Studies on substrate specificity of S-adenosylmethionine: protein-carboxyl methyltransferase from calf brain

The Timing and Duration of Folate Restriction Differentially Impacts Colon Carcinogenesis

Diet plays a crucial role in the development of colorectal cancer (CRC). Of particular importance, folate, present in foods and supplements, is a crucial modulator of CRC risk. The role of folate, and, specifically, the synthetic variant, folic acid, in the primary prevention of CRC has not been fully elucidated. Animal studies varied considerably in the timing, duration, and supplementation of folates, leading to equivocal results. Our work attempts to isolate these variables to ascertain the role of folic acid in CRC initiation, as we previously demonstrated that folate restriction conferred protection against CRC initiation in a β-pol haploinsufficient mouse model. Here we demonstrated that prior adaptation to folate restriction altered the response to carcinogen exposure in wild-type C57BL/6 mice. Mice adapted to folate restriction for 8 weeks were protected from CRC initiation compared to mice placed on folate restriction for 1 week, irrespective of antibiotic supplementation. Through analyses of mTOR signaling, DNA methyltransferase, and DNA repair, we have identified factors that may play a critical role in the differential responses to folate restriction. Furthermore, the timing and duration of folate restriction altered these pathways differently in the absence of carcinogenic insult. These results represent novel findings, as we were able to show that, in the same model and under controlled conditions, folate restriction produced contrasting results depending on the timing and duration of the intervention.

Epigenetic Regulation of Fanconi Anemia Genes Implicates PRMT5 Blockage as a Strategy for Tumor Chemosensitization

Strengthened DNA repair pathways in tumor cells contribute to the development of resistance to DNA-damaging agents. Consequently, targeting proteins in these pathways is a promising strategy for tumor chemosensitization. Here, we show that the expression of a subset of Fanconi anemia (FA) genes is attenuated in glioblastoma tumor cells deficient in methylthioadenosine phosphorylase (MTAP), a common genetic alteration in a variety of cancers. Subsequent experiments in cell line models of different cancer types illustrate that this reduced transcription of FA genes can be recapitulated by blockage of Protein Arginine Methyltransferase 5 (PRMT5), a promising therapeutically targetable epigenetic regulator whose enzymatic activity is compromised in MTAP-deficient cells. Further analyses provide evidence to support that PRMT5 can function as an epigenetic regulator that contributes to the increased expression of FA genes in cancer cells. Most notably and consistent with the essential roles of FA proteins in resolving DNA damage elicited by interstrand crosslinking (ICL) agents, PRMT5 blockage, as well as MTAP loss, sensitizes tumor cells to ICL agents both in vitro and in xenografts. Collectively, these findings reveal a novel epigenetic mechanism underlying the upregulated expression of FA genes in cancer cells and suggest that therapeutically targeting PRMT5 can have an additional benefit of chemosensitizing tumor cells to ICL agents. IMPLICATIONS: PRMT5 positively regulates the expression of FA genes. Inhibition of PRMT5 attenuates FA-dependent DNA repair pathway and sensitizes tumor cells to ICL agents.

Caffeic acid and chlorogenic acid cytotoxicity, genotoxicity and impact on global DNA methylation in human leukemic cell lines

Dietary phenolic compounds such as caffeic and chlorogenic acid exert an antiproliferative effect and modulate the gene-specific DNA methylation status in human breast tumor cells, but it remains unclear whether they interfere with global DNA methylation in human leukemia cells. We examined whether caffeic and chlorogenic acid (1-250 µM) exert antitumor action in human promyelocytic leukemia cells (HL-60) and human acute T-cell leukemia cells (Jurkat). Caffeic and chlorogenic acid did not reduce cell viability in the two cell lines, as assessed using the neutral red uptake and MTT assays. These phenolic acids (1-100 μM) neither induced DNA damage (comet assay) nor increased the micronuclei frequency (micronucleus assay) in HL-60 and Jurkat cells, indicating that they were not genotoxic or mutagenic. Analysis of global DNA methylation levels using a 5-mC DNA ELISA kit revealed that chlorogenic acid at a non-cytotoxic concentration (100 μM) induced global DNA hypomethylation in Jurkat cells, but not in HL-60 cells, suggesting that it exerts a cell-specific effect. Caffeic acid did not change global DNA methylation. As other phenolic compounds, chlorogenic acid probably modulates DNA methylation by targeting DNA methyltransferases. The hypomethylating action of chlorogenic acid can be beneficial against hematological malignances whose pathogenic processes involve impairment of DNA methylation.

Methyl Donor Deficiency Blocks Colorectal Cancer Development by Affecting Key Metabolic Pathways

Our understanding of the role of folate one-carbon metabolism in colon carcinogenesis remains incomplete. Previous studies indicate that a methyl donor-deficient (MDD) diet lacking folic acid, choline, methionine, and vitamin B12 is associated with long-lasting changes to the intestinal epithelium and sustained tumor protection in Apc-mutant mice. However, the metabolic pathways by which the MDD diet affects these changes are unknown. Colon samples harvested from Apc^Δ14/+ mice fed the MDD diet for 18 weeks were profiled using a GC-MS and LC-MS/MS metabolomics platform. Random forest and pathway analyses were used to identify altered metabolic pathways, and associated gene expression changes were analyzed by RT-PCR. Approximately 100 metabolites affected by the MDD diet were identified. As expected, metabolites within the methionine cycle, including methionine (-2.9-fold, P < 0.001) and betaine (-3.3-fold, P < 0.001), were reduced. Elevated homocysteine (110-fold, P < 0.001) was associated with increased flux through the transsulfuration pathway. Unexpectedly, levels of deoxycholic acid (-4.5-fold, P < 0.05) and several other secondary bile acids were reduced. There were also unexpected reductions in the levels of carnitine (-2.0-fold, P < 0.01) and a panel of acylcarnitines involved in fatty acid β-oxidation. Finally, metabolites involved in redox balance, including ascorbate and hypotaurine, were found to be persistently elevated. These findings provide clues to the molecular changes underlying MDD-mediated tumor protection and identify regulatable metabolic pathways that may provide new targets for colon cancer prevention and treatment. IMPLICATIONS: Metabolomic profiling reveals molecular changes underlying MDD-induced tumor protection and may provide new targets for colorectal cancer prevention and treatment.

MTAP Loss Promotes Stemness in Glioblastoma and Confers Unique Susceptibility to Purine Starvation

Homozygous deletion of methylthioadenosine phosphorylase (MTAP) is one of the most frequent genetic alterations in glioblastoma (GBM), but its pathologic consequences remain unclear. In this study, we report that loss of MTAP results in profound epigenetic reprogramming characterized by hypomethylation of PROM1/CD133-associated stem cell regulatory pathways. MTAP deficiency promotes glioma stem-like cell (GSC) formation with increased expression of PROM1/CD133 and enhanced tumorigenicity of GBM cells and is associated with poor prognosis in patients with GBM. As a combined consequence of purine production deficiency in MTAP-null GBM and the critical dependence of GSCs on purines, the enriched subset of CD133+ cells in MTAP-null GBM can be effectively depleted by inhibition of de novo purine synthesis. These findings suggest that MTAP loss promotes the pathogenesis of GBM by shaping the epigenetic landscape and stemness of GBM cells while simultaneously providing a unique opportunity for GBM therapeutics. SIGNIFICANCE: This study links the frequently mutated metabolic enzyme MTAP to dysregulated epigenetics and cancer cell stemness and establishes MTAP status as a factor for consideration in characterizing GBM and developing therapeutic strategies.

16 Inhibition of mammalian protein methyltransferases by 5'-methylthioadenosine (MTA): A mechanism of action of dietary same?

5'-deoxy-5'-methylthioadenosine (5'-methylthioadenosine, MTA) is a naturally occurring metabolite. As an experimental reagent, it has proved useful in providing investigators a window onto the role of protein methylation reactions in intact cells, although its mode of action is poorly understood in most cases. This chapter reevaluates its utility as a reagent. It appears now that MTA is at best a poor direct inhibitor of methyltransferases and that its effectiveness in intact cells may depend on its ability to inhibit S-adenosyl-l-homocysteine hydrolase. This chapter reviews recent evidence that points to an important role for MTA as an intermediary in the beneficial pharmaceutical action of orally ingested S-adenosyl-l-methionine (AdoMet, SAMe). These new results suggest that oral AdoMet may function not by enhancing the activity of cellular methyltransferases, as has been previously surmised, but by inhibiting their action. Such inhibition, particularly of protein methyltransferases involved in intracellular communication, may attenuate signal transduction pathways otherwise leading to inflammatory damage to tissues.

Inhibitory effect of arginine-derivatives from ginseng extract and basic amino acids on protein-arginine N-methyltransferase

Protein-arginine N-methyltransferase (protein methylase I) catalyzes methylation of arginyl residues on substrate protein posttranslationally utilizing S-adenosyl-L-methionine as the methyl donor and yields NG-methylarginine residues. Arginyl-fructose and arginyl-fructosyl-glucose from Korean red ginseng were found to inhibit protein methylase I activity in vitro. This inhibitory activity was shown to be due to arginyl moiety in the molecules, rather than that of carbohydrates. Several basic amino acids as well as polyamines were also found to inhibit protein methylase I activity. Interestingly, the intensity of the inhibitory activity was correlated with the number of amino-group in polyamines, thus, in the order of spermine > spermidine > putrescine > agmatine-sulfate, with IC50 at approximately 15 mM, 25 mM, 35 mM, and 50 mM, respectively. On the other hand, neutral amino acids or NaCl did not inhibit the enzyme activity. Lineweaver-Burk plot analysis of the protein methylase I activity in the presence of arginine and spermidine indicated that the inhibition was competitive in nature in respect to protein substrate, with the Ki values of 24.8 mM and 11.5 mM, respectively.

Recent advances in protein methylation: enzymatic methylation of nucleic acid binding proteins

Heterogeneous nuclear RNP protein A1, one of the major proteins in hnRNP particle (precursor for mRNA), is known to be posttranslationally arginine-methylated in vivo on residues 193, 205, 217 and 224 within the RGG box, the motif postulated to be an RNA binding domain. Possible effect of NG-arginine methyl-modification in the interaction of protein A1 to nucleic acid was investigated. The recombinant hnRNP protein A1 was in vitro methylated by the purified nuclear protein/histone-specific protein methylase I (S-adenosylmethionine:protein-arginine N-methyltransferase) stoichiometrically and the relative binding affinity of the methylated and the unmethylated protein A1 to nucleic acid was compared: Differences in their binding properties to ssDNA-cellulose, pI values and trypsin sensitivities in the presence and absence of MS2-RNA all indicate that the binding property of hnRNP protein A1 to single-stranded nucleic acid has been significantly reduced subsequent to the methylation. These results suggest that posttranslational methyl group insertion to the arginine residue reduces protein-RNA interaction, perhaps due to interference of H-bonding between guanidino nitrogen arginine and phosphate RNA.

Induction of apoptosis in leukemia U937 cells by 5'-deoxy-5'-methylthioadenosine, a potent inhibitor of protein carboxylmethyltransferase

We found dramatic changes in leukemia U937 cells treated with 5'-deoxy-5'-methylthioadenosine (MTA), a potent inhibitor of protein carboxylmethyltransferase (protein methylase II). Initiation of cell death was observed by 1 day after MTA treatment, and it was induced in a dose- and time-dependent manner. However, cell viability measured by trypan blue exclusion was not consistent with the actual percentage of cell death. These results indirectly indicated that the type of cell death is apoptosis rather than necrosis. Nuclear fragmentation and DNA condensation of MTA-treated U937 cells were analyzed by both fluorescent and electron microscopy. MTA-treated cells first began to arrest in the M phase of the cell cycle, and they then exhibited a mitotic-like nuclear fragmentation process with partially membraneless chromatin. Furthermore, agarose gel electrophoresis of DNA extracted from cells treated with MTA showed DNA laddering with production of fragments of approximately 200 bp multiples. These studies indicated that cell death induced by MTA has the characteristics of apoptosis, although nuclear fragmentation is atypical. It seems likely that the process of apoptosis in U937 cells induced by MTA correlates with incomplete assembly of the nuclear envelope, since MTA itself could inhibit the carboxylmethylation of nuclear lamin B and delayed incorporation of lamin B into the nuclear envelope.

Protein damage and methylation-mediated repair in the erythrocyte

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Mechanism of erythrocyte accumulation of methylation inhibitor S-adenosylhomocysteine in uremia

We have recently demonstrated that methyl esterification of erythrocyte membrane proteins, a reaction involved in recognition and repair of specifically damaged proteins, is impaired in uremia. This is accompanied by a significant increase in intracellular S-adenosylhomocysteine (AdoHcy), a potent inhibitor of methyltransferases. AdoHcy accumulation is normally prevented by its enzymatic hydrolysis to homocysteine (Hcy) and adenosine, a reversible reaction catalyzed by AdoHcy hydrolase. To assess the contribution that Hcy offers in the elevation of AdoHcy, we measured plasma and red blood cell Hcy, AdoHcy, adenosine, and S-adenosylmethionine (AdoMet) intracellular concentrations, as well as RBC AdoHcy hydrolase specific activity, in standard hemodialysis patients and normal subjects. Plasma and red blood cell Hcy levels are significantly higher in the dialysis group, and are positively correlated to AdoHcy levels. Adenosine and AdoMet levels, and AdoHcy hydrolase specific activity are not significantly different between the two groups. The enzymatic formation of labeled AdoHcy from Hcy and tracer adenosine appears to be significantly increased, in vitro, in erythrocytes from both control and uremic patients, when 50 microM Hcy (concentration comparable to plasma levels actually found in vivo in uremic patients) is added to the incubation medium. When erythrocytes from uremic patients are incubated in vitro in absence of Hcy, a significant reduction of intracellular AdoHcy is observed with time compared to identical samples incubated in presence of 50 microM Hcy, with a T1/2 of approximately 270 minutes. The results allow us to conclude that plasma and red cell Hcy levels actually found in uremia can be effectively responsible for the intracellular accumulation of the toxic compound AdoHcy.

Enzymatic methyl esterification of erythrocyte membrane proteins is impaired in chronic renal failure. Evidence for high levels of the natural inhibitor S-adenosylhomocysteine

The enzyme protein carboxyl methyltransferase type II has been recently shown to play a crucial role in the repair of damaged proteins. S-adenosylmethionine (AdoMet) is the methyl donor of the reaction, and its demethylated product, S-adenosylhomocysteine (AdoHcy), is the natural inhibitor of this reaction, as well as of most AdoMet-dependent methylations. We examined erythrocyte membrane protein methyl esterification in chronic renal failure (CRF) patients on conservative treatment or hemodialyzed to detect possible alterations of the methylation pattern, in a condition where a state of disrupted red blood cell function is present. We observed a significant reduction in membrane protein methyl esterification in both groups, compared to control. The decrease was particularly evident for cytoskeletal component ankyrin, which is known to be involved in membrane stability and integrity. Moreover, we observed a severalfold rise in AdoHcy levels, while AdoMet concentration was comparable to that detected in the control, resulting in a lower [AdoMet]/[AdoHcy] ratio (P < 0.001). Our findings show an impairment of this posttranslational modification of proteins, associated with high AdoHcy intracellular concentration in CRF. The data are consistent with the notion that, in CRF, structural damages accumulate in erythrocyte membrane proteins, and are not adequately repaired.

Enzymatic methyl esterification of synthetic tripeptides: structural requirements of the peptide substrate. Detection of the reaction products by fast-atom-bombardment mass spectrometry

Eukaryotic protein carboxyl methyltransferase catalyzes a two-substrates reaction in which the methyl group of S-adenosylmethionine is transferred to the free carboxyl group of D-aspartyl and L-isoaspartyl-containing peptide or protein substrates. It has been previously shown that at least three binding sites are required for the interaction of adenosylmethionine with the enzyme and/or the protein substrate [Oliva A., Galletti P., Zappia V., Paik W. K. & Kim S. (1980) Eur. J. Biochem. 104, 595-602], while very little is known concerning the structural requirements of the protein substrate. In this study several synthetic tripeptides were selected in order to elucidate the structural requirements of the methyl-accepting substrates. The results obtained with this series of peptides suggested that: (1) three residues appear to be the minimal length, so far identified, required for a productive enzyme-substrate interaction, several dipeptides being ineffective as substrates [McFadden P. N. & Clarke S. (1986) J. Biol. Chem. 261, 11,503-11,511]; (2) the isoaspartyl residue is not recognized unless its alpha-amino group is involved in a carboamide bond; (3) an hydrogen atom on the amide linkage following the isoaspartyl residue is essential for both recognition and catalysis; (4) oligopeptides containing both D-aspartyl and D-isoaspartyl residues are not recognized by this methyltransferase. On the basis of these results, interaction sites between the peptide substrate and the enzyme molecule have been proposed. This paper also reports the first application of fast-atom-bombardment mass spectrometry to the detection of the products of the enzymatic methyl esterification reaction. By this soft ionization technique, the methyl-esterified peptides as well as the corresponding cyclic imides generated during the spontaneous demethylation process have been identified.

Association of SIBA treatment and a Met-depleted diet inhibits in vitro growth and in vivo metastatic spread of experimental tumor cell lines

We have used 5'-deoxy-5'-S isobutyl-thioadenosine (SIBA), an analog of S-adenosylhomocysteine, alone or in association with a methionine-depleted diet in order to obtain an antitumoral effect in two different tumor models: a transplantable rat rhabdomyosarcoma (RMS-J1) induced by i.m. injection of nickel and the well-known Lewis lung carcinoma (3LL) of C57BL/6 mice. Since SIBA has been reported to inhibit the methyl group transfer from methionine to S-adenosylhomocysteine, among other activities, its association with a reduction of methyl donors, achieved by methionine depletion of the diet (in vivo) or the culture medium (in vitro), should logically lead to an additive effect. In vitro, 3LL and RMS-J1 were sensitive to the cytotoxic effect of SIBA and were methionine-dependent for their proliferation. Fibroblast proliferation was not affected by these two treatments alone or in association. In vivo, either SIBA treatment or a low methionine diet led to a significant decrease in the metastatic character of these two tumors; however, local tumor growth was not significantly affected. The median number of 3LL metastases counted in the lungs was reduced from 100 to 18 by SIBA treatment, and to 27 by the low methionine diet. No additive effect could be detected when the treatments were given simultaneously. RMS-J1-bearing rats treated with SIBA and fed a low Met diet underwent primary tumor excision. The median numbers of lung metastatic nodules were 27, 26, 14 and 8 for the control, SIBA-treated rats, methionine-deprived rats and rats receiving the combined therapy. Expressed as percentages 20 per cent were cured, 23 per cent showed a low number of lung metastases (P less than 10), whereas all the rats in the control group developed more than 10 pulmonary nodules. No cytotoxic effect could be observed on the treated rats. The role of SIBA and methionine depletion, as agents interfering with transmethylation processes, in regard to the control of tumor development, namely metastatic invasiveness, is discussed.

Polyamines, DNA methylation and cell differentiation

The cellular concentration of AdoMet is normally very much higher than that of dcAdoMet, the aminopropyl group donor in polyamine synthesis. However, when cells are depleted of their putrescine and spermidine, i.e. the aminopropyl group acceptors, the dcAdoMet concentration increases dramatically, to a level that may greatly exceed that of AdoMet. Using a highly purified DNA methyltransferase and its preferred substrates, a defined hemimethylated duplex oligodeoxynucleotide or poly(dI-dC)-poly(dI-dC), we demonstrate that dcAdoMet is a poor methyl group donor, and that it starts to inhibit DNA methylation when its concentration exceeds that of AdoMet. At a dcAdoMet/AdoMet ratio of 5:1 there is very little methyl transfer. This study suggests that the antiproliferative and differentiative effects brought about by inhibitors of polyamine synthesis may be partly attributable to dcAdoMet-mediated inhibition of DNA methylation.

Selective deamidation and enzymatic methylation of seminal ribonuclease

Isoenzymatic forms alpha 2, alpha beta, and beta 2 of bovine seminal ribonuclease are generated by the transformation of beta-type into alpha-type subunit through deamidation of a single amide group [Di Donato, A., & D'Alessio, G. (1981) Biochemistry 20, 7232-7237]. The residue involved in this selective deamidation has been identified as Asn67. Deamidation occurs by formation of a cyclic imide intermediate involving the Gly at position 68. Opening of the cyclic imide may occur on either side of the nitrogen, generating both the normal alpha-aspartyl and an isoaspartyl residue at position 67. The alpha-carboxyl of the isoaspartyl residue is effectively methylated by bovine brain protein carboxylmethyltransferase.

Enzymatic basis for the calcium-induced decrease of membrane protein methyl esterification in intact erythrocytes. Evidence for an impairment of S-adenosylmethionine synthesis

The effect of Ca2+ loading, induced by the ionophore A23187, on methyl esterification of membrane proteins (i.e. bands 2.1, 3, 4.1 and 4.5) has been investigated in intact human erythrocytes. When the cells were incubated with L-[methyl-3H]methionine, 40 microM CaCl2 and 10 microM A23187 induce a 50% inhibition of membrane protein methyl esterification. This effect is selectively due to the increased intracellular Ca2+ concentration, as it is antagonized by 10 mM EGTA, and other divalent cations such as Mn2+ do not exert any inhibition. In order to clarify the mechanism(s) of the reported inhibition, the various events involved in the methyl esterification process in vivo were analyzed. L-Methionine uptake as well as protein methylase II activity are not directly affected by altered intracellular Ca2+ concentrations. Conversely in the Ca2+-loaded erythrocytes the conversion of [3H]methionine into [3H]AdoMet, catalyzed by AdoMet synthetase, decreases up to 25%. When the undialyzed erythrocyte cytosolic fraction is assayed in vitro for AdoMet synthetase the activity of the enzyme from the CaCl2/A23187-treated erythrocytes is significantly lower than the control, up to 5 mM ATP. This result suggests that in the Ca2+-loaded erythrocytes the ATP intracellular concentration is significantly lowered. The direct evaluation of ATP intracellular concentration, by HPLC, confirms a significant drop of ATP level, as a consequence of the Ca2+ loading. The removal of Ca2+ from the cells quantitatively restores both the AdoMet synthesis and the methyl esterification levels. The possible role of altered ATP intracellular concentrations as a regulatory factor in the AdoMet-dependent reactions as well as in post-translational protein methylation related to the ageing process is also discussed.

Protein O-carboxylmethylation in relation to male gamete production and function

Protein O-carboxylmethyltransferase (PCM) activity of differentiating male germ cells in the testis and of spermatozoa is strikingly high. PCM catalyzes the methylesterification by S-adenosylmethionine of dicarboxylic amino acid residues in proteins. PCM appears to be the only type of protein methyltransferase present in mature spermatozoa. Mammalian sperms contain considerable amounts of S-adenosylmethionine and can apparently synthesize this nucleoside from L-methionine and ATP. Spermatozoa are rich in S-adenosylhomocysteine hydrolase. The characteristics of this enzyme in testicular germ cells and in sperms are very similar to those in other mammalian tissues; the very sub-stoichiometric extent of methylation of various pure protein substrates, and the rapid spontaneous hydrolysis of the protein methyl ester products at physiological and especially higher pH values, are particularly remarkable. From studies on processes related to protein O-carboxylmethylation in rat spermatozoa from different regions of the epididymis, and in ejaculated spermatozoa from normal and infertile men, unequivocal evidence could not be obtained for hypotheses of other investigators that PCM-catalyzed reactions are of regulatory importance for the acquisition of a potentiality for motility in sperms during their transit and maturation in the epididymis, or for the locomotion of ejaculated sperms. The findings are discussed in the light of the recent hypothesis of S. Clarke that PCM catalyzes methylesterification of D-aspartyl residues that accumulate in certain proteins as a result of slow spontaneous racemization of L-aspartyl residues, and that the methyl esterification of D-aspartyl residues may be related to disposal or repair of proteins damaged in this fashion.

Studies on the metabolic effects of methylthioformycin

5'-Methylthioformycin, a structural analog of 5'-methylthioadenosine in which the N-C glycosidic bond is substituted by a C-C bond, has been synthesized by a newly developed procedure. Membrane permeability of the molecule has been compared to that of methylthioadenosine in intact human erythrocytes and Friend erythroleukemia cells. The formycinyl compound is taken up with a rate significantly lower than that of 5'-methylthioadenosine and is not metabolized by the cells. 5'-Methylthioformycin inhibits Friend erythroleukemia cells' growth: the effect is dose-dependent, fully reversible and not caused by cytotoxicity. Several enzymes related to methylthioadenosine metabolism are inhibited by methylthioformycin. Rat liver methylthioadenosine phosphorylase is competitively inhibited with a Ki value of 2 microM. Among the propylamine transferases tested only rat brain spermine synthase is significantly inhibited, while rat brain spermidine synthase is less sensitive. Rat liver S-adenosylhomocysteine hydrolase is irreversibly inactivated with 50% inhibition at 400 microM methylthioformycin. 5'-Methylthioformycin does not exert any significant effect on protein carboxyl-O-methyltransferase. Inferences about the mechanism of the antiproliferative effect of the drug have been drawn from the above results.

Increased methyl esterification of membrane proteins in aged red-blood cells. Preferential esterification of ankyrin and band-4.1 cytoskeletal proteins

The enzymatic carboxyl methyl esterification of erythrocyte membrane proteins has been investigated in three different age-related fractions of human erythrocytes. When erythrocytes of different mean age, separated by density gradient centrifugation, were incubated under physiological conditions (pH 7.4, 37 degrees C) in the presence of L-[methyl-3H]methionine, the precursor in vivo of the methyl donor S-adenosylmethionine, a fourfold increase in membrane-protein carboxyl methylation was observed in the oldest cells compared with the youngest ones. The identification of methylated species, based on comigration of radioactivity with proteins stained with Coomassie blue, analyzed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, shows, in all cell fractions, a pattern similar to that reported for unfractionated erythrocytes. However in the membrane of the oldest erythrocytes the increase in methylation of the cytoskeletal proteins, bands 2.1 and 4.1, appears to be significantly more marked compared with that observed in the other methylated polypeptides. Furthermore the turnover rate of incorporated [3H]methyl groups in the membrane proteins of the oldest cells markedly increases during cell ageing. Particularly in band 4.1 the age-related increase in methyl esterification is accompanied by a significant reduction of the half-life of methyl esters. The activity of cytoplasmic protein methylase II does not change during cell ageing, while the isolated ghosts from erythrocytes of different age show an age-related increased ability to act as methyl-accepting substrates, when incubated in presence of purified protein methylase II and methyl-labelled S-adenosylmethionine, therefore the relevance of membrane structure in determining membrane protein methylation levels can be postulated. Finally the possible correlation of this posttranslational protein modification with erythrocyte ageing is discussed.

Occurrence of 5'-deoxy-5'-methylthioadenosine phosphorylase in the mammalian CNS: distribution and kinetic studies on the rat brain enzyme

5'-Deoxy-5'-methylthioadenosine (MTA) phosphorylase catalyzes the cleavage of MTA, a secondary product of polyamine biosynthesis, to 5-methylthioribose-1-phosphate and adenine. The occurrence and the general properties of the enzyme were studied in mammalian brain with the following results. (1) Cerebral tissues contained levels of MTA phosphorylase that were comparable to those occurring in other mammalian tissues. (2) Interspecies differences in the enzyme distribution were quite limited, with the highest specific activity values observed in pig brain. Moreover, the enzyme seemed to be generally more concentrated in the cerebellar fractions. (3) Rat brain MTA phosphorylase was highly localized in the cellular soluble fraction. In the first days of rat life, its specific activity in the whole brain was observed to decline significantly from a value of 17.6 units/mg at 1-5 days of age to 13.7 units/mg at 6-10 days of age, remaining then fairly constant up to maturity. (4) Kinetic studies performed with the soluble enzyme extracted from rat brain showed: a pH optimum of 7.4; a Km value for MTA of about 10 microM; an inhibitory effect of the MTA analog 5'-deoxy-5'-isobutylthioadenosine; and a remarkable resistance of the enzyme to heat treatment.

Transport and metabolism of 5'-methylthioadenosine in human erythrocytes

The transport and metabolism of 5'-deoxy-5'-S-methylthioadenosine have been studied in intact human erythrocytes. The sulfur nucleoside is rapidly accumulated into red cells and the extent of uptake largely exceeds the theoretical equilibrium between inner and outer compartment owing to its conversion into a non-permeable compound, namely 5-methylthioribose 1-phosphate. To characterize the nucleoside transport, phosphate-depleted erythrocytes, in which the methylthioadenosine metabolism is negligible, have been employed. The results indicate that: (i) the transport occurs via a facilitated-diffusion mechanism; (ii) the process is not energy-dependent and (iii) no specific cation is required. The kinetic analyses of both the transport and the metabolism show that the uptake of methylthioadenosine is a result of the tandem action of a transport step of high capacity (Vmax = 604 +/- 51 pmol/10(6) cells per min) and low affinity (Km = 3270 +/- 321 microM) followed by a metabolic step of low capacity (Vmax = 6.6 pmol/10(6) cells per min) and high affinity (Km = 30 microM). Furthermore, a substrate inhibition exerted by methylthioadenosine at high concentration (over 200 microM) on its specific phosphorylase is reported for the first time. Experiments performed with several analogs of the thioether indicate that the adenine amino group and the hydrophobic substituent at the 5'-position are critical for the transport carrier recognition. Adenine is the most powerful inhibitor of methylthioadenosine transport.

Purification and properties of protein methylase II from wheat germ

Protein methylase II (S-adenosylmethionine: protein-carboxyl O-methyltransferase, EC 2.1.1.24) was purified from wheat germ approximately 1200-fold with a yield of 4.6% by employing gel filtration. The enzyme, using S-adenosyl-L-methionine, methyl-esterifies the free carboxyl group of protein. Since the enzymatic product was unstable and hydrolyzed non-enzymatically to yield methanol, methanol was identified from the hydrolysate by the formation of the methyl ester of 3.5-dinitrobenzoate. The pH optimum for the wheat germ enzyme was 7.0 in contrast to the 6.0 for the methylase II of calf brain. The wheat germ enzyme had a molecular weight of 41 000 compared to 25 000 for the corresponding rat erythrocyte enzyme [S. Kim (1975) Arch. Biochem. Biophys. 161, 652-657]. The enzyme also differed with the mammalian enzyme in protein substrate preference: the mammalian enzyme showed equal preference to histone and immunoglobulin G while the wheat germ enzyme transferred a methyl group 4.5 times more to histone than to immunoglobulin G. The Km values for histone and S-adenosyl-L-methionine were 0.2 mM and 5 microM. S-Adenosyl-L-homocysteine and its analogues, sinefungin and A9145C, were competitive inhibitors with Ki values 1.5 microM, 0.4 microM and 0.1 microM, respectively. To investigate the identity of endogenous substrate(s) for protein methylase II, the crude wheat germ extract was incubated with S-adenosyl-L-[methyl-3H]methionine and the methylated proteins were analyzed by acid/urea and sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Among other methyl acceptors in the wheat germ, histone polypeptides were the major endogenous substrates.

Accumulation of decarboxylated S-adenosyl-L-methionine in mammalian cells as a consequence of the inhibition of putrescine biosynthesis

Biological transmethylation reactions and polyamine biosynthesis share the substrate S-adenosyl-L-methionine. Under normal conditions, decarboxylated S-adenosyl-L-methionine, the aminopropyl donor for polyamine biosynthesis, does not accumulate because of its rapid utilization in spermidine and spermine synthesis. Alteration of polyamine synthesis by DL-alpha-difluoromethylornithine, an enzyme-activated irreversible inhibitor of L-ornithine decarboxylase, leads to a striking accumulation of decarboxylated S-adenosyl-L-methionine in rat hepatoma cells cultured in vitro and in rat ventral prostate. This increase is due both to lack of putrescine and spermidine for the aminopropyltransferase reactions and to the elevation of S-adenosyl-L-methionine decarboxylase activity. The biological implications of accumulation of decarboxylated S-adenosyl-L-methionine are discussed with regard to the regulation of S-adenosyl-L-methionine decarboxylase activity and to the antiproliferative effects of DL-alpha-difluoromethylornithine.

Methylation at D-aspartyl residues in erythrocytes: possible step in the repair of aged membrane proteins

Reversibly methylated aspartyl residues in human erythrocyte membrane proteins are shown to be in the "unnatural" D configuration. This is demonstrated by treatment of proteolytically derived aspartic acid beta-[3H]methyl ester with L- and D-amino-acid oxidases and by the resolution of diastereomeric L-leucyl dipeptides containing either L- or D-aspartic acid beta-methyl ester by ion-exchange chromatography. Based on this observation, we propose a novel role for eukaryotic protein carboxyl methyltransferases (protein O-methyltransferase; S-adenosyl-L-methionine:protein O-methyltransferase, EC 2.1.1.24). We suggest that these widely distributed enzymes function to recognize aspartyl residues that have racemized spontaneously for a subsequent repair reaction. This repair function is postulated to couple ester hydrolysis with the restoration of the original L configuration of the aspartyl residue. It is possible that similar types of racemization repair processes may occur by reversible covalent modifications at other residues. Other possible functions for D-aspartic acid beta-methyl ester residues in proteins are considered.

Cytochrome c specific methylase from wheat germ

The cytochromes c of plants (e.g., wheat germ) possess two trimethyllysines, residues 72 and 86. In order to investigate the nature of these methylations, we have purified a cytochrome c specific methylase S-adenosylmethionine: protein(lysine) N-methyltransferase (protein methylase III) from wheat germ 135-fold. The in vitro site of methylation by both the purified enzyme and crude wheat germ extract toward various forms of horse heart cytochrome c was localized by two dimensional peptide mapping, Aminex A-5 column peptide analysis, and CNBr cleavage analysis to be the residue 72 lysine. However, no additional sites, in particular residue 86, were seen to be methylated. Although the enzyme is highly specific toward cytochrome c as an in vitro protein substrate, avian cytochromes c are seen to be much better substrates than those from mammalian sources. The enzyme possesses an extremely low Km for apocytochrome c (1.21 microM), suggesting that methylation may occur before heme attachment in vivo. Various S-adenosyl-L-homocysteine analogues were tested for their inhibitor capability toward the enzyme; it was observed that only the D and L forms of S-adenosylhomocysteine are inhibitors while analogues modified in the adenine or homocysteine moieties do not possess inhibitory capability. Results from the Aminex A-5 column chromatography of horse heart cytochrome c chymotryptic digest showed the N epsilon-methyl-, N epsilon-dimethyl-m and N epsilon-trimethyllysine forms of the residue 68-74 peptide to elute earlier than the unmethylated form. This results suggest that the methylated peptides are less basic than the unmethylated form.

Repair of O6-ethylguanine in DNA by a chromatin fraction from rat liver: transfer of the ethyl group to an acceptor protein

Incubation of O6-[3H]ethylguanine-containing DNA with a rat liver chromatin fraction resulted in a decrease in the O6-ethylguanine content of the DNA. Analysis of the products of this reaction showed that the ethyl group had been transferred from the O6-ethylguanine to a protein acceptor. When the incubation mixture was separated on a cesium chloride gradient, the radioactivity removed from O6-ethylguanine appeared in a low-density band. This material has been isolated and subjected to trypsin digestion and high-pressure liquid chromatography analysis; it was sensitive to trypsin and the digest contained new high-pressure liquid chromatography peaks characteristic of oligopeptides. Radioactive peaks from the trypsin digestion have been digested further to the amino acid level and have been shown to contain S-[3H]ethylcysteine. Thus, we conclude that the repair activity in rat liver chromatin removes the ethyl group from O6-ethylguanine and transfers it to a cysteine moiety contained in an acceptor protein.