High-performance liquid chromatographic separation of natural adenosyl-sulphur compounds

Investigation of the molecular origins of protein-arginine methyltransferase I (PRMT1) product specificity reveals a role for two conserved methionine residues

Protein-arginine methyltransferases aid in the regulation of many biological processes by methylating specific arginyl groups within targeted proteins. The varied nature of the response to methylation is due in part to the diverse product specificity displayed by the protein-arginine methyltransferases. In addition to site location within a protein, biological response is also determined by the degree (mono-/dimethylation) and type of arginine dimethylation (asymmetric/symmetric). Here, we have identified two strictly conserved methionine residues in the PRMT1 active site that are not only important for activity but also control substrate specificity. Mutation of Met-155 or Met-48 results in a loss in activity and a change in distribution of mono- and dimethylated products. The altered substrate specificity of M155A and M48L mutants is also evidenced by automethylation. Investigation into the mechanistic basis of altered substrate recognition led us to consider each methyl transfer step separately. Single turnover experiments reveal that the rate of transfer of the second methyl group is much slower than transfer of the first methyl group in M48L, especially for arginine residues located in the center of the peptide substrate where turnover of the monomethylated species is negligible. Thus, altered product specificity in M48L originates from the differential effect of the mutation on the two rates. Characterization of the two active-site methionines provides the first insight into how the PRMT1 active site is engineered to control product specificity.

Reversed-phase high-performance liquid chromatography procedure for the simultaneous determination of S-adenosyl-L-methionine and S-adenosyl-L-homocysteine in mouse liver and the effect of methionine on their concentrations

An improved reversed-phase high-performance liquid chromatography (HPLC) procedure with ultraviolet detection is described for the simultaneous determination of S-adenosyl-L-methionine (SAM) and S-adenosyl-L-homocysteine (SAH) in mouse tissue. The method provides rapid resolution of both compounds in a 25-microl perchloric acid extract of the tissue. The limits of detection in 25-microl injection volumes were 22 and 20 pmol for SAM and SAH, respectively. The limits of quantitation in 25-microl injection volumes were 55 and 50 pmol for SAM and SAH, respectively, with recovery consistently >98%. The assay was validated over linear ranges of 55-11000 pmol for SAM and 50-10000 pmol for SAH. The intra-day precision and accuracy were < or =6.4% relative standard deviation (RSD) and 99.9-100.0% for SAH and < or =6.7% RSD and 100.0-100.1% for SAM. The inter-day precision and accuracy were < or =5.9% RSD and 99.9-100.6% for SAH and < or =7.0% RSD and 99.5-100.1% for SAM. Compared to earlier procedures, the HPLC method demonstrated significantly better separation, detection limit and linear range for SAM and SAH determination. The assay demonstrated applicability to monitoring in mice the time-course of the effect of methionine on SAM and SAH levels in the liver. Administering methionine to mice increased by 10-fold the liver concentration of SAM and SAH within 2 h, which then rapidly decreased to the control levels by 8 h. This indicated that methionine was promptly converted to SAM and then rapidly catabolized into SAH. Thus, the metabolism of methionine to SAM should be considered in the supplementation of methionine to maintain SAM levels in the body.

Reconciling structure and function in HhaI DNA cytosine-C-5 methyltransferase

Pre-steady state partitioning analysis of the HhaI DNA methyltransferase directly demonstrates the catalytic competence of the enzyme.DNA complex and the lack of catalytic competence of the enzyme.S-adenosyl-L-methionine (AdoMet) complex. The enzyme.AdoMet complex does form, albeit with a 50-fold decrease in affinity compared with the ternary enzyme.AdoMet.DNA complex. These findings reconcile the distinct binding orientations previously observed within the binary enzyme.AdoMet and ternary enzyme. S-adenosyl-L-homocysteine.DNA crystal structures. The affinity of the enzyme for DNA is increased 900-fold in the presence of its cofactor, and the preference for hemimethylated DNA is increased to 12-fold over unmethylated DNA. We suggest that this preference is partially due to the energetic cost of retaining a cavity in place of the 5-methyl moiety in the ternary complex with the unmethylated DNA, as revealed by the corresponding crystal structures. The hemi- and unmethylated substrates alter the fates and lifetimes of discrete enzyme.substrate intermediates during the catalytic cycle. Hemimethylated substrates partition toward product formation versus dissociation significantly more than unmethylated substrates. The mammalian DNA cytosine-C-5 methyltransferase Dnmt1 shows an even more pronounced partitioning toward product formation.

Two-dimensional high-performance liquid chromatographic method for assaying S-adenosyl-L-methionine and its related metabolites in tissues

S-Adenosyl-L-methionine (SAM) is a methyl-donor compound which is actively involved in a variety of biochemical reactions. An assay has been developed permitting the quantitative measurement of SAM and its related metabolites (S-adenosylhomocysteine, decarboxylated SAM, methylthioadenosine, adenosine and adenine) in liver and cell cultures. As gradient reversed-phase chromatographic or cation-exchange chromatographic methods often resulted in overlapping peaks, a two-dimensional high-performance liquid chromatographic (HPLC) procedure was developed involving gradient reversed-phase chromatographic separation followed by ion-exchange chromatography. After precipitating large molecules in the sample by perchloric acid, gel permeation was carried out on a Sephadex G 25 column to separate small water-soluble metabolites from proteins and membrane fragments. The freeze-dried sample was injected onto an ODS column and a 0-10% acetonitrile gradient in 10 mM ammonium formate buffer (pH 2.9) (20 min, linear) was applied. The relevant fractions were collected and injected onto a cation-exchange column (Partisil SCX, 10 microns, 250 mm x 4.6 mm I.D.). Elution and quantification were carried out using ammonium formate buffers of various concentration (15-400 mM), pH 2.9. The detector response (254 nm) as a function of concentration was linear over the concentration range 30-500 pmol. The detection limits of the compounds after the two-dimensional chromatographic procedure ranged from 10 to 60 pmol and the recovery was higher than 70%. The reproducibility of the results obtained from given samples was within 9-22% for rat liver and 6-24% for mast cells.

A comparative study of the reversed-phase HPLC retention behaviour of S-adenosyl-L-methionine and its related metabolites on Hypersil ODS and Supelcosil LC-ABZ stationary phases

S-Adenosyl-L-methionine (SAM) and its metabolites S-adenosyl-L-homocysteine (SAH) and methyl-thioadenosine (MTA) are endogenous compounds that are heavily involved in a variety of biochemical processes, and have therefore been the target for several assays in body fluids and tissues. Reversed-phase chromatographic behaviour of SAM and its metabolites has been studied by using Supelcosil LC-ABZ column, specially designed for analysis of acidic, basic, zwitterionic and neutral compounds, and on a Hypersil ODS column as a function of mobile phase pH. The retentions of the compounds, expressed by the capacity ratio (k'), are measured on both column with mobile phases comprised of 10% acetonitrile and 10 mM ammonium formate buffer with pH values ranging from 2 to 9. Higher selectivity is observed on Supelcosil LC-ABZ within pH range 4-6. Different retention properties are observed at very low pH and seemed as if the Supelcosil LC-ABZ column reduced the effect of the mobile phase pH by about 1 pH unit. Whilst the Supelcosil column can be recommended for the routine analysis of SAM and its related metabolites in biological fluids by using mobile phase pH 5, the Hypersil ODS column may be suggested for use with mobile phase pH values of 3-4.

Assay of phenylethanolamine N-methyltransferase activity using high-performance liquid chromatography with ultraviolet absorbance detection

A simple and rapid method for measuring phenylethanolamine N-methyltransferase (PNMT) activity by high-performance liquid chromatography (HPLC) with ultraviolet (UV) detection is described. This assay requires a partially purified PNMT preparation derived from bovine adrenals, with noradrenaline and S-adenosyl-L-methionine (SAM) as co-substrates. After incubation, the reaction is stopped by addition of acid and the reaction mixture is analysed directly by HPLC. The enzymatically formed S-adenosyl-L-homocysteine (SAH) is detected at 258 nm and determined. Under optimum conditions, the stability of SAH allowed automation of the HPLC detection. This assay was validated by the determination of the kinetic properties of PNMT. Km values for noradrenaline and SAM defined in this assay (16 and 5.7 microM, respectively) are consistent with previously published values. This assay is simple enough to be used for large series of measurements of PNMT activity testing new methyl acceptors, potential inhibitors or PNMT activity in adrenal medulla.

Quantification of erythrocyte S-adenosyl-L-methionine levels and its application in enzyme studies

A highly selective high-performance liquid chromatographic method for the quantification of human erythrocyte S-adenosyl-L-methionine levels is described. A strong cation-exchange sorbent with propylsulphonic acid functional groups was used to extract S-adenosyl-L-methionine and S-adenosylethionine (internal standard) from erythrocytes. Quantification of erythrocyte S-adenosyl-L-methionine levels was achieved by using reversed-phase high-performance liquid chromatography and ultraviolet detection at 254 nm. This method was adapted to measure methionine-adenosyltransferase activity in erythrocytes, which enables us to study the possible role of altered methylation in different diseases.

EcoRI DNA methyltransferase-DNA interactions

We present a novel strategy with synthetic hemimethylated DNA substrates containing uracil for thymine and inosine for guanosine replacements and EcoRI DNA methyltransferase to characterize the importance of major groove hydrophobic groups to the sequence-specific modification of DNA. The bacterial Mtase uses S-adenosyl-L-methionine to methylate the double-stranded DNA site 5'GAATTC3' at the N6 position of the central adenosine of each strand. Uracil substitution in either strand at the outer thymine (5'GAATUC3') causes 2.2- and 1.7-fold improvements in specificity (kcat/KmDNA). The fact that the specificity constant for the substrate containing uracil in both strands is identical to the value expected for noninteracting substitutions suggests that no significant methyltransferase-DNA interactions are altered beyond the site of either substitution. Similar analysis of the internal thymine (5'GAAUTC3') also shows these methyl groups to make a negative contribution to specificity, although the observed nonadditivity with the doubly modified substrate clearly shows methyltransferase-DNA interactions beyond the site of substitution to be affected in this case. To further probe the effect of analogue incorporation on methyltransferase-DNA interactions beyond the site of substitution, the relatively "silent" and additive uracil changes (5'GAATUC3') were combined with inosine for guanosine substitutions (e.g., 5'IAATTC3') known to have significant negative effects on specificity. In contrast to the additivity observed with the outer thymines, these studies show significant changes in methyltransferase-DNA interactions caused by the removal of the thymine methyls. Our results implicate a complex and flexible methyltransferase-DNA interface in which subtle structural changes in the substrate are transmitted over the entire canonical site.(ABSTRACT TRUNCATED AT 250 WORDS)

Non-additivity of sequence-specific enzyme-DNA interactions in the EcoRI DNA methyltransferase

We describe a novel strategy to characterize protein-DNA interactions involving monomeric enzymes such as DNA methyltransferases (Mtases). This strategy is applied to our investigation of the EcoRI DNA Mtase, which binds its double stranded recognition site 5'-G-AATTC-3' and methylates the central adenosine of each strand using S-adenosyl-L-methionine as the methyl donor. We show that prior methylation of adenosine in either strand does not perturb catalysis. In contrast, substrates substituted with deoxyinosine at either guanosine position (T-BMI5 and TI5-BM) show the minor groove residing N2 amino group of both guanosines contribute to DNA recognition since specificity constants for the modified substrates are reduced 13 and 39 fold. Similar analysis of a substrate containing deoxyinosine at both positions (TI5-BMI5) clearly shows that some communication occurs between the sites. To determine the extent to which structural changes in the DNA alone contribute to this lack of additivity, we performed DNA melting analysis of the singly and doubly substituted substrates, and also found non-additivity. Although our functional and structural analyses suggest that deoxyinosine incorporation causes long range conformational effects, the similarity of KmAdoMet for all substrates suggests that no large-scale structural changes occur in the Mtase-DNA-AdoMet complex. Our results support the following conclusions: 1) The non-additivity shown in this system contrasts with the widespread demonstration of additivity involving repressors [Lehming et al., 1990; Takeda et al., 1989; Ebright et al., 1987], suggesting that sequence discrimination by enzymes may involve more complex mechanisms. Further, this non-additivity precludes quantitative assignment of individual interactions and we suggest that future analyses of this and related enzyme systems with base analogs include detailed information about the long range structural consequences of individual substitutions. 2) Although TI5-BM and T-BMI5 are shown to be radically different by thermodynamic analysis, the similar specificity constants with the Mtase suggest that the underlying structural differences (e.g., altered helical parameters of the DNA) are not critical for sequence-recognition. 3) The significance of minor groove Mtase-DNA interactions to specificity is confirmed.

Kinetic mechanism of the EcoRI DNA methyltransferase

We present a kinetic analysis of the EcoRI DNA N6-adenosine methyltransferase (Mtase). The enzyme catalyzes the S-adenosylmethionine (AdoMet)-dependent methylation of a short, synthetic 14 base pair DNA substrate and plasmid pBR322 DNA substrate with kcat/Km values of 0.51 X 10(8) and 4.1 X 10(8) s-1 M-1, respectively. The Mtase is thus one of the most efficient biocatalysts known. Our data are consistent with an ordered bi-bi steady-state mechanism in which AdoMet binds first, followed by DNA addition. One of the reaction products, S-adenosylhomocysteine (AdoHcy), is an uncompetitive inhibitor with respect to DNA and a competitive inhibitor with respect to AdoMet. Thus, initial DNA binding followed by AdoHcy binding leads to formation of a ternary dead-end complex (Mtase-DNA-AdoHcy). We suggest that the product inhibition patterns and apparent order of substrate binding can be reconciled by a mechanism in which the Mtase binds AdoMet and noncanonical DNA randomly but that recognition of the canonical site requires AdoMet to be bound. Pre-steady-state and isotope partition analyses starting with the binary Mtase-AdoMet complex confirm its catalytic competence. Moreover, the methyl transfer step is at least 10 times faster than catalytic turnover.

Acid-soluble purine and pyrimidine derivatives of growing, encysting and encystment-inhibited amoebae

The intracellular acid-soluble purine and pyrimidine derivatives of myxamoebae-swarm cells of Physarum flavicomum were investigated during growth, microcyst formation, and during adenine-inhibition of encystment, using high performance liquid chromatography (HPLC). We also studied the incorporation of exogenous radioactive adenine into the acid soluble purine derivatives and S-adenosyl-sulphur compounds separated by HPLC. The most abundant ribonucleoside monophosphate was AMP in the growing and 15 h encysting cells (NC), while it was UMP in the 15 h adenine-inhibited cells (AIC). ADP was the nucleoside diphosphate present in the greatest quantity in the growing and NC cells but it was CDP in the AIC. The nucleoside triphosphate in highest concentration was ATP, UTP, and GTP in growing, NC, and AIC, respectively. Guanosine was the most abundant nucleoside in all cells. The nucleobase occurring in greatest concentration was cytosine, cytosine and guanine, and adenine in the growing, NC, and AIC, respectively. The AMP content in the 15 h AIC was 2.1-fold higher than that of adenosine. The 15 h NC had the lowest adenylate energy charge, a value of 0.54 +/- 0.02, while the values for growing cells and the AIC were 0.62 +/- 0.02 and 0.76 +/- 0.01, respectively. [14C]-Adenine labelling studies (15 h) revealed the occurrence of purine nucleotide interconversion, as the label was detected not only in adenosine, AMP, ADP, ATP, but also in guanine, guanosine, GMP, GDP, GTP, as well as, in inosine monophosphate and xanthosine monophosphate. The percentage incorporation of the radiolabelled adenine into AMP was higher than into adenosine. An increased intracellular level of guanine nucleotides is associated with the inhibition of encystment. The extracellular adenine, rather than internal adenine sources, appears to be the primary precursor of nucleotide for S-adenosylmethionine synthesis during adenine-inhibition of encystment.

The genetics of polyamine synthesis in Neurospora crassa

New mutations of the polyamine pathway of Neurospora crassa fell into three categories. The majority affected ornithine decarboxylase and lay at the previously defined spe-1 locus. One mutation, JP100, defining the new spe-2 locus, eliminated S-adenosyl-methionine decarboxylase and led to putrescine accumulation. Revertants of this mutation suggested that the locus encodes the enzyme. Two other mutations, LV105 and JP120, defined a third locus, spe-3. Strains with these mutations also accumulated putrescine and were presumed to lack spermidine synthase activity, which catalyzes the formation of spermidine from putrescine and decarboxylated S-adenosylmethionine. The three spe loci lay within about 20 map units of one another on the right arm of Linkage Group V in the order: centromere-spe-2-spe-1-spe-3. The requirement for spermidine for growth was much less in spe-2 and spe-3 mutants than in spe-1 mutants, which do not accumulate putrescine. This suggested that putrescine fulfills many, but not all, of the functions of spermidine, or that high levels of putrescine render spermidine more effective in its essential roles.

Biosynthesis and metabolism of 9-[5'-deoxy-5'-(methylthio)-beta-D-xylofuranosyl]adenine, a novel natural analogue of methylthioadenosine

The biosynthesis of 9-[5'-deoxy-5'-(methylthio)-beta-D-xylofuranosyl]adenine (xylosyl-MTA), a naturally occurring analogue of 5'-deoxy-5'-methylthioadenosine (MTA) recently characterized, was studied in the nudibranch mollusc Doris verrucosa. Experiments performed in vivo with putative labelled precursors such as [8-14C]adenine, [Me-14C]methionine and [Me-14C]MTA indicate that xylosyl-MTA originates from MTA. Experiments with MTA double-labelled at critical positions are consistent with a 3'-isomerization of the nucleoside through the formation of a 3'-oxo intermediate. In addition, experiments with the newly synthesized [3'-3H]xylosyl-MTA are indicative for a very low turnover rate of this molecule, which therefore accumulates in the mollusc.

Comparison of polyamine and S-adenosylmethionine contents of growing and encysted Acanthamoeba isolates

We have used High Performance Liquid Chromatography to determine metabolite characteristics of three recent isolates of Acanthamoeba which exhibit cultural characteristics consistent with those of established potential pathogens. Growing amoebae and dormant cysts of these isolates were explored in regard to their qualitative and quantitative intracellular levels of polyamine and S-adenosylmethionine metabolites. The polyamine found in the greatest concentration in the growing cells was 1,3-diaminopropane (DAP), followed by spermidine (SPD). A low level of putrescine was also found in the growing cells. These polyamines significantly decreased in concentration as the amoebae differentiated to cysts. N8-acetylspermidine and acetylspermine were found in both developmental stages while acetylcadaverine was found only in growing amoebae and N1-acetylspermidine only in cysts. Acetylputrescine was present in both stages of two isolates but only in the growing amoebae of the third isolate. Spermine was not detected in any of the isolates. S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH) were present in growing amoebae but SAM was undetectable or barely detectable in cysts. SAH also decreased in concentration during encystation of two of the isolates to a level comparable to that of the other isolate. The developmental transition from growing amoebae to dormant cysts is characterized metabolically by a threshold adjustment in concentration of SAM, SAH and of the polyamines (esp., DAP and SPD).

A method for the measurement of S-adenosylmethionine in small volume samples of cerebrospinal fluid or brain using high-performance liquid chromatography-electrochemistry

A method for the measurement of S-adenosylmethionine in cerebrospinal fluid and brain using high-performance liquid chromatography with electrochemical detection is described. The method is based upon the catechol O-methyltransferase-catalyzed methylation of dihydroxybenzylamine to its 3- and 4-methoxy derivatives and measurement of the latter. Detection limits are 0.1 pmol for 3- and 4-methoxyhydroxybenzylamine, recovery is 100%, and variability is less than 8%. HPLC measurement of 3- and 4-methoxyhydroxybenzylamine is linear up to at least 100 pmol injected. The use of a nonbiological catechol in the assay minimizes interference from endogenous and exogenous catechols and the wide limits of linearity allows the assay of small volumes of cerebrospinal fluid or brain extract without further concentration or dilution.

High-performance liquid chromatographic determination of a naturally occurring xylosyl thioether, 9-[5'-deoxy-5'(methylthio)-beta-D-xylofuranosyl]adenine

A high-performance liquid chromatographic (HPLC) method for the separation and purification of 9-[5'-deoxy-5'(methylthio)-beta-D-xylofuranosyl]adenine (xylosyl-MTA), a naturally occurring analogue of 5'-deoxy-5'-methylthioadenosine (MTA), has been developed. The compound was purified from perchloric extracts of biological samples by reversed-phase HPLC on an Ultrasil ODS RP-18 column. Isocratic elution resulted in a complete separation between the xylosyl thioether and MTA. The determination of xylosyl-MTA in the nudibranch mollusc Doris verrucosa was also performed. Its concentration ranged from 330 to 380 nmol/g, considerably exceeding the cellular levels of MTA.

Modulation of polyamine-biosynthetic activity by S-adenosylmethionine depletion

The methionine-analogue inhibitor of S-adenosylmethionine (AdoMet) synthetase, L-2-amino-4-methoxy-cis-but-3-enoic acid (L-cisAMB), was used to study the early effects of AdoMet depletion on polyamine biosynthesis. In the presence of decreased methionine (30 microM) in the medium, treatment of cultured L1210 cells with 1 mM-L-cisAMB resulted in a near-total (95%) depletion of cellular AdoMet pools by 4 h. This was accompanied by a 3-fold increase in ornithine decarboxylase (ODC) activity, a 2.5-fold increase in AdoMet decarboxylase (AdoMetDC) activity and a 20% decrease in spermidine and spermine pools. The increase in enzyme activities seemed to be partially due to prolongation of enzyme activity half-life, since that of ODC was extended from 30 to 50 min and that of AdoMetDC from 65 to 310 min. By temporal sequence characterization (0-4 h), the onset of elevations of enzyme activity (0.5-1 h) seemed to be causally related to an earlier (0-0.5 h) decline in AdoMet pools, as opposed to the 20% decrease in spermidine and spermine pools, which occurred much later (2-4 h); the latter are known to regulate decarboxylase activities negatively. Drug-induced elevations in ODC and, to a lesser extent, AdoMetDC activities were reversed by later treatment with exogenous AdoMet. However, because the latter also increased spermine pools (which could not be prevented with various enzyme inhibitors), the reversal of elevations in enzyme activities could not be directly linked to AdoMet. Although not definitive, the data raise the interesting possibility that, in addition to being negatively regulated by polyamines, ODC and AdoMetDC activities may also be subject to negative control by cellular AdoMet (or an AdoMet metabolite). The net effect of either or both of these influences would be to conserve polyamine-biosynthetic activity in the face of declining AdoMet supplies.

Relative effects of S-adenosylmethionine depletion on nucleic acid methylation and polyamine biosynthesis

Treatment of cultured L1210 cells with 1 mM-L-2-amino-4-methoxy-cis-but-3-enoic acid (L-cisAMB), a methionine-analogue inhibitor of S-adenosylmethionine (AdoMet) synthetase (EC 2.5.1.6), produced a rapid and near-total depletion of AdoMet by 4 h. After this, the pools recovered to 60% of control by 48 h, apparently because of an increase in AdoMet synthetase activity. Both AdoMet depletion and the accompanying increase in synthetase activity were substantially enhanced by lowering methionine concentrations in the media from 100 microM to 30 microM, the minimal concentration that supports cell growth at control values. During a 4 h incubation in media containing 30 microM-methionine, 1-5 mM-L-cisAMB depleted cellular AdoMet to undetectable values, and inhibited nucleic acid methylation by 44-72% and RNA methylation by 60-87%. Under these same treatment conditions, putrescine pools increased by about 3-fold, whereas spermidine pools decreased by only 20% and spermine pools remained the same. Pool changes were accompanied by a 2-4-fold increase in ornithine decarboxylase activities and AdoMet activities. Thus the rapid depletion of AdoMet pools by L-cisAMB results immediately in a decrease in methyl-transfer reactions involving nucleic acids, whereas, by contrast, biosynthesis of higher polyamines appears to be minimally affected, owing to compensatory increases in key enzyme activities.

S-adenosylmethionine and transmethylation reactions in trichomonads

S-Adenosylmethionine (SAM) levels in trichomonads, a range of trypanosomatids and mouse liver were measured using HPLC techniques. The concentrations were found to be similar in each with the exception of Herpetomonas muscarum ingenoplastis, which contained approximately ten-fold more. Living trichomonads were found to incorporate exogenous L-methionine into intracellular SAM and its methyl carbon was also detected in lipids and nucleic acids, presumably through its involvement in transmethylation reactions. Norleucine and cycloleucine inhibited L-methionine uptake and incorporation into living Trichomonas vaginalis. Both the rates of incorporation of exogenous L-methionine into intracellular SAM and its involvement in transmethylation reactions were greater for Trichomonas vaginalis than for Tritrichomonas foetus. The results suggest that Trichomonas vaginalis and other trichomonads contain enzymes equivalent to SAM synthetase (EC 2.5.1.6) and SAM-dependent methyltransferases (EC 2.1.1).

A rapid nonchromatographic assay for aminopropyltransferases

Aminopropyltransferases are key enzymes in the biosynthesis of the polyamines spermidine and spermine. A procedure is described for assaying these enzymes by differential charcoal adsorption of 14C-labeled decarboxylated adenosylmethionine substrate from the labeled polyamine product. This assay is linear with time and enzyme concentration, and is suitable for use with a variety of amine acceptors. This procedure has the advantage, over those previously used, that it is extremely rapid yet very sensitive.

Antitumour imidazotetrazines-X. Effect of 8-carbamoyl-3-methylimidazo[5,1-d]-1,2,3,5-tetrazin-4-(3H)-one (CCRG 81045; M & B 39831; NSC 362856) on DNA methylation during induction of haemoglobin synthesis in human leukaemia cell line K562

Treatment of K562 human erythroleukaemia cells with 8-carbamoyl-3-methylimidazo[5,1-d]-1,2,3,5-tetrazin-4-(3H)-one (CCRG 81045) caused a concentration-dependent increase in the number of cells producing haemoglobin after 3 days of treatment. The ethyl analogue (CCRG 82019) was inactive in the induction of erythroid characteristics. The concentration of 5-methyl-cytosine in the DNA of CCRG 81045 treated cells decreased 3 days after treatment, and was directly proportional to the number of benzidine-positive cells in the cultures, suggesting a direct correlation between hypomethylation of DNA and the induction of haemoglobin synthesis. Although the mechanism of this drug-induced hypomethylation of DNA is not known, the methyl analogue (CCRG 81045) also appeared to reduce template activity of isolated DNA to a greater extent than the ethyl analogue, suggesting that the extent or position of alkylation of DNA bases be important in inhibiting DNA-recognition enzymes.

Growth inhibition by methionine analog inhibitors of S-adenosylmethionine biosynthesis in the absence of polyamine depletion

Four methionine analog inhibitors of methionine adenosyltransferase, the enzyme which catalyzes S-adenosylmethionine biosynthesis, were tested in cultured L1210 cells for their effects on cell growth, leucine incorporation, S-adenosylmethionine (AdoMet) formation and polyamine biosynthesis. The IC50 values were as follows: selenomethionine, 0.13 mM; L-2-amino-4-methoxy-cis-but-3-enoic acid (L-cis-AMB), 0.4 mM; cycloleucine, 5 mM and 2-aminobicyclo[2.1.1]hexane-2-carboxylic acid, 5 mM. At IC50 levels, the analogs significantly reduced AdoMet pools by approximately 50% while not similarly affecting leucine incorporation or polyamine biosynthesis. In combination with inhibitors of polyamine biosynthesis, growth inhibition was greatly increased with methylglyoxal bis(guanylhydrazone), an inhibitor of AdoMet decarboxylase, but only slightly increased with alpha-difluoromethylornithine, an inhibitor of ornithine decarboxylase. Overall, the data indicate that the methionine analogs, and particularly L-cis-AMB, seem to inhibit cell growth by interference with AdoMet biosynthesis. Since polyamine biosynthesis is not affected, the antiproliferative effect may be mediated through perturbations of certain transmethylation reactions.

A rapid high-performance liquid chromatographic procedure for the simultaneous determination of methionine, ethionine, S-adenosylmethionine, S-adenosylethionine, and the natural polyamines in rat tissues

A method for the analysis of S-adenosyl-L-methionine (SAM) and S-adenosyl-L-ethionine (SAE) and their major metabolites by high-performance liquid chromatography is described. The procedure allows the simultaneous analysis of the natural polyamines, putrescine, spermidine, and spermine, and some of the major amino acids, methionine, tyrosine, and tryptophan. The uv absorbance at 254 nm is used for the determination of the SAM and SAE analogs, whereas the polyamines and amino acids are analyzed by fluorescence detection after postcolumn derivatization with o-phthalaldehyde. The method allows SAM and polyamine determinations by direct injection of the tissue extracts without prepurification. The procedure is applied to study the effects of DL-ethionine treatment on the SAM, SAE, methionine, and polyamine levels in various tissues of rats.

High-performance liquid chromatographic analysis of S-adenosylmethionine and its metabolites in rat tissues: interrelationship with changes in biogenic catechol levels following treatment with L-dopa

A method is described for the simultaneous analysis of S-adenosylmethionine (SAM) and its metabolites, S-adenosylhomocysteine (SAH) and decarboxylated S-adenosylmethionine along with the natural polyamines, putrescine, spermidine and spermine. The separation is obtained by a reversed-phase ion-pair liquid chromatographic procedure with gradient elution followed by dual detection. The UV absorbance at 254 nm is used for the analysis of SAM and of the SAM metabolites, whereas the polyamines and some major amino acids, e.g., methionine, tyrosine and tryptophan, are analyzed by fluorescence detection after UV-cell derivatization with o-phthalaldehyde. A separate ion-pair reversed-phase high-performance liquid chromatographic (HPLC) procedure using isocratic elution and electrochemical detection is employed to analyse in the same tissue extracts the catechols and 5-hydroxyindoles, 3,4-dihydroxyphenylalanine (DOPA), dopamine, norepinephrine, 3,4-dihydroxyphenylacetic acid, homovanillic acid, 4-hydroxy-3- methoxyphenylalanine , tryptophan, 5-hydroxytryptophan, serotonin and 5- hydroxyindolacetic acid. The sample preparation for the two HPLC procedures requires only homogenization of the tissues in perchloric acid and centrifugation before injection onto the column. The two chromatographic procedures have been applied to study the interrelationship, in various tissues of rats, between the SAM and SAH levels and the biogenic catechols after different treatments with L-DOPA alone or in combination with alpha- monofluoromethyl -DOPA, a potent enzyme-activated irreversible inhibitor of aromatic L-amino acid decarboxylase.

Utilization of preformed and endogenously synthesized methionine by cells in tissue culture

Some malignant and transformed cell lines are unable to proliferate in vitro in a L-methionine-depleted medium supplemented with L-homocysteine. To investigate the utilization of preformed and endogenously synthesized methionine 4 cell lines have been chosen with a range of abilities to proliferate under such nutritional conditions. The order of the ability of these cell lines to proliferate in an L-methionine-depleted medium containing 0.1 mM L-homocysteine parallels the minimal concentration of L-methionine required for optimal growth; L-methionine auxotrophs having a greater minimal requirement. In the presence of 0.1 mM L-homocysteine all of the cell lines synthesize macromolecules from [5-14C]methyltetrahydrofolic acid during a 24 h period, and the cell line with the highest methionine requirement shows the most extensive incorporation of radiolabel into DNA and RNA, both in depleted medium and in medium containing 6.7 microM L-methionine. Double-label experiments using [5-14C]methyltetrahydrofolic acid and L-(methyl-3H) methionine show preferential incorporation of preformed over endogenously synthesized methionine by methionine auxotrophs. There is no alteration in the intracellular level of S-adenosyl-L-homocysteine (SAH) or SAH hydrolase activity in cells incubated for 24 h in methionine-depleted medium supplemented with 0.1 mM L-homocysteine. These results suggest that certain cell lines are unable to effectively use endogenously synthesized methionine.

Potentiation of the growth inhibitory effects of adenosine 3',5'-monophosphate analogues by homocysteine

The growth inhibitory effects of N6-monobutyryl adenosine 3',5'-monophosphate (mbcAMP) and N6,O2'-dibutyryl adenosine 3',5' monophosphate (dbcAMP) towards Walker carcinoma in vitro are significantly potentiated by the addition of L-homocysteine to the culture medium. This effect is not seen with L-cysteine or when exogenous cAMP or prostaglandin E2(PGE2) replace the butyrylated cyclic nucleotide. Combinations of mbcAMP or dbcAMP and L-homocysteine significantly inhibit nucleic acid methylations. Both the butyrylated cyclic nucleotides cause an elevation of the intracellular level of S-adenosyl-L-homocysteine (SAH), a potent inhibitor of S-adenosyl-L-methionine (SAM) dependent methyltransferases, and this is significantly enhanced in combination with L-homocysteine. The increase in SAH level produced by such combinations is proportional to the inhibition of methyl group incorporation into 5-methyl cytosine and 7-methyl guanine. These results suggest that L-homocysteine potentiates accumulation of SAH in the presence of mbcAMP and dbcAMP and that the resultant inhibition of methylation accounts for the enhanced growth inhibition.

Effect of inhibition of polyamine synthesis on the content of decarboxylated S-adenosylmethionine

1. The content of decarboxylated S-adenosylmethionine (AdoMet) in transformed mouse fibroblasts (SV-3T3 cells) was increased 500-fold to about 0.4fmol/cell when ornithine decarboxylase was inhibited by alpha-difluoromethylornithine. This increase was due to the absence of putrescine and spermidine, which serve as substrates for aminopropyltransferases with decarboxylated AdoMet as an aminopropyl donor, and to the enhanced activity of AdoMet decarboxylase brought about by depletion of spermidine. The increase in decarboxylated AdoMet content was abolished by addition of putrescine, but not by 1,3-diaminopropane. 2. 5'-Methylthiotubercidin also increased decarboxylated AdoMet content, presumably by direct inhibition of aminopropyl-transferase activities, but the increase in its content and the decline in spermidine content were much less than those produced by alpha-difluoromethylornithine. 3. Decarboxylated AdoMet content of regenerating rat liver was measured in rats treated with inhibitors of ornithine decarboxylase. The content was increased by 60% 32h after partial hepatectomy in control rats, by 90% when alpha-difluoromethylornithine was given to the partially hepatectomized rats, and by 330% when 1,3-diaminopropane was used to inhibit putrescine and spermidine synthesis. After 48h of exposure to 1,3-diaminopropane, which completely prevented the increase in spermidine after partial hepatectomy, there was a 5-fold rise in hepatic decarboxylated AdoMet concentration. These increases were prevented by treatment with putrescine or with methylglyoxal bis(guanylhydrazone), an inhibitor of AdoMet decarboxylase. 4. These results show that changes in AdoMet metabolism result from the administration of specific inhibitors of polyamine synthesis. The possible consequences of the accumulation of decarboxylated AdoMet, which could, for example, interfere with normal cellular methylation or lead to depletion of cellular adenine nucleotides, should be considered in the interpretation of results obtained with such inhibitors.

Reversed-phase ion-pair liquid chromatographic procedure for the simultaneous analysis of S-adenosylmethionine, its metabolites and the natural polyamines

A method using reversed-phase ion-pair liquid chromatography with dual detection was developed for the simultaneous determination of the S-adenosylmethionine (SAM) analogues and the natural polyamines. The separation is obtained with a gradient elution and by adjusting the concentration of octanesulfonic acid used as ion-pairing agent, the ionic strength of the eluent, the pH and the acetonitrile content of the eluents. The SAM analogues are analyzed by UV detection at 254 nm and the polyamines by fluorescence detection after post-column derivatization with o-phthalaldehyde. The method allows the determination of the SAM analogues and the polyamines in one single run by direct injection of tissue extracts. The procedure is applied to the study in rats and in hepatoma tissue culture cells of the biochemical effects of alpha difluoromethylornithine, a potent enzyme-activated irreversible inhibitor of ornithine decarboxylase.