S-adenosylmethionine Analog Research Articles

Preparation of carbocyclic S-adenosylazamethionine accompanied by a practical synthesis of (-)-aristeromycin.

For the preparation of a carbocyclic nitrogen analogue of S-adenosylmethionine (carba-AdoazaMet, 4), a practical synthesis of (-)-aristeromycin (7) has been developed using variations of literature procedures. This approach called for a stereospecific synthesis of (3aR,6aR)-2,2-dimethyl-3a,6a-dihydrocyclopenta[1,3]dioxol-4-one ((4R, 5R)-4,5-O-isopropylidene-2-cyclopentenone) (8), which was achieved by modifying reported procedures from D-(-)-ribose.

Stable organophosphorus analogues of S-adenosylmethionine and S-methylmethionine

The organophosphorus analogues of the biologically significant sulfonium compounds S-adenosylmethionine and S-methylmethionine are much more stable than their carboxylic prototypes.

The structural basis for substrate specificity and inhibition of human S-adenosylmethionine decarboxylase.

S-Adenosylmethionine decarboxylase belongs to a small class of amino acid decarboxylases that use a covalently bound pyruvate as a prosthetic group. It is an essential enzyme for polyamine biosynthesis and provides an important target for the design of anti-parasitic and cancer chemotherapeutic agents. We have determined the structures of S-adenosylmethionine decarboxylase complexed with the competitive inhibitors methylglyoxal bis(guanylhydrazone) and 4-amidinoindan-1-one-2'-amidinohydrazone as well as the irreversible inhibitors 5'-deoxy-5'-[N-methyl-N-[(2-aminooxy)ethyl]amino]adenosine, 5'-deoxy-5'-[N-methyl-N-(3-hydrazinopropyl)amino]adenosine, and the methyl ester analogue of S-adenosylmethionine. These structures elucidate residues important for substrate binding and show how those residues interact with both covalently and noncovalently bound inhibitors. S-Adenosylmethionine decarboxylase has a four-layer alphabeta betaalpha sandwich fold with residues from both beta-sheets contributing to substrate and inhibitor binding. The side chains of conserved residues Phe7, Phe223, and Glu247 and the backbone carbonyl of Leu65 play important roles in binding and positioning the ligands. The catalytically important residues Cys82, Ser229, and His243 are positioned near the methionyl group of the substrate. One molecule of putrescine per monomer is observed between the two beta-sheets but far away from the active site. The activating effects of putrescine may be due to conformational changes in the enzyme, to electrostatic effects, or both. The adenosyl moiety of the bound ligand is observed in the unusual syn conformation. The five structures reported here provide a framework for interpretation of S-adenosylmethionine decarboxylase inhibition data and suggest strategies for the development of more potent and more specific inhibitors of S-adenosylmethionine decarboxylase.

A Radical-Based Strategy for the Synthesis of Higher Homologues of Sinefungin

Homosinefungin 5, which can be considered as an analogue of S-adenosylmethionine (SAM) and of S-adenosylhomocysteine (SAH), has been synthesized by means of a sequence in which the key step was the addition of a radical, produced by the simple treatment of an iodide precursor with a zinc-copper couple, to suitably activated olefins.

Effects of Intermediates of Methionine Metabolism and Nucleoside Analogs on S-Adenosylmethionine Transport by Trypanosoma brucei brucei and a Drug-resistant Trypanosoma brucei rhodesiense

The effects of purine nucleoside analogs, polyamines, and established trypanocidal agents on the uptake of [8- 14C]adenosine and S-[ methyl- 3H]adenosylmethionine (AdoMet) by bloodform trypanosomes of drug-susceptible Trypanosoma brucei brucei and a drug-resistant Trypanosoma brucei rhodesiense clinical isolate were compared. AdoMet uptake was not antagonized by ornithine or methionine (500 μM), adenosine (100 μM), or other purine nucleosides, including methylthioadenosine (MTA) at 500 μM. Hydroxyethylthioadenosine (HETA), a trypanocidal analog of methylthioadenosine, and sinefungin, an analog of AdoMet, were competitive with AdoMet transport in both isolates. Dipyridamole, an antagonist of the adenosine P 2 transporter, also competed with AdoMet transport in both isolates. The trypanocidal diamidines pentamidine, Berenil, CGP 40215, and the decarboxylated S-adenosylmethionine (dAdoMet) analog MDL 73811 (5′-{[( Z)-4-amino-2-butenyl]}methyl- amino}-5′-deoxyadenosine) competed with P 2 adenosine transport but did not inhibit AdoMet transport at 100 μM. Methylglyoxalbis(guanylhydrazone) (MGBG), an analog of dAdoMet, was a strong competitive inhibitor of adenosine transport at 100 μM, but did not inhibit AdoMet transport. The polyamines putrescine, spermine, and spermidine (1 mM) were examined for competition with adenosine and AdoMet transport. Putrescine significantly inhibited P 2 adenosine transport in both strains (in the presence of saturating inosine), but AdoMet transport was not affected by these polyamines. P 2 adenosine transport in both strains was highly inhibited by melarsen oxide and melamine, its key organic component, whereas AdoMet uptake was not affected by these agents. These findings further characterize distinguishing features of the unique AdoMet transporter in African trypanosomes, and indicate that the P 2 adenosine transporter remains functional in melarsen- and diamidine-resistant clinical isolates.

Proton-ATPase activities involved in the uptake of an S-adenosylmethionine analogue

Characteristics of the transport of sinefungin (SF) were studied in Leishmania donovani promastigotes grown in vitro in a semi-defined medium. The uptake is time and pH dependent, temperature sensitive, saturable and independent of the growth phase. Metabolic inhibitors decrease the influx, indicating that sinefungin uptake is an energy requiring process. The presence of Na + is unnecessary for activity. The uptake is sensitive to valinomycin and nigericin and to the H +-ATPases inhibitors such as N′ N′-dicyclohexylcarbodiimide, bafilomycin A and oligomycin. Sulfhydryl group(s) are involved in carrier activity. Use of SF analogues shows, stereospecificity of the transporter, recognition of the 6′-amino group and to a lesser degree of the 9′-amino group of the lateral chain, whereas the 9′-carboxyl group of the lateral chain is not implicated in the recognition. Adenosine and ornithine do not interfere with the uptake. No significant amount of SF is tightly bound to macromolecules. In a SF-resistant clone, though the uptake of SF is reduced (the apparent V max is 276 pmoles mg protein −1 30 min −1 compared with 2061 pmoles mg protein −1 30 min −1 for the wild-type clone), the apparent affinity for SF is similar to that of wild-type cells ( K m 0.7 and 0.6 μM respectively). This lower uptake activity is not the reflection of an increased efflux of the drug. In these resistant cells, the susceptibility of SF uptake to variation of the external pH, as well as to azide, NaF, and valinomycin are decreased, that to nigericin is lost.

S-(5'-deoxy-5'-adenosyl)-1-aminoxy-4-(methylsulfonio)-2-cyclopentene (AdoMao): an irreversible inhibitor of S-adenosylmethionine decarboxylase with potent in vitro antitrypanosomal activity.

The S-adenosylmethionine (AdoMet) analogue S-(5'-deoxy-5'-adenosyl)-1-aminoxy-4-(methylsulfonio)-2-cycl opentene (AdoMao) was synthesized in two of its four possible diastereomeric forms using a facile chemoenzymatic route. The trans-1R,4R- and trans-1S,4S-diastereomers of AdoMao, as well as the corresponding diastereomers of the unmethylated precursor molecule nor-AdoMao, were then evaluated as inhibitors of S-adenosylmethionine decarboxylase (AdoMet-DC) from both bacterial and human sources. All four of the analogues acted as time-dependent, irreversible inhibitors of AdoMet-DC from Escherichia coli, exhibiting remarkably constant Ki values ranging between 20.6 and 23.7 microM. These analogues also inhibited the human form of AdoMet-DC, although this form of the enzyme was able to discriminate between AdoMao (Ki values of 21.2 microM for the trans-1R,4R form and 19.6 microM for the trans-1S,4S form) and nor AdoMao (Ki values of 95.2 microM for the trans-1R,4R form and 30.9 microM for the trans-1S,4S form). The trans diastereomers of AdoMao and nor-AdoMao were next evaluated for their ability to inhibit trypanosomal growth in vitro against cultured Trypanosoma brucei brucei bloodforms. All four of these analogues were effective growth inhibitors, with IC50 values ranging between 0.9 and 10.1 microM. The two most effective analogues, trans-1S,4S-AdoMao (IC50 0.9 microM) and trans-1S,4S-AdoMao (IC50 3.0 microM) were also effective against two clinical isolates of the pathogenic organism Trypanosoma brucei rhodesiense, KETRI 243 and KETRI 269. The most promising analogue in all respects was trans-1S,4S-AdoMao, which was subsequently found to have minimal effects on cell growth, AdoMet-DC activity, and intracellular polyamine levels in the sensitive human promyelocytic leukemia cell line HL60. Thus, the S-adenosylmethionine analogue trans-1S,4S-AdoMao acts as an effective inhibitor of AdoMet-DC and appears to serve as a parasite-specific trypanocidal agent in vitro.

Irreversible inhibition of human S-adenosylmethionine decarboxylase by the pure diastereomeric forms of S-(5′-deoxy-5′-adenosyl)-1-ammonio-4-methylsulfonio-2-cyclopentene (AdoMac)

The S-adenosylmethionine (AdoMet) analogue AdoMac [ S-(5′-deoxy-5′-adenosyl)-1-ammonio-4-methylsulfonio-2-cyclopentene], an enzyme-activated, irreversible inhibitor of the Escherichia coli form of S-adenosylmethionine decarboxylase (AdoMet-DC), also acts as a potent inhibitor of the human form of the enzyme. This analogue has been resolved recently into its four possible diastereomeric forms, and each pure diastereomer has now been evaluated as an inhibitor of human AdoMet-DC. As was the case for the bacterial enzyme, kinetic analysis revealed that the four pure diastereomeric forms of AdoMac differentially inhibit human S-adenosylmethionine decarboxylase ( K i values ranging between 11 and 63μM). Although the human and bacterial forms of the enzyme each discriminated between the four diastereomers of AdoMac, each form appeared to bind optimally to a distinctly different diastereomer of the inhibitor. These data suggest that the active sites of human and bacterial AdoMet-DC are distinctly different, and that it may be possible to design inhibitors that are specific for a given form of the enzyme.

α-cyano-substituted analogoues of decarboxylated S-adenosylmethionine as enzyme activated, irreversible inhibitors of S-adenosylmethionine decarboxylase

The S-adenosylmethionine analogues 2a and 2b were synthesized and evaluated as enzyme activated, irreversible inhibitors of the pyruvoyl enzyme S-adenosylmethionine decarboxylase. Each compound inactivated both the Escherichia coli and human forms of the enzyme in a time dependent manner. The synthesis of these compounds and the associated enzyme kinetic studies are described.

Preparation of the pure diastereomeric forms of S- (5′-deoxy-5′-adenosyl)-1-ammonio-4-methylsulfonio-2- cyclopentene and their evaluation as irreversible inhibitors of S-adenosylmethionine decarboxylase from Escherichia coli

The conformationally restricted S-adenosylmethionine analogue AdoMac ( S-(5′-deoxy-5′-adenosyl)-1-ammonio-4-methylsulfonio-2-cyclopentene has been shown to act as an enzyme activated, irreversible inhibitor of the Escherichia coli form of the enzyme S-adenosylmethionine decarboxylase. Inactivation of the enzyme is presumably initiated by formation of an imine linkage between the inhibitor and the terminal pyruvate of the enzyme, followed by base-catalyzed elimination of methylthioadenosine and generation of a latent electrophile. Removal of the driving force for the elimination of methylthioadenosine resulted in a reversibly binding inhibitor. Thus, the thioether analogue corresponding to AdoMac, and the corresponding dihydro derivative (H 2-AdoMac), reversibly inhibit the enzyme. AdoMac was resolved into its four pure diastereomeric forms, and each diastereomer was evaluated as an irreversible inhibitor of the enzyme. The K I values for the individual diastereomers range between 3.83 and 39.6 μM, with the cis-1 S,4 R diastereomer being the most potent inhibitor. However, the k inact values for the four diastereomers are not significantly different, suggesting that the binding of each diastereomer to the enzyme is configuration-dependent, while the subsequent inactivation likely proceeds through a single intermediate which is formed from each of the four diastereomers. Since each pure diastereomer represents a distinct conformational mimic exhibiting restricted sidechain rotation, the data suggests that these and related analogues may be useful as conformational probes for the catalytic site of AdoMet-DC.

Effects of Sinefungin on Growth and Sterol Composition of Leishmania Promastigotes

The S-adenosylmethionine analogue sinefungin was tested in vitro against promastigotes of various strains of Leishmania. The IC50 values for the Leishmania mexicana,Leishmania major, and Leishmania donovani strains used were of the order of 10 ng/ml but the Leishmania amazonensis strain tested was more resistant to the drug, the IC50 value being 6 μg/ml. Sterol profiles, in which 24-alkyl (C28) sterols predominated, were relatively unaffected by sinefungin. Incorporation of label derived from either [methyl-2H3]methionine or [methyl-14C]methionine into sterols was not appreciably affected by treatment at a growth-inhibiting concentration of sinefungin. It was concluded that sinefungin had only a limited effect on sterol production at the 24-transmethylation step and it is unlikely that this is the primary cause of cell death.

Restricted rotation analogs of S-adenosylmethionine: synthesis, evaluation as inhibitors of S-adenosylmethionine decarboxylase, and potential use as selective antitrypanosomal agents

A series of restricted rotation analogs of S-adenosylmethionine were synthesized and evaluated as inhibitors of S-adenosylmethionine decarboxylase from Escherichia coli. These analogs were also evaluated for inhibitory activity against cultured mammalian cells, and against the parasite Trypanosoma brucei brucei. All analogs tested appeared to selectively inhibit trypanosomal growth. In particular, compound 5 (AdoMac) inhibits the growth of Trypanosoma brucei brucei with an IC 50 of 5.2μM.

Trypanosoma brucei spliced-leader RNA methylations are required for trans splicing in vivo.

The Trypanosoma brucei spliced leader (SL) RNA donates its 5' leader sequence to all nuclear pre-mRNAs via trans RNA splicing. The SL RNA is a small-nuclear U RNA-like molecule which is present in the cell as part of a small ribonucleoprotein particle. However, unlike the trimethylguanosine-capped small nuclear U RNAs, the SL RNA has a highly modified 5' terminus containing an m7G cap and methylations on the first four transcribed nucleotides. Here, we show that incubation of procyclic-form T. brucei in the presence of the S-adenosylmethionine analog, sinefungin, leads to a rapid inhibition of SL RNA methylation. A concomitant inhibition of trans splicing and an accumulation of high-molecular-weight tubulin transcripts were also observed. The effects of sinefungin on SL RNA methylation and on trans splicing were correlated by labeling of cells incubated in the presence of the antibiotic. The results indicate that 5' modifications of the SL RNA are necessary for it to participate in trans splicing. SL RNA modification is not required for assembly of the core SL ribonucleoprotein, as these Cs2SO4-resistant particles can be formed with either methylated or undermethylated SL RNA.

Trypanosoma brucei spliced-leader RNA methylations are required for trans splicing in vivo.

The Trypanosoma brucei spliced leader (SL) RNA donates its 5' leader sequence to all nuclear pre-mRNAs via trans RNA splicing. The SL RNA is a small-nuclear U RNA-like molecule which is present in the cell as part of a small ribonucleoprotein particle. However, unlike the trimethylguanosine-capped small nuclear U RNAs, the SL RNA has a highly modified 5' terminus containing an m7G cap and methylations on the first four transcribed nucleotides. Here, we show that incubation of procyclic-form T. brucei in the presence of the S-adenosylmethionine analog, sinefungin, leads to a rapid inhibition of SL RNA methylation. A concomitant inhibition of trans splicing and an accumulation of high-molecular-weight tubulin transcripts were also observed. The effects of sinefungin on SL RNA methylation and on trans splicing were correlated by labeling of cells incubated in the presence of the antibiotic. The results indicate that 5' modifications of the SL RNA are necessary for it to participate in trans splicing. SL RNA modification is not required for assembly of the core SL ribonucleoprotein, as these Cs2SO4-resistant particles can be formed with either methylated or undermethylated SL RNA.

Purification and biochemical characterization of the EcaI DNA methyltransferase.

The EcaI GGTNACC-specific DNA-adenine modification methyltransferase has been purified to apparent homogeneity. The active form of the DNA methyltransferase is a single polypeptide. The enzyme has a pH optimum at pH 8.0 and a temperature optimum at 25 degrees C. EcaI DNA methyltransferase transfers one methyl group to the adenine of the recognition site in a single binding event. The Km was 170 nM for DNA and 1.8 microM for the methyl donor S-adenosylmethionine. Methylated DNA is a competitive inhibitor with respect to DNA (Ki = 3.5 nM). The other product of the DNA-methylation reaction, S-adenosylhomocysteine was found to be a competitive inhibitor with respect to S-adenosylmethionine (Ki = 2.7 microM). The S-adenosylmethionine analog sinefungin was shown to be a very strong inhibitor (Ki = 3.5 nM) of the DNA methyltransferase reaction.

Purification and characterization of the Mβ RsrI DNA methyltransferase from Escherichia coli

The gene ( rsrIM) encoding the RsrI DNA methyltransferase (Mβ RsrI) from Rhodobacter sphaeroides was cloned and expressed in Escherichia coli. Under the control of a bacteriophage T7 promoter, 2% of the total protein in a crude extract was Mβ RsrI. This level of expression represents an approximately 50-fold increase over that present in the natural host. Chromatography using DNA cellulose and the S-adenosylmethionine analogue, sinefungin, was useful in purifying the enzyme to homogeneity. The purification yielded 100 times more enzyme than was obtained from the same quantity of R. sphaeroides cell paste. Mβ RsrI deposits one methyl group per productive DNA-binding event, as does its functional but sequence-nonhomologous analogue, Mβ EcoRI. Unlike Mβ EcoRI, the R. sphaeroides enzyme is a dimer at micromolar concentrations.

Irreversible inhibition of S-adenosylmethionine decarboxylase of trypanosoma bru cei bru cei by S-adenosylmethionine analogues

S-Adenosylmethionine analogues designed as active-site directed inhibitors were tested in vitro for their effects on S-adenosylmethionine decarboxylase (AdoMetDC) of Trypanosoma brucei brucei. These analogues contained a tertiary nitrogen atom in place of the sulfonium and had a side chain of variable length ending in a reactive group (hydrazino-, aminooxy-, hydrazido- or a methylnitrosourea). The hydrazino- derivatives were the most potent inhibitors with ic 50 values in the range of 40–100 nM. The most active compound ( ic 50 of 0.04 μM) was 5'-deoxy-5'-[(2-hydrazinoethyl)-methylamino] adenosine (MHZEA). Addition of MHZEA produced a time-dependent inactivation with an apparent K i, of 0.4 μM, and the enzyme half-life at a saturating concentration of MHZEA was 0.4 min. Increasing the length of the side chain or changing the methyl group attached to the nitrogen to an ethyl group reduced the potency. Replacement of the hydrazino moiety with an aminooxy group resulted in about a 30- to 35-fold decrease in inhibition potency. However, the relative order of activities of these aminooxy analogues was similar to that found in the hydrazino series with 5'-deoxy-5'-[(2-arninooxyethyl) methylamino]adenosine (MAOEA), which had an ic 50 of 1.3 μM, being the most active. The hydrazido analogs were even less effective with 5'-deoxy-5'-[(3-hydrazino-3-oxopropyl)-methylamino] adenosine, the best inhibitor, having an ic 50, value of 8.7 μM. The methylnitrosourea derivatives were inactive. The inactivation of trypanosomal AdoMetDC with MHZEA or MAOEA was irreversible and was greatly stimulated by putrescine, a known activator of the enzyme, indicating that the compounds bind to the active site and form a covalent bond with the enzyme. These inhibitors may have considerable potential as chemotherapeutic agents against trypanosomiasis and other protozoal infections and may also be useful in studying the role of AdoMetDC in the regulation of polyamine levels in these organisms.

Restricted rotation analogs of decarboxylated S-adenosylmethionine as inhibitors of polyamine biosynthesis.

The synthesis and preliminary biological evaluation of decarboxylated S-adenosylmethionine analogs 1 and 2 is described. In an L1210 cultured cell assay, these compounds, designed as conformationally restricted active site probes for spermidine and spermine synthase, exhibit LD 50 values of 0.1 and 1.0 mM, respectively. In addition, these compounds have significant effects on levels of cellular polyamines and S-adenosylmethionine.

Metabolism, cellular actions, and cytotoxicity of selenomethionine in cultured cells

Selenomethionine metabolism and the biochemical basis for its cytotoxicity were analyzed in cultured human and murine lymphoid cells. The metabolic pathways were also addressed, using purified mammalian enzymes and crude tissue extracts. Selenomethionine was found to be effectively metabolized to S-adenosylmethionine analog, and that analog was further metabolized in transmethylation reactions and in polyamine synthesis, similarly to the corresponding sulphur metabolites of methionine. Selenomethionine did not block these pathways, nor was there a specific block on the synthesis of DNA, RNA, or proteins when added to the culture medium. Selenomethionine showed cytotoxicity at above 40 microM levels. Yet, low selenomethionine levels (10 microM) could replace methionine and support cell growth in the absence of methionine. Selenomethionine toxicity took place concomitantly with changes in S-adenosylmethionine pools. D-form was less cytotoxic than L-form. Methionine concentration modified the cytotoxicity. Together, this indicates that selenomethionine uptake and enzymic metabolism are involved in the cytotoxicity in a yet unknown way.

Identification of a critical cysteine in EcoRI DNA methyltransferase by mass spectrometry.

EcoRI DNA methyltransferase (MTase) is rapidly inactivated by N-ethylmaleimide with concomitant incorporation of 2 mol of N-ethyl[2-3H]maleimide/mol of functional monomer. Preincubation of the enzyme with either S-adenosylmethionine or DNA reduces the rate of activity loss, whereas preincubation with DNA and the S-adenosylmethionine analog sinefungin completely protects the enzyme from inactivation. An endo proteinase Glu-C digest of N-ethyl[2-3H]maleimide-modified enzyme was prepared and separated by high pressure liquid chromatography. Modified and unmodified cysteine-containing peptides were located and identified by radioactivity, mass spectrometry, and tandem mass spectrometry. In the absence of any ligands, cysteines 25, 116, and 223 are modified by N-ethylmaleimide; in the presence of DNA and sinefungin, Cys-223 is essentially unmodified. Thus, N-ethylmaleimide modification of Cys-223 in EcoRI DNA MTase is responsible for the loss of enzyme activity. Cys-223 is preceded by Asn, and this (or Cys-Asn) occurs with high frequency in adenine and cytosine (N-4) DNA MTases. Direct involvement of cysteine in methyl transfer reactions to adenine N-6 and cytosine N-4 is supported by the similarity of the reactions catalyzed by adenine N-6 and cytosine N-4 DNA MTases, the frequent presence of Asn-flanking Cys, and the importance of Cys-223 to EcoRI MTase function.