S-adenosylmethionine decarboxylase: a brief review

Cotton Leaf Curl Multan virus C4 protein suppresses both transcriptional and post-transcriptional gene silencing by interacting with SAM synthetase

Gene silencing is a natural antiviral defense mechanism in plants. For effective infection, plant viruses encode viral silencing suppressors to counter this plant antiviral response. The geminivirus-encoded C4 protein has been identified as a gene silencing suppressor, but the underlying mechanism of action has not been characterized. Here, we report that Cotton Leaf Curl Multan virus (CLCuMuV) C4 protein interacts with S-adenosyl methionine synthetase (SAMS), a core enzyme in the methyl cycle, and inhibits SAMS enzymatic activity. By contrast, an R13A mutation in C4 abolished its capacity to interact with SAMS and to suppress SAMS enzymatic activity. Overexpression of wild-type C4, but not mutant C4R13A, suppresses both transcriptional gene silencing (TGS) and post-transcriptional gene silencing (PTGS). Plants infected with CLCuMuV carrying C4R13A show decreased levels of symptoms and viral DNA accumulation associated with enhanced viral DNA methylation. Furthermore, silencing of NbSAMS2 reduces both TGS and PTGS, but enhanced plant susceptibility to two geminiviruses CLCuMuV and Tomato yellow leaf curl China virus. These data suggest that CLCuMuV C4 suppresses both TGS and PTGS by inhibiting SAMS activity to enhance CLCuMuV infection in plants.

The Uptake and Metabolism of Amino Acids, and Their Unique Role in the Biology of Pathogenic Trypanosomatids

Trypanosoma brucei, as well as Trypanosoma cruzi and more than 20 species of the genus Leishmania, form a group of flagellated protists that threaten human health. These organisms are transmitted by insects that, together with mammals, are their natural hosts. This implies that during their life cycles each of them faces environments with different physical, chemical, biochemical, and biological characteristics. In this work we review how amino acids are obtained from such environments, how they are metabolized, and how they and some of their intermediate metabolites are used as a survival toolbox to cope with the different conditions in which these parasites should establish the infections in the insects and mammalian hosts.

Hydroxylamine derivatives for regulation of spermine and spermidine metabolism

The biogenic polyamines spermine, spermidine, and their precursor putrescine are present in micro-to-millimolar concentrations in all cell types and are vitally important for their normal growth. High intracellular content of spermine and spermidine determines the multiplicity of the cellular functions of the polyamines. Many of these functions are not well characterized at the molecular level, ensuring the ongoing development of this field of biochemistry. Tumor cells have elevated polyamine level if compared with normal cells, and this greatly stimulates the search for new opportunities to deplete the intracellular pool of spermine and spermidine resulting in decrease in cell growth and even cell death. O-Substituted hydroxylamines occupy their own place among chemical regulators of the activity of the enzymes of polyamine metabolism. Varying the structure of the alkyl substituent made it possible to obtain within one class of chemical compounds highly effective inhibitors and regulators of the activity of all the enzymes of putrescine, spermine and spermidine metabolism (with the exception of FAD-dependent spermine oxidase and acetylpolyamine oxidase), effectors of the polyamine transport system, and even actively transported in cells "proinhibitor" of ornithine decarboxylase. Some principles for the design of specific inhibitors of these enzymes as well as the peculiarities of cellular effects of corresponding O-substituted hydroxylamines are discussed.

Expression and distribution of genes encoding for polyamine-metabolizing enzymes in the different zones of male and female mouse kidneys

The role of polyamines in renal physiology is only partially understood. Moreover, most of the data on the enzymes of polyamine metabolism come from studies using whole kidneys. The aim of the present study was to analyze the mRNA abundance of the genes implicated in both the polyamine biosynthetic and catabolic pathways in different renal zones of male and female mice, by means of the quantitative reverse transcription-polymerase chain reaction. Our results indicate that there is an uneven distribution of the different mRNAs studied in the five renal zones: superficial cortex, deep cortex, outer stripe of the outer medulla (OS), inner stripe of the outer medulla (IS), and the inner medulla + papilla (IM). The biosynthetic genes, ornithine decarboxylase (ODC) and spermine synthase, were more expressed in the cortex, whereas the mRNAs of the catabolic genes spermine oxidase (SMO) and diamine oxidase were more abundant in IS and IM. The genes involved in the regulation of polyamine synthesis (AZ1, AZ2 and AZIN1) were expressed in all the renal zones, predominantly in the cortex, while AZIN2 gene was more abundant in the OS. ODC, SMO, spermidine synthase and spermidine/spermine acetyl transferase expression was higher in males than in females. In conclusion, the genes encoding for the polyamine metabolism were specifically and quantitatively distributed along the corticopapillary axis of male and female mouse kidneys, suggesting that their physiological role is essential in defined renal zones and/or nephron segments.

Conformational stabilization of rat s-adenosylmethionine decarboxylase by putrescine

The activity and processing of mammalian S-adenosylmethionine decarboxylase (AdoMetDC) is stimulated by putrescine. To obtain new insights into the mechanism through which putrescine stimulates AdoMetDC, we investigated conformational changes in rat prostate AdoMetDC in the presence or absence of putrescine. We examined the reactivity of purified rat prostate AdoMetDC to the SH-reagent iodoacetic acid (IAA) and its susceptibility to proteolysis in the presence or absence of putrescine using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). The activity of AdoMetDC treated with IAA in the absence of putrescine was reduced, but about 80% of its activity remained after treatment with IAA in the presence of putrescine. In the presence of putrescine, IAA incorporation was 1.9 mol IAA/mol of AdoMetDC α-subunit, while there was no incorporation of IAA in the β-subunit of AdoMetDC. In the absence of putrescine, 5.0 mol of IAA/mol of α-subunit and 0.9 mol of IAA/mol of β-subunit were incorporated. Only Cys292 and Cys310 were carboxymethylated by IAA in the presence of putrescine. In contrast, in the absence of putrescine all cysteines were carboxymethylated by IAA. In addition, putrescine slowed the rate of AdoMetDC degradation by trypsin. These results demonstrate that the conformation of AdoMetDC purified from rat prostate is stabilized by putrescine.

Spermine synthase activity affects the content of decarboxylated S-adenosylmethionine

dcAdoMet (decarboxylated S-adenosylmethionine) is an essential intermediate in the synthesis of polyamines. Its content is normally very low, amounting to less than 5% of that of S-adenosylmethionine itself. It was found that in mice lacking spermine synthase there was a large increase in dcAdoMet and that overexpression of spermine synthase reduced the amount of this nucleoside. There was also an increase in dcAdoMet in cells derived from patients with Snyder–Robinson syndrome, a rare X-linked recessive human disease caused by SMS gene mutations that greatly reduce the content of spermine synthase. These results suggest that there is an inverse relationship between the amount of spermine synthase protein and the content of dcAdoMet and raise the possibility that some of the abnormalities seen in mammals deficient in spermine synthase might be due to changes in dcAdoMet pools.

Structural basis for putrescine activation of human S-adenosylmethionine decarboxylase

Putrescine (1,4-diaminobutane) activates the autoprocessing and decarboxylation reactions of human S-adenosylmethionine decarboxylase (AdoMetDC), a critical enzyme in the polyamine biosynthetic pathway. In human AdoMetDC, putrescine binds in a buried pocket containing acidic residues Asp174, Glu178, and Glu256. The pocket is away from the active site but near the dimer interface; however, a series of hydrophilic residues connect the putrescine binding site and the active site. Mutation of these acidic residues modulates the effects of putrescine. D174N, E178Q, and E256Q mutants were expressed and dialyzed to remove putrescine and studied biochemically using X-ray crystallography, UV-CD spectroscopy, analytical ultracentrifugation, and ITC binding studies. The results show that the binding of putrescine to the wild type dimeric protein is cooperative. The D174N mutant does not bind putrescine, and the E178Q and E256Q mutants bind putrescine weakly with no cooperativity. The crystal structure of the mutants with and without putrescine and their complexes with S-adenosylmethionine methyl ester were obtained. Binding of putrescine results in a reorganization of four aromatic residues (Phe285, Phe315, Tyr318, and Phe320) and a conformational change in the loop 312-320. The loop shields putrescine from the external solvent, enhancing its electrostatic and hydrogen bonding effects. The E256Q mutant with putrescine added shows an alternate conformation of His243, Glu11, Lys80, and Ser229, the residues that link the active site and the putrescine binding site, suggesting that putrescine activates the enzyme through electrostatic effects and acts as a switch to correctly orient key catalytic residues.

Genetic analysis of Drosophila melanogaster susceptibility to intestinal Vibrio cholerae infection

We previously demonstrated that Vibrio cholerae is able to colonize the intestine of the fly to produce a lethal infection. Here we present the results of a genetic screen undertaken to identify factors that alter susceptibility of the fly to intestinal V. cholerae infection. In this model of infection, the Eiger/Wengen signalling pathway protects the fly against infection. Furthermore, mutations within the IMD signalling pathway increase resistance to intestinal V. cholerae infection and increase programmed cell death within the intestinal epithelium during infection. We propose that programmed cell death protects the intestinal epithelium against V. cholerae infection and therefore that the fly may serve as a useful model in which to study modulation of intestinal epithelial cell survival by commensal and pathogenic intestinal bacteria as well as the pathological processes leading to erosion of the intestinal epithelium and intestinal malignancy.

Polyamine metabolism of filaria and allied parasites

Putrescine and the polyamines spermidine and spermine occur both in prokaroytes and in eukaryotes where they seem intimately involved in regulatory processes of cellular growth and differentiation. They seem to play an important role related to the biosynthesis of nucleic acids and proteins, although at the molecular level their precise function remains unclear. In general, prokaryotes utilize putrescine and spermidine while eukaryotes tend to have higher concentrations of spermidine and spermine compared to putrescine(1-3.) Differences in polyamine metabolism between parasites and their hosts suggest several potential targets for chemotherapeutic attack As Rolf Walter discusses here, such approaches have already been exploited for African trypanosomes and also offer some leads for the chemotherapy of helminth infections.

Extremely rapid turnover of S-adenosylmethionine decarboxylase inCrithidia fasciculata

The activity of S-adenosylmethionine decarboxylase (AdoMetDC) in Crithidia fasciculata was shown to be correlated to the growth of the parasite. An increase in activity was observed during exponential growth. Inhibition of protein synthesis induced an extremely rapid decay of AdoMetDC activity. The half-life of the enzyme was estimated to be about 3 min, which is the shortest half-life ever recorded for an eukaryotic AdoMetDC. The reduction in AdoMetDC activity was correlated with a decrease in AdoMetDC protein content, demonstrating a rapid turnover of the enzyme. No polyamine-mediated feedback regulation of AdoMetDC was observed in the parasite.

S-adenosylmethionine decarboxylase from Leishmania donovani. Molecular, genetic, and biochemical characterization of null mutants and overproducers

The polyamine biosynthetic enzyme, S-adenosylmethionine decarboxylase (ADOMETDC) has been advanced as a potential target for antiparasitic chemotherapy. To investigate the importance of this protein in a model parasite, the gene encoding ADOMETDC has been cloned and sequenced from Leishmania donovani. The Delta adometdc null mutants were created in the insect vector form of the parasite by double targeted gene replacement. The Delta adometdc strains were incapable of growth in medium without polyamines; however, auxotrophy could be rescued by spermidine but not by putrescine, spermine, or methylthioadenosine. Incubation of Delta adometdc parasites in medium lacking polyamines resulted in a drastic increase of putrescine and glutathione levels with a concomitant decrease in the amounts of spermidine and the spermidine-containing thiol trypanothione. Parasites transfected with an episomal ADOMETDC construct were created in both wild type and Delta adometdc parasites. ADOMETDC overexpression abrogated polyamine auxotrophy in the Delta adometdc L. donovani. In addition, ADOMETDC overproduction in wild type parasites alleviated the toxic effects of 5'-(((Z)-4-amino-2-butenyl)methylamino)-5'-deoxyadenosine (MDL 73811), but not pentamidine, berenil, or methylglyoxyl bis(guanylhydrazone), all inhibitors of ADOMETDC activities in vitro. The molecular, biochemical, and genetic characterization of ADOMETDC establishes that it is essential in L. donovani promastigotes and a potential target for therapeutic validation.

Polyamines as cancer markers: applicable separation methods

Spermine, spermidine, putrescine and cadaverine are aliphatic amines widely spread in the human body. Their concentrations together with their acetyl conjugates increase significantly in the biological fluids and the affected tissues of cancer patients. Their concentrations decrease with the improvement in the patient's condition on multiple therapy. Various chromatographic techniques are frequently used in monitoring concentrations of di- and polyamines in cancer. Among these techniques, thin-layer chromatography and liquid chromatography using pre- or postcolumn derivatization, separating on a reversed-phase or an ion-exchange column are the most commonly used. Besides, high-resolution capillary column gas chromatography (GC) is increasingly used over packed column GC, and in recent years, capillary zone electrophoresis has also gained some importance in polyamine determinations. The review examines the prospects and the limitations of polyamines as cancer markers using chromatographic and electrophoretic techniques.

Spermine is not essential for growth of Saccharomyces cerevisiae: identification of the SPE4 gene (spermine synthase) and characterization of a spe4 deletion mutant

Spermine, ubiquitously present in most organisms, is the final product of the biosynthetic pathway for polyamines and is synthesized from spermidine. In order to investigate the physiological roles of spermine, we identified the SPE4 gene, which codes for spermine synthase, on the right arm of chromosome XII of Saccharomyces cerevisiae and prepared a deletion mutant in this gene. This mutant has neither spermine nor spermine synthase activity. Using the spe4 deletion mutant, we show that S. cerevisiae does not require spermine for growth, even though spermine is normally present in the wild-type organism. This is in striking contrast to the absolute requirement of S. cerevisiae for spermidine for growth, which we had previously reported using a mutant lacking the SPE3 gene (spermidine synthase) [Hamasaki-Katagiri, N., Tabor, C. W., Tabor, H., 1997. Spermidine biosynthesis in Saccharomyces cerevisiae: Polyamine requirement of a null mutant of the SPE3 gene (spermidine synthase). Gene 187, 35-43].

Methotrexate in rheumatoid arthritis: an update with focus on mechanisms involved in toxicity

OBJECTIVES

To provide an update of the current knowledge of the mechanism of action of low-dose methotrexate (MTX) in the treatment of patients with rheumatoid arthritis (RA), with an emphasis on the mechanisms involved in toxicity. We also considered strategies currently used to prevent or decrease toxicity of MTX.

METHODS

We reviewed the literature dealing with the subjects of MTX treatment of RA, the mechanisms of action of low-dose MTX regarding efficacy and toxicity, and strategies used to prevent or decrease MTX toxicity.

RESULTS

MTX is a fast working and effective second-line antirheumatic agent (SLA). Its use is limited mainly because of side effects. The mechanisms of action regarding efficacy and toxicity are probably determined by different metabolic pathways. Recent data indicate that the antiinflammatory effect of MTX is mediated by adenosine. However, MTX side effects can only partly be explained by folate antagonism and may also depend on its action on other related metabolic pathways. The latter include the homocysteine-methionine-polyamine pathway and purine metabolism. Variants in these metabolic routes (ie, the C677T mutation in the methylene-tetrahydrofolate reductase [MTHFR] gene), may predispose to the development of side effects. Currently the most promising strategy to decrease or prevent toxicity of MTX is concomitant prescription of folic acid or folinic acid. Other strategies are currently under investigation.

CONCLUSIONS

MTX benefits a majority of RA patients. Approximately 30% of patients, however, abandon treatment because of drug-related side effects. Folic acid or folinic acid likely reduces MTX toxicity. More data, however, are needed to evaluate a potential detrimental effect on the antirheumatic efficacy of MTX.

Significant increases in the steady states of putrescine and spermidine/spermine N1-acetyltransferase mRNA in HeLa cells accompanied by growth arrest

Polyamines are intrinsic polycations which play critical roles in cell proliferation. Ornithine decarbolylase (ODC) catalyzes the first step of polyamine biosynthesis converting ornithine to putrescine. In addition to polyamine degradation, spermidine/spermine acetyltransferase (SSAT) regulates interconversion pathway of spermine and spermidine to putrescine. We quantified the polyamines and mRNAs of ODC and SSAT in HeLa S3 cells at various stages during exponential and plateau phases of culturing. Unexpectedly, putrescine and SSAT mRNA levels increased remarkably at the plateau phase, in contrast to the decrease of ODC mRNA level. It will be suggested that the putrescine has a novel function linked to the arrest of cell growth in which the SSAT-mediated pathway producing putrescine takes part.

Processing of mammalian and plant S-adenosylmethionine decarboxylase proenzymes

S-Adenosylmethionine decarboxylase (AdoMetDC) is a pyruvoyl enzyme, and the pyruvate is formed in an intramolecular reaction that cleaves a proenzyme precursor and converts a serine residue into pyruvate. The wild type potato AdoMetDC proenzyme processed much faster than the human proenzyme and did not require putrescine for an optimal rate of processing despite the presence of three acidic residues (equivalent to Glu11, Glu178, and Glu256) that were demonstrated in previous studies to be required for the putrescine activation of human AdoMetDC proenzyme processing (Stanley, B. A., Shantz, L. M., and Pegg, A. E. (1994) J. Biol. Chem. 269, 7901-7907). A fourth residue that is also needed for the putrescine stimulation of human AdoMetDC proenzyme processing was identified in the present studies, and this residue (Asp174) is not present in the potato sequence. The site of potato AdoMetDC proenzyme processing was found to be Ser73 in the conserved sequence, YVLSESS, which is the equivalent of Ser68 in the human sequence. Replacement of the serine precursor with threonine or cysteine by site-directed mutagenesis in either the potato or the human AdoMetDC proenzyme did not prevent processing but caused a significant reduction in the rate. Although the COOH-terminal regions of the known eukaryotic AdoMetDCs are not conserved, only relatively small truncations of 8 residues from the human protein and 25 residues from the potato proenzyme were compatible with processing. The maximally truncated proteins show no similarity in COOH-terminal amino acid sequence but each contained 46 amino acid residues after the last conserved sequence, suggesting that the length of this section of the protein is essential for maintaining the proenzyme conformation needed for autocatalytic processing.

Spontaneous oxidation of methionine: effect on the quantification of plasma methionine levels

Plasma methionine (Met), methionine sulfoxide (MSO), and total Met concentrations were determined by reversed-phase chromatography and fluorescence detection after automated precolumn derivatization with an o-phthalic aldehyde mercaptoethanol reagent. Addition of pure, MSO-free L-Met to plasma samples resulted in the anticipated linear increase in plasma Met concentrations, but simultaneously effected a dose-dependent, linear increase in MSO levels. In contrast, the addition of pure L-MSO to plasma samples rendered linear calibration curves for MSO, while the Met concentration remained constant. A strong buffering effect against the spontaneous or hydrogen peroxide induced oxidation of Met to MSO was observed in plasma samples. This protective effect could be neutralized by preincubating the plasma samples with sodium azide. The addition of relatively low concentrations of red cell lysates to plasma samples, prior to hydrogen peroxide oxidation, strongly inhibited the conversion of Met to MSO. Plasma samples from 127 healthy female volunteers were analyzed: MSO concentrations (mean, 3.6 +/- 2.1 microM) exhibited a weak positive correlation (r = 0.352) with Met levels (mean, 21.3 +/- 6.1 microM) but, after the exclusion of two probable outliers from the data set, no correlation was observed. Our results suggest that plasma Met concentrations should be corrected for oxidative losses incurred during storage, sample processing and because of the action of a variety of in situ oxidants, present in plasma, in order to obtain a reliable estimate of the methionine status of an individual.

Regulation of O6-methylguanine-DNA methyltransferase by methionine in human tumour cells

Methionine (MET)-dependent cell lines require MET to proliferate, and homocysteine (HCY) does not act as a substitute for this requirement. From six O6-methylguanine-DNA methyltransferase (MGMT)-efficient (mer+) cell lines tested, two medulloblastomas (Daoy and D-341) and a lung non-small-cell adenocarcinoma with metastatic potential (H-1623) were most sensitive to MET deprivation, while two glioblastomas (U-138, D-263) and a small-cell lung carcinoma H-1944 were moderately to weakly dependent. Regardless of the degree of MET dependence, all of these lines down-regulated their MGMT activity within 48-72 h of transfer from MET+HCY- to MET-HCY+ media, long before the eradication of the culture. Reduction of MGMT activity was due to a decline of both MGMT mRNA and protein levels. However, the reduction was not related to the methylation status of the MGMT promoter at the SmaI site or the HpaII sites in the body of the gene; such sites have been shown to be associated in MGMT regulation and in defining the mer phenotype. MET-dependent, mer+ tumour cells cultured in MET-HCY+ were more sensitive to BCNU (IC50 = 5-10 microM) than those cultured in MET+HCY-(IC50 = 45-90 microM), while MET-independent or mer- cell lines were unaffected. This indicates that reduction of MGMT, imposed by the absence of MET, renders mer+ tumour cells more susceptible to alkylating agents. The relatively selective suppression of MGMT activity in mer+ MET-dependent tumour cells, in combination with the inability of such cells to proliferate in the absence of MET, may lead to the development of more effective treatment strategies for mer+ MET-dependent tumours.

Role of the 5'-untranslated region of mRNA in the synthesis of S-adenosylmethionine decarboxylase and its regulation by spermine

S-Adenosylmethionine decarboxylase (AdoMetDC), a rate-limiting enzyme in polyamine biosynthesis, is regulated by polyamines at the levels of both transcription and translation. Two unusual features of AdoMetDC mRNA are a long (320 nt) 5'-untranslated region (5'UTR), which is thought to contain extensive secondary structure, and a short (15 nt) open reading frame (ORF) within the 5'UTR. We have studied the effects of altering these elements on both the expression of AdoMetDC and its regulation by n-butyl-1,3-diaminopropane (BDAP), a spermine synthase inhibitor. Human AdoMetDC cDNAs containing alterations in the 5'UTR, as well as chimaeric constructs in which the AdoMetDC 5'UTR was inserted ahead of the luciferase-coding region, were transfected into COS-7 cells. Construct pSAM320, which contains all of the 5'UTR, the AdoMetDC protein-coding region and the 3'UTR, was expressed poorly (2-fold over the endogenous activity). Deletion of virtually the entire 5'UTR, leaving nt -12 to -1, increased expression 59-fold, suggesting that 5'UTR acts as a negative regulator. The same effect was seen when the 27 nt at the extreme 5' end were removed (pSAM293, 47-fold increase), or when the internal ORF which is present in this region was destroyed by changing the ATG to CGA (pSAM320-ATG, 38-fold increase). The expression and regulation of pSAM44 (made by deleting nt -288 to -12), which has very little predicted secondary strucutre, was very similar to that of pSAM320 indicating that the terminal 27 nt including the internal ORF rather than extensive secondary structure may be responsible for the low basal levels of AdoMetDC expression. These results, confirmed using luciferase constructs, suggest that the negative effect on expression is predominantly due to the internal ORF. Depletion of spermine by BDAP increased the expression from pSAM320 more than 5-fold without affecting AdoMetDC mRNA levels. Expression from pSAM293 was unchanged by spermine depletion, whereas that from pSAM320-ATG was increased 2.5-fold. These results indicate the presence of a spermine response element in the first 27 nt of the 5'UTR that may include but is not entirely due to the internal ORF.

Characterization of S-adenosylmethionine decarboxylase induced by human cytomegalovirus infection

Infection of human diploid embryonic lung (MRC5) cells by human cytomegalovirus (HCMV), strain AD169, increased the activity of a key enzyme in the synthesis of polyamines: S-adenosylmethionine decarboxylase (E.C. 4.1.1.50). The initial peak of S-adenosylmethionine decarboxylase activity occurred about 15 h postinfection. S-Adenosylmethionine decarboxylase was purified using a highly specific affinity chromatography step from HCMV-infected and control uninfected MRC5 cells. No difference was found between the two enzymes in their stability to heat or effect of pH on activity. Both enzymes were activated only by putrescine. The appKm for S-adenosylmethionine for the virus-induced enzyme was 1.7 times higher than the appKm for the control enzyme. The most dramatic difference observed was in the effect of high salt concentration on enzyme activity. S-Adenosylmethionine decarboxylase from HCMV-infected cells was unaffected by 0.8 M NaCl, whereas the enzyme from uninfected cells was inhibited by 50% at 0.45 M NaCl and was significantly inhibited at a concentration of 0.8 M NaCl. Thus, different forms of S-adenosylmethionine decarboxylase probably exist in infected and uninfected MRC5 cells.

S-adenosyl-L-methionine decarboxylase of Acanthamoeba castellanii (Neff): purification and properties

S-Adenosyl-L-methionine decarboxylase (AdoMetDC) has been purified to near homogeneity from the Neff strain of Acanthamoeba castellanii. The holoenzyme molecular mass is 88.8 kDa, including two copies each of a 32.8 kDa alpha-subunit and a 10-15 kDa beta-subunit. The alpha-subunit contains the active site. It has an N-terminal pyruvoyl group, and the first 19 amino acids are 63 and 74% identical with comparable sequences from yeast and mammals, respectively. The apparent Km for S-adenosylmethionine (AdoMet) in the presence of 2 mM putrescine was 30.0 microM. The enzyme was stimulated 2-fold by putrescine, but was unaffected by spermidine. It was inhibited by the following anti-metabolites, listed with their Ki values: Berenil (0.17 microM), pentamidine (19.4 microM), propamidine (334 microM), hydroxystilbamidine (357 microM), methylglyoxal bis(guanylhydrazone) (604 microM) and ethidium bromide (1.3 mM). Activity of the enzyme fell to undetectable levels during cell differentiation (encystment).

Regulation of S-adenosylmethionine decarboxylase activity by alterations in the intracellular polyamine content

The effects of addition of exogenous spermidine and spermine and of two inhibitors of polyamine biosynthesis, alpha-difluoromethylornithine (DFMO), which decreases spermidine concentrations, and n-butyl-1,3-diaminopropane, which depletes spermine, on the expression of S-adenosylmethionine decarboxylase (AdoMetDC) activity were studied in mammalian cell lines (HT29, CHO and COS-7). AdoMetDC levels were inversely related to the polyamine content, and spermine was the more potent repressor of AdoMetDC activity, but only spermidine affected the amount of AdoMetDC mRNA. Transfection of COS-7 cells or CHO cells with plasmid constructs containing a chloramphenicol acetyltransferase (CAT) reporter gene driven by portions of the AdoMetDC promoter region indicated that CAT expression was altered by spermidine, but not by spermine, suggesting that there is a spermidine-responsive element in this promoter. Transient transfection of COS-7 cells with pSAMh1, a plasmid containing the AdoMetDC cDNA in a vector with the SV40 promoter and origin of replication, led to a large increase in AdoMetDC expression. Although treatment of COS-7 cells with n-butyl-1,3-diaminopropane greatly increased endogenous AdoMetDC activity, the spermine depletion brought about by this inhibitor did not stimulate AdoMetDC expression from pSAMh1. The pSAMh1 cDNA is missing 72 nucleotides from the 5' end of the AdoMetDC mRNA, and it is possible that translational regulation by spermine involves this region. The expression of AdoMetDC from pSAMh1 in COS-7 cells was greatly inhibited by DFMO treatment, although endogenous AdoMetDC activity was increased. The expression of other plasmids containing the SV40 origin of replication was also inhibited by DFMO in COS-7 cells, but not in CHO cells. DFMO treatment did not interfere with the expression of plasmids driven by the RSV promoter. These results suggest that low spermidine levels interfere with the replication of plasmids containing the SV40 origin of replication.

S-adenosylmethionine decarboxylase as an enzyme target for therapy

The polyamine biosynthetic pathway has attracted much interest as a therapeutic target. Many studies have shown the potential value of inhibitors of the first enzyme in the biosynthetic pathway, ornithine decarboxylase, which forms putrescine. In order to convert putrescine into the polyamines, spermidine and spermine, the aminopropyl donor, decarboxylated S-adenosylmethionine, is needed. Therefore, S-adenosylmethionine decarboxylase (AdoMetDC, EC 4.1.1.50) is essential for polyamine synthesis. Early studies of the inhibition of this enzyme were carried out with compounds such as methylglyoxal bis(guanylhydrazone) that lack specificity and also lack potency since they are competitive inhibitors whose effects are overcome by a compensatory increase in the amount of the target enzyme. Recently, powerful irreversible inhibitors of AdoMetDC have become available including 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine, an enzyme activated inhibitor and 5'-deoxy-5'-[(3-hydrazinopropyl)methylamino]adenosine which binds to the active site and forms a covalent bond with the pyruvate prosthetic group. This review describes the current state of knowledge of the structure and properties of AdoMetDC, the available inhibitors of this enzyme, their mechanism of action and their effects on polyamines and on the growth of tumors and protozoan parasites. These effects indicate that AdoMetDC inhibitors may be of therapeutic value either alone or in combination with ornithine decarboxylase inhibitors and that further trials of these compounds should be considered.

Molecular cloning of the mouse S-adenosylmethionine decarboxylase cDNA: specific protein binding to the conserved region of the mRNA 5'-untranslated region

The nucleotide sequence of a cDNA encoding the proenzyme of mouse S-adenosylmethionine decarboxylase (AdoMetDC) including 257 nucleotides of the 5' untranslated region has been determined. Comparison of the nucleotide sequence of the mouse 5' untranslated region with those of other mammals shows it to be highly conserved. The 52 nucleotides upstream from the translation initiation codon are identical in human, rat, bovine and mouse. The polyamines, spermidine and spermine, have been shown to inhibit AdoMetDC mRNA translation. An RNA gel retardation assay demonstrated that a cytoplasmic extract from mouse brain forms an RNA-protein complex with the completely conserved 5' untranslated sequence and that the complex formation is highly dependent on the presence of spermine. Crosslinking by UV irradiation shows that the complex contains a 39-kDa protein interacting with the 5' untranslated sequence. These data demonstrate spermine-dependent specific protein binding to a highly conserved 5' untranslated region of an mRNA translationally regulated by polyamines.

Putrescine activated S-adenosylmethionine decarboxylase from Trypanosoma brucei brucei

Trypanosoma brucei brucei contained a S-adenosyl-L-methionine decarboxylase (AdoMetDC) strongly activated by putrescine. The enzyme was also activated to a lesser extent by cadaverine and 1,3-diaminopropane. Spermidine and spermine had no effect on basal activity of the enzyme. However, they interfered with putrescine activation of trypanosomal AdoMetDC. The trypanosomal enzyme could not be precipitated with antiserum against human AdoMetDC. The trypanosomal AdoMetDC enzyme subunit was labeled by reaction with 35S-decarboxylated AdoMet in the presence of NaCNBH4, and found to have a molecular weight of 34 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The subunit was readily degraded on storage to a form with a molecular weight of 26 kDa. The specificity of labeling of AdoMetDC by this procedure was confirmed by the prevention of 35S-decarboxylated S-adenosylmethionine (AdoMet) binding in the presence of specific AdoMetDC inhibitors [either methylglyoxal bis(guanylhydrazone (MGBG), a reversible inhibitor, or 5'-deoxy-5'-[(2-hydrazinoethyl)methylamino]adenosine (MHZEA), an irreversible inactivator]. As compared to human AdoMetDC, the trypanosomal enzyme showed weaker binding to a column of MGBG-Sepharose and also was significantly less sensitive to inhibition by MGBG and its congener ethylglyoxal bis(guanylhydrazone) (EGBG). Thus, the trypanosomal AdoMetDC differs significantly from its mammalian and bacterial counterparts and may therefore be exploited as a specific target for chemotherapy of trypanosomiasis.

Cytostasis induced in L1210 murine leukaemia cells by the S-adenosyl-L-methionine decarboxylase inhibitor 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine may be due to hypusine depletion

The effects of inhibition of the capacity to form spermidine and spermine on cell growth were investigated using murine leukaemia L1210 cells and 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine (MDL 73811, AbeAdo), an enzyme-activated irreversible inhibitor of S-adenosyl-L-methionine decarboxylase. Putrescine levels were increased 80-fold, and spermidine and spermine levels were greatly reduced after a 3-day exposure to a maximally inhibitory dose of 10 microM-AbeAdo. Addition of AbeAdo to the culture medium inhibited the growth of L1210 cells measured 3 days later in a dose-dependent manner, but, even at a dose of 10 microM, which was maximally effective, exposure to AbeAdo was not immediately cytostatic. However, the growth rate of L1210 cells chronically exposed to 10 microM-AbeAdo declined steadily until day 12, when the cells stopped growing. L1210 cells exposed to AbeAdo for 12 days could not be rescued from cytostasis by removal of the drug from the culture, but could be rescued by exposure to exogenous spermidine or spermine, indicating that the growth-inhibitory effects of AbeAdo were a result of spermidine and/or spermine depletion. It is suggested that elevated intracellular putrescine in AbeAdo-treated cells sustained limited growth in the absence of physiological levels of spermidine and spermine until certain critical and specific physiological role(s) fulfilled by spermidine (and/or spermine) became deficient resulting in cytostasis. N-(3-Aminopropyl)-1,4-diamino-cis-but-2-ene, a spermidine analogue that is a substrate for deoxyhypusine synthase, was able to mimic the effects of spermidine in reversing AbeAdo-induced cytostasis. Spermidine analogues such as 5,5-dimethylspermidine, which are not substrates for deoxyhypusine synthase, were not active in this way. These results provide evidence that the formation of hypusine in the protein-synthesis initiation factor eIF-5A may be a critical role of spermidine essential for cell growth.

Irreversible inhibition of S-adenosylmethionine decarboxylase of Trypanosoma brucei brucei 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 IC50 values in the range of 40-100 nM. The most active compound (IC50 of 0.04 microM) was 5'-deoxy-5'-[(2-hydrazinoethyl)-methylamino]adenosine (MHZEA). Addition of MHZEA produced a time-dependent inactivation with an apparent Ki of 0.4 microM, 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-aminooxyethyl)methylamino]adenosine (MAOEA), which had an IC50 of 1.3 microM, 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 IC50 value of 8.7 microM. 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.

Purification of human S-adenosylmethionine decarboxylase expressed in Escherichia coli and use of this protein to investigate the mechanism of inhibition by the irreversible inhibitors, 5'-deoxy-5'-[(3-hydrazinopropyl)methylamino]adenosine and 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine

Human S-adenosylmethionine decarboxylase (AdoMetDC) was expressed in high yield in Escherichia coli using the pIN-III(lppP-5) expression vector and purified to apparent homogeneity using affinity chromatography on methylglyoxal bis(guanylhydrazone)-Sepharose. The inactivation of the purified enzyme by 5'-deoxy-5'-[(3-hydrazinopropyl)methylamino]adenosine (MHZPA) was accompanied by an increase in absorbance at 260 nm of the large subunit. This increase was equivalent to the addition of 1 molecule of MHZPA. After digestion with the protease Lys-C, a peptide that contained the bound MHZPA was isolated and found to have the amino acid composition consistent with that expected from the amino terminus of the large subunit. These results indicate that MHZPA inactivates AdoMetDC by forming a hydrazone derivative at the pyruvate prosthetic group. Inactivation of AdoMetDC by 5'-([(Z)-4-amino-2-butenyl]methylamino]-5'-deoxyadenosine (AbeAdo) led to the appearance of a new peptide peak in the Lys-C protease digest. This peptide had the sequence ASMFVSK. This agrees with the expected sequence from the amino terminus, which is pyruvoyl-SMFVSK, with the exception that the pyruvate has been converted to alanine. Direct gas-phase sequencing of the large subunit of the enzyme also indicated the presence of alanine at the amino terminus after inactivation with AbeAdo. These results indicate that this inhibitor leads to transamination of the pyruvate prosthetic group. Since the pyruvate is covalently linked to the protein, its replacement by alanine leads to an irreversible inactivation of AdoMetDC.

Developmental expression of ornithine and S-adenosylmethionine decarboxylases in mouse brain

The activities of the two key enzymes in mammalian polyamine synthesis, ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (AdoMetDC) in mouse brain show distinct, but inverse, changes during ontogeny. The level of ODC activity is about 70 fold higher at the time of birth than in the adult mouse, whereas AdoMetDC activity is very low after birth and increases as the brain matures. The correlation between the changes in enzyme activities and in the levels of the corresponding mRNAs diminishes dramatically during development. The increase in AdoMetDC mRNA level exceeds the increase in enzyme activity by 100%. Whereas ODC mRNA level falls initially, in concert with decreasing enzyme activity, but then shows an abrupt rise to a very high level during the late period of brain maturation while the enzyme activity continues to decrease to an almost undetectable level. These data suggest the development-dependent appearance of post-transcriptional regulation mechanisms.

Nucleotide sequence of hamster S-adenosylmethionine decarboxylase cDNA

The nucleotide sequence of a cDNA encoding the proenzyme of hamster S-adenosylmethionine decarboxylase including 169 nucleotides of the 5' untranslated region has been determined. The deduced amino acid sequence shows a remarkable similarity to the human proenzyme with only seven differences out of 334 amino acids. The nucleotide sequence of the 5' untranslated region showed 93% homology with the corresponding rat and human sequences suggesting that this region may play an important role in the regulation of S-adenosylmethionine decarboxylase expression.

Structure and organization of the gene encoding rat S-adenosylmethionine decarboxylase

The gene for S-adenosylmethionine decarboxylase (AdoMetDC) was isolated from a rat genomic library using AdoMetDC cDNA as a probe. Nucleotide sequence analysis shows that the rat AdoMetDC gene consists of 8 exons which encode a protein identical to that inferred by a rat AdoMetDC cDNA sequence. The exons range in length from 43 to 1964 base pairs spanning 15672 bases of chromosomal DNA. All of the exon/intron junctions were found to conform to the consensus splice donor and acceptor sequences. Exon 8 corresponds to the 3' noncoding region of the 2 species of AdoMetDC mRNA which are formed by alternative utilization of 2 polyadenylation signals separated from each other by 1272 nucleotides. The transcription initiation site was located by S1 nuclease protection and by primer extension analysis, -325 nucleotides upstream of the translation initiation codon. The promoter region of the rat AdoMetDC gene contains a TATA box at -29 base pairs. No typical GC or CAAT boxes are located in the promoter, but six GC boxes and several putative binding sites for both tissue-specific and non-specific transcription factors are found in the proximal part of intron 1. Southern blot analyses reveal a complex hybridization pattern suggesting that there are multiple copies of the AdoMetDC gene in the rat genome.

Antitrypanosomal effects of polyamine biosynthesis inhibitors correlate with increases in Trypanosoma brucei brucei S-adenosyl-L-methionine

We reported recently that administration of ([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine (MDL 73811), an enzyme-activated irreversible inhibitor of S-adenosyl-L-methionine decarboxylase (AdoMetDC; EC 4.1.1.50), a key enzyme in the synthesis of spermidine, cures African trypanosome infections in mice. The precise mechanism of action of MDL 73811 was not clear because a rapid disappearance of trypanosomes from the bloodstream of treated rats occurred before significant depletion of spermidine. Administration of MDL 73811 to Trypanosoma brucei brucei-infected rats resulted in a 70% decrease in parasitaemia within 1 h and a complete disappearance of parasites by 5 h. The reduction in parasitaemia was accompanied by complete inhibition of AdoMetDC activity by 10 min after injection of MDL 73811; inhibition was sustained for at least 4 h. Polyamine levels in trypanosomes were unaffected during the first 1 h in which the marked decrease in parasitaemia was observed, but parasite AdoMet levels increased 20-fold within this time. In contrast, exposure of cultured mammalian cells to MDL 73811 resulted in only a 1.5-2-fold increase in AdoMet levels over a 6 h time course. Experiments with inhibitors of ornithine decarboxylase (ODC) also suggested that the increased AdoMet levels might be an important factor for antitrypanosomal efficacy. Trypanosomes taken from rats treated for 36 h with eflornithine, an inhibitor of ODC, were depleted of putrescine and had markedly decreased spermidine levels. These organisms also had less than 10% of control AdoMetDC activity, and had elevated decarboxy AdoMet (greater than 4000-fold) and AdoMet (up to 50-fold) levels. The methyl ester of alpha-monofluromethyl-3,4-dehydro-ornithine (delta-MFMO-CH3), which cures murine T. b. brucei infections, and the ethyl ester analogue of this compound (delta-MFMO-C2H5), which does not cure this infection, become ODC inhibitors upon hydrolysis and thus were tested for their effects on trypanosomal polyamines, AdoMet and decarboxy AdoMet levels. Although both esters of delta-MFMO depleted trypanosomal polyamines, AdoMet and decarboxy AdoMet levels were elevated in T. b. brucei from infected mice treated with delta-MFMO-CH3 but not in parasites from mice treated with the delta-MFMO-C2H5. These data suggest that inhibition of AdoMetDC, either directly with MDL 73811 or indirectly with inhibitors of ODC, apparently leads to a trypanosome-specific elevation of AdoMet. It is possible that major changes in AdoMet, rather than changes in polyamines, may be responsible for the antitrypanosomal effects of these drugs.

Regulation of S-adenosylmethionine decarboxylase during the germination of sporangiospores of Mucor rouxii

General properties of S-adenosylmethionine decarboxylase (SAMDC) from Mucor rouxii were studied. Dormant spores of the fungus did not contain detectable levels of the enzyme, but it started to be synthesized at early stages of spore germination. Kinetics of synthesis changed before emergence of the germ tube, with a corresponding increase in a second SAMDC activity which, in contrast to the one originally synthesized, was not activated by putrescine. Development of the second enzyme activity required de novo protein synthesis. Neither enzymic activity was stimulated by Mg2+. Addition of the SAMDC inhibitor methylglyoxal bis-(guanylhydrazone) (MGBG) stopped fungal development in growth phase Ia: cells became spherical and showed ultrastructural alterations. Although MGBG inhibited polyamine formation, it barely inhibited protein and RNA biosynthesis during the first hour of incubation. However, at later periods, biosynthesis of both macromolecules was strongly decreased. When MGBG was added to growth media 3 h after inoculation of spores, it did not affect spore germination and outgrowth. A hypothesis for two different roles of spermidine and putrescine in spore germination is discussed.

Irreversible inhibition of rat S-adenosylmethionine decarboxylase by 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine

5'-([(Z)-4-Amino-2-butenyl]methylamino)-5'-deoxyadenosine [Z)-AbeAdo) was tested in vitro and in vivo as a potential inhibitor of S-adenosyl-L-methionine decarboxylase (AdoMetDC), a pyruvoyl-containing enzyme, purified from rat liver. In vitro (Z)-AbeAdo produces a time- and dose-dependent irreversible inhibition of the enzyme. Saturation kinetics are observed when the enzyme is preincubated with (Z)-AbeAdo in the presence of 50 microM putrescine, a known activator of AdoMetDC. Under these conditions kinetic constants were measured (Ki = 0.56 +/- 0.04 microM; tau 1/2 = 0.51 +/- 0.03 min). The inhibition is not relieved by prolonged dialysis of the inactivated enzyme. The turnover number for (Z)-AbeAdo, i.e. the number of inactivator molecules required to inactivate one enzyme molecule, is approximately 1.5. The selectivity of (Z)-AbeAdo was explored: the compound is not a substrate of adenosine deaminase, mitochondrial monoamine oxidase and diamine oxidase, but is slowly oxidized by benzylamine oxidase from rat aorta. The (E)-isomer of AbeAdo, is at least 100-fold less active than (Z)-AbeAdo as a time-dependent inhibitor of rat liver AdoMetDC. In rats, intraperitoneal administration of (Z)-AbeAdo produces a rapid, long-lasting and dose-dependent decrease of AdoMetDC activity in ventral prostate, testis and brain.

Regulation of the Na(+)-dependent and the Na(+)-independent polyamine transporters in renal epithelial cells (LLC-PK1)

We have studied the regulation of the Na(+)-dependent and Na(+)-independent polyamine transport pathways in the renal LLC-PK1 cell line. Most of the experiments were performed in the presence of 5 mM DL-2-difluoromethylornithine (DFMO) in order to inhibit the cellular synthesis of polyamines. The activity of both transporters as measured by putrescine uptake was increased by growth-promoting stimuli and decreased by exogenous polyamines. The time course of the increase in uptake activity induced by fetal calf serum could be fitted by a single exponential, and the process was three times faster for the Na(+)-dependent than for the Na(+)-independent transporter. Maximum activity was reached after more than 24 h. This increase could be inhibited by actinomycin D and by cycloheximide. Other growth-promoting stimuli, such as subconfluent cell density, as well as growth factors also induced an increase in the transport activity. Particularly, there was a marked stimulation of the Na(+)-dependent pathway by epidermal growth factor in combination with insulin. On the other hand, the transport activity decayed very rapidly upon addition of exogenous polyamines (t1/2 less than 60 min). The diamine putrescine was much less effective in this respect than the polyamines spermidine and spermine. The non-metabolizable substrate methylglyoxal bis(guanylhydrazone) did not induce a decay of the transport activity, but it protected the Na(+)-dependent pathway against the polyamine-induced decay. Inhibition of the protein synthesis by cycloheximide did not induce a rapid decrease of the transport activity; neither did it affect the polyamine-induced decay. These observations suggest that this polyamine-induced decay is not owing to an inhibitory effect on the rate of synthesis of the transporters, but rather to a degradation or an inactivation of the transporters. The polyamine-induced decay slowed down at lower cell density. This effect was particularly pronounced for the Na(+)-dependent transporter. Since the uptake of polyamines was increased at low cell density, the decreased rate of decay in this condition pleads against a simple mechanism of transinhibition by the substrate. In conclusion, both transport pathways were similarly affected by the regulatory parameters, but the Na(+)-dependent transporter was more rapidly and more effectively regulated. The numerous interacting regulatory steps furthermore suggest a physiological role for these transporters, such as an involvement in urinary polyamine disposal.

Nucleotide sequence of rat S-adenosylmethionine decarboxylase cDNA. Comparison with an intronless rat pseudogene

Due to two different polyadenylation signals, two forms of S-adenosylmethionine decarboxylase (AdoMetDC) mRNA (2.1 and 3.4 kb) are present in human and rodent tissues. The nucleotide sequences of rat and human cDNAs corresponding to the shorter mRNA were published previously by us [Pajunen et al., J. Biol. Chem. 263 (1988) 17040-17049]. These sequences covered the coding regions but were incomplete at their 5' ends. Here we report the sequence of rat cDNA spanning the entire longer mRNA with a substantially extended leader region, and compare the sequence with that of a rat psi AdoMetDC pseudogene isolated from a rat genomic library. Relative to the mRNA, the pseudogene has multiple base changes as well as insertions, and deletions. Furthermore, it lacks introns, and is flanked by a short direct repeat. These are typical characteristics of a processed retrogene.

Ascaris suum and Onchocerca volvulus: S-adenosylmethionine decarboxylase

Putrescine-dependent S-adenosylmethionine decarboxylase (EC 4.1.1.50) was demonstrated in Ascaris suum and Onchocerca volvulus; activation was found to be about fourfold by putrescine. Mg2+ did not affect the enzyme activity. A. suum was taken as a model nematode and its S-adenosylmethionine decarboxylase was partially purified and characterized. The molecular weight was estimated to be 220,000. The apparent Km-value for adenosylmethionine was determined to be 17 microM. Methylglyoxal bis(guanylhydrazone) and berenil competitively inhibited the enzyme activity; the apparent Ki-values were found to be 0.24 microM and 0.11 microM, respectively. The dependence of filarial worms on uptake and interconversion of putrescine and polyamines as well as properties of the S-adenosylmethionine decarboxylase, different from the host enzyme, points to the polyamine metabolisms as a useful target for chemotherapy.

Detection of proenzyme form of S-adenosylmethionine decarboxylase in extracts from rat prostate

Previous work in which the synthesis of S-adenosylmethionine decarboxylase was studied by translation of its mRNA indicated that it was formed as a proenzyme having a M.W. of about 37,000 that was cleaved to form the enzyme sub-unit of M.W. 32,000 in a putrescine-stimulated reaction. The extent to which the proenzyme accumulates in vivo and is affected by the putrescine concentration was studied by subjecting prostate extracts to Western immunoblotting procedures. The proenzyme form was readily detectable in control prostates (about 4% of the total) and this proportion was increased to 25% when the rats were pretreated for 3 days with the ornithine decarboxylase inhibitor, alpha-difluoromethylornithine. Conversely, it was decreased to almost undetectable levels after treatment with methylglyoxal bis(guanylhydrazone). These results indicate that the processing of the proenzyme form of S-adenosylmethionine decarboxylase is regulated by the cellular putrescine concentration. This conversion provides another step at which polyamine biosynthesis may be controlled.

Regulation of mammalian S-adenosylmethionine decarboxylase

S-Adenosylmethionine decarboxylase is a key enzyme in the biosynthesis of polyamines that is the rate limiting step in the formation of spermidine and spermine. The activity of S-adenosylmethionine decarboxylase is known to be regulated negatively by these polyamines and positively by their precursor, putrescine. A specific antiserum to S-adenosylmethionine decarboxylase was raised by immunizing rabbits with the homogeneous enzyme purified from rat prostate and a specific radioimmunoassay for the protein was set up. Using this radioimmunoassay it was found that a number of inhibitors of other steps in the polyamine biosynthetic pathway lead to increases in the amount of S-adenosylmethionine decarboxylase protein. These changes were caused by both a decreased rate of degradation and an increased rate of synthesis of the protein. The increased synthesis was due to two factors; a rise in the amount of translatable mRNA and an enhanced translation efficiency. The mRNA content of the prostate was substantially increased by treatment for 3 days with alpha-difluoromethylornithine (2% in drinking water). The translation of mRNA for S-adenosylmethionine decarboxylase was studied using a polyamine-depleted reticulocyte lysate supplemented with mRNA from rat prostate and the antiserum to precipitate the proteins corresponding to S-adenosylmethionine decarboxylase. These studies indicated that the enzyme was synthesized as an inactive precursor of Mr 37,000 which was converted to the enzyme sub-unit of Mr 32,000. The conversion of the precursor to the active sub-unit in vitro was increased by putrescine. The precursor could also be detected by immunoblotting of extracts from prostates of rats depleted of putrescine by treatment with the ornithine decarboxylase inhibitor, alpha-difluoromethylornithine. The translation of the S-adenosylmethionine decarboxylase mRNA in the reticulocyte lysates was strongly inhibited by the addition of spermidine or spermine demonstrating that polyamines directly inhibit the synthesis of S-adenosylmethionine decarboxylase. cDNA clones corresponding to S-adenosylmethionine decarboxylase were isolated using prostatic mRNA from polysomes enriched in S-adenosylmethionine decarboxylase by immunopurification. The use of these probes showed that rat ventral prostate contains two S-adenosylmethionine decarboxylase mRNA species of approximately 3.4 and 2.1 kb which differ in the 3' non-translated sequence. The sequence of these cDNAs will enable the amino acid sequence of the precursor to be obtained. This will provide evidence on the origin of the pyruvate prosthetic group of S-adenosylmethionine decarboxylase.(ABSTRACT TRUNCATED AT 400 WORDS)

Messenger RNAs coding for enzymes of polyamine biosynthesis are induced during the G0-G1 transition but not during traverse of the normal G1 phase

The events occurring during emergence of cells from quiescence ("G0") are not necessarily identical to those in the G1 phase of continuously dividing cells. Cellular levels of the mRNAs coding for ornithine decarboxylase (ODC) and S-adenosyl-methionine decarboxylase (SDC), key enzymes in polyamine synthesis, increased maximally within 5 h after addition of serum to resting 3T3 cells, following a kinetic course similar to that of c-myc mRNA. In a pure early G1 population of cells, prepared by centrifugal elutriation of growing fibroblasts, the levels of ODC and SDC mRNAs were not significantly lower than in other phases of the cell cycle and approximated serum-induced levels rather than the reduced values found in serum-starved cells. Thus, we conclude that the mRNAs coding for the polyamine biosynthetic enzymes, like c-myc, are growth controlled, but not regulated during traverse of a normal cell cycle.

Polyamine metabolism in filarial worms

The human and animal filarial parasites Onchocerca volvulus, Dirofilaria immitis, Brugia patei and Litomosoides carinii contained low levels of putrescine but much higher levels of spermidine and spermine as estimated by ion-pair high pressure liquid chromatography; N-acetylated polyamines were present only in minute amounts. Enzyme activities of ornithine decarboxylase (EC 4.1.1.17) and arginine decarboxylase (EC 4.1.1.19), respectively, were not detectable. Experiments carried out with O. volvulus and D. immitis demonstrated the uptake and bioconversion of labeled polyamines. There is evidence for the existence of a complete reverse pathway generating putrescine from spermidine and spermine, respectively, in both worms. N-Acetylating enzyme activities were detected in 100,000 X g preparations of homogenates from D. immitis which were capable to acetylate putrescine, spermidine and spermine. Long term incubation of the worms in the presence of labeled polyamines resulted in the excretion of putrescine and N-acetylputrescine.

Increase in S-adenosylmethionine decarboxylase in SV-3T3 cells treated with S-methyl-5'-methylthioadenosine

Treatment of SV-3T3 cells with the spermine synthase inhibitor S-methyl-5'-methylthioadenosine [AdoS+(CH3)2] led to a large increase in the activity of S-adenosylmethionine decarboxylase (AdoMetDC) without affecting ornithine decarboxylase. The elevation of AdoMetDC activity was due to an increased amount of enzyme protein, as demonstrated by radioimmunoassay and by immunoblotting. The increase in AdoMetDC protein was caused by at least three factors: an increase in the amount of translatable mRNA, an increased translation efficiency of the mRNA and an increase in the half-life of the protein. The depletion of spermine brought about by AdoS+(CH3)2 appeared to be responsible for the increased synthesis of AdoMetDC and for part of the decrease in its rate of degradation. An additional stabilization of the enzyme protein was probably due to the binding of AdoS+(CH3)2, which is also a weak inhibitor of AdoMetDC. These results demonstrate the importance of cellular spermine concentrations in regulating the activity of AdoMetDC, which is a key enzyme controlling polyamine synthesis.

Putrescine-activated S-adenosylmethionine decarboxylase from Acanthamoeba culbertsoni

Acanthamoeba culbertsoni, the free living pathogenic amoeba responsible for fatal meningoencephalitis, contains an S-adenosylmethionine decarboxylase (EC 4.1.1.50) which is strongly activated by putrescine and to a lesser extent by cadaverine; spermidine, spermine, diaminopropane and 1,6-diaminohexane are inactive. Methylglyoxal bis-(guanylhydrazone) competitively inhibited the enzyme with a Ki value of 123 microM. The enzyme was strongly inhibited by berenil (Ki = 0.5 microM) and to a lesser extent by pentamidine. The putrescine-activated enzyme is inhibited by MgCl2. The apparent molecular weight of 110,000 and its enzymatic properties indicate that the enzyme has characteristics intermediate between the bacterial and eukaryotic S-adenosylmethionine decarboxylases.

Effect of putrescine on the synthesis of S-adenosylmethionine decarboxylase

The synthesis of S-adenosylmethionine (AdoMet) decarboxylase was studied by translating the rat prostate mRNA for this enzyme in a reticulocyte lysate. The protein was formed as a precursor of Mr 37,000, which was converted into the enzyme subunit of Mr 32,000 in the lysates. The presence of putrescine had no effect on the synthesis of the precursor of AdoMet decarboxylase, but accelerated its conversion into the enzyme subunit. Spermidine, spermine, decarboxylated AdoMet, AdoMet and methylglyoxal bis(guanylhydrazone) were not able to substitute for putrescine in this effect. These results indicate that, in addition to its direct activation of mammalian AdoMet decarboxylase, putrescine could increase the amount of the enzyme by increasing its production.