related metabolism of L-ethionine in acute experiments with rats

A method is described for a simple column chromatographic determination of N-acetylethionine, N-acetylethionine sulfoxide, and alpha-keto-gamma-ethiolbutyric acid on AG 1. With this analytical method and with chromatography on AG 50W, the urinary excretion pattern of ethionine, composed of all the aforementioned compounds, ethionine sulfoxide and S-adenosylethionine was studied in acute experiments in female rats as a function of the dose of L-[ethyl-1-14C]ethionine. Whereas N-acetylethionine sulfoxide is the major urinary metabolite at low ethionine doses, at higher doses increased amounts of unchanged ethionine and ethionine sulfoxide are found and account for the major portion of the administered dose at 460-613 mumol (75-100 mg)/100 g body wt. The urinary excretion pattern of S-adenosylethionine shows a close relationship to the concentration pattern of this metabolite in the kidney. The extent of t-RNA ethylation and the amount of carbon dioxide formed from the ethyl group of ethionine peak at doses of 77 (12.5) and 306 mumol (50 mg)/100 g body wt respectively, and do not increase further at higher doses.

In vivo D-ethionine inversion and its inhibition

The quantitative study of the inversion of D-ethionine into L-ethionine in vivo, measured in the liver of rats by the formation of S-adenosylethionine from L-ethionine, demonstrates high efficiency of this conversion when lower doses of D-ethionine are used. At higher doses (> 25 mg (80 mumol)/100 g body wt) the accumulation of S-adenosylethionine is retarded. It is suggested that this decrease of S-adenosylethionine formation is due to an unknown as yet effect of D-ethionine on the L-ethionine metabolism. At these higher doses, the alternative assay of the inversion, based on determining the dilution of radioactive L-ethionine probes by inverted D-ethionine, demonstrates considerably higher inversion, supporting the previous assumption. The inversion can be inhibited by administering D-ethionine simultaneously with either kojic acid or sodium benzoate, both inhibitors of D-amino acid oxidase. Sodium benzoate administration is tolerated very well by rats and therefore may be used in chronic experiments.

Exchange of tubulin dimer into rings in microtubule assembly--disassembly

We have prepared native radioactive tubulin dimer from two species: [35S]tubulin dimer, by in vivo labeling of rat brain, and porcine [3H]ethyltubulin, as previously described [Zeeberg, B., Cheek, J., & Caplow, M. (1980) Anal. Biochem. 104, 321--327]. After microtubule assembly with radioactive tubulin dimer and nonradioactive dimer and rings, the tubulin in the rings and the dimer obtained upon disassembly have approximately equal specific activities. Therefore, during the reaction sequence dimer + rings leads to 37 degrees C microtubules leads to 0 degrees C dimer + rings the tubulin initially in rings becomes indistinguishable from tubulin initially in dimer. Under nonpolymerizing conditions (0 degrees C) radioactive tubulin dimer and radioactive guanine nucleotide are incorporated into rings at approximately equal rates. This indicates that there is a pathway for nucleotide incorporation into rings under nonpolymerizing conditions which involves the incorporation of dimer-bound nucleotide. We also report results on the lack of the mirror image equilibrium during the disassembly process, using porcine [3H]-ethyltubulin dimer, rat [35S]tubulin dimer, and a [3H]-GDP.porcine tubulin dimer complex. In all three cases there is no significant disassembly-dependent incorporatioin of radioactivity into rings when microtubules are disassembled in the presence of radioactive dimer. These results demonstrate that, for rat and porcine tubulin, rings are formed during microtubule disassembly by direct cleavage of intact rings, without a tubulin dimer intermediate.

Incorporation of amino acid analogs during the biosynthesis of Escherichia coli aspartate transcarbamylase

Amino acid-requiring mutants capable of producing derepressed levels of aspartate transcarbamylase (carbamoylphosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2) were obtained and used for the incorporation in this enzyme of eight different amino acid analogs. These amino acid replacements enabled the biosynthesis of a series of modified aspartate transcarbamylases altered in their catalytic or regulatory properties. The enzyme in which phenylalanine was rereplaced by 2-fluorophenylalanine was purified to homogeneity and appeared to have the same specific activity as normal asparate transcarbamylase but lacking both homotropic and heterotropic interactions.