Substrate specificity of 5'-methylthioadenosine phosphorylase from human prostate

Crystal structures of a new class of pyrimidine/purine nucleoside phosphorylase revealed a Cupin fold

Nucleotides metabolism is a fundamental process in all organisms. Two families of nucleoside phosphorylases (NP) that catalyze the phosphorolytic cleavage of the glycosidic bond in nucleosides have been found, including the trimeric or hexameric NP-I and dimeric NP-II family enzymes. Recent studies revealed another class of NP protein in Escherichia coli named Pyrimidine/purine nucleoside phosphorylase (ppnP), which can catalyze the phosphorolysis of diverse nucleosides and yield d-ribose 1-phosphate and the respective free bases. Here, we solved the crystal structures of ppnP from E. coli and the other three species. Our studies revealed that the structure of ppnP belongs to the RlmC-like Cupin fold and showed as a rigid dimeric conformation. Detail analysis revealed a potential nucleoside binding pocket full of hydrophobic residues, and the residues involved in the dimer and pocket formation are all well conserved in bacteria. Since the Cupin fold is a large superfamily in the biosynthesis of natural products, our studies provide the structural basis for understanding, and the directed evolution of NP proteins.

Metabolome analysis using cerebrospinal fluid from narcolepsy type 1 patients

Narcolepsy type 1 (NT1) is a hypersomnia characterized by excessive daytime sleepiness and cataplexy. Inappropriate regulation of fatty acid metabolism has been suggested to be involved in the pathophysiology of NT1, but the detailed mechanisms remain uncertain. Here we performed a metabolomic analysis of cerebrospinal fluid samples from 14 NT1 and 17 control subjects using a novel capillary electrophoresis coupled with Fourier transform mass spectrometry. A total of 268 metabolites were identified and the amount of histidine was the most significantly increased in NT1 patients (p = 4.0 × 10-4). Validation analysis using high-performance liquid chromatography (HPLC) including independent replication samples also identified the association of histidine (p = 2.02 × 10-3). Further, levels of histamine, which is synthesized from histidine, were also examined using HPLC and were found to be significantly decreased in NT1 patients (p = 6.12 × 10-4). Pathway analysis with nominally significant metabolites identified several pathways related to the metabolism of glycogenic amino acids, suggesting that glycogenesis is enhanced in NT1 as a compensatory mechanism for fatty acid metabolism. We performed further exploratory analysis, searching for metabolites associated with sleep variables from polysomnography and the multiple sleep latency test. As a result, 5'-deoxy-5'-methylthioadenosine showed a significant association with apnea-hypopnea index (p = 2.66 ×10-6). Moreover, gamma aminobutyric acid displayed a negative correlation with rapid eye movement sleep latency (REML), and thus might represent an intriguing target for future studies to elucidate how the controlling circuit of REM sleep is associated with abnormally short REML in NT1.

Enzymatic Transition States and Drug Design

Transition state theory teaches that chemically stable mimics of enzymatic transition states will bind tightly to their cognate enzymes. Kinetic isotope effects combined with computational quantum chemistry provides enzymatic transition state information with sufficient fidelity to design transition state analogues. Examples are selected from various stages of drug development to demonstrate the application of transition state theory, inhibitor design, physicochemical characterization of transition state analogues, and their progress in drug development.

Transition-State Analogues of Campylobacter jejuni 5'-Methylthioadenosine Nucleosidase

Campylobacter jejuni is a Gram-negative bacterium responsible for food-borne gastroenteritis and associated with Guillain-Barré, Reiter, and irritable bowel syndromes. Antibiotic resistance in C. jejuni is common, creating a need for antibiotics with novel mechanisms of action. Menaquinone biosynthesis in C. jejuni uses the rare futalosine pathway, where 5'-methylthioadenosine nucleosidase ( CjMTAN) is proposed to catalyze the essential hydrolysis of adenine from 6-amino-6-deoxyfutalosine to form dehypoxanthinylfutalosine, a menaquinone precursor. The substrate specificity of CjMTAN is demonstrated to include 6-amino-6-deoxyfutalosine, 5'-methylthioadenosine, S-adenosylhomocysteine, adenosine, and 5'-deoxyadenosine. These activities span the catalytic specificities for the role of bacterial MTANs in menaquinone synthesis, quorum sensing, and S-adenosylmethionine recycling. We determined inhibition constants for potential transition-state analogues of CjMTAN. The best of these compounds have picomolar dissociation constants and were slow-onset tight-binding inhibitors. The most potent CjMTAN transition-state analogue inhibitors inhibited C. jejuni growth in culture at low micromolar concentrations, similar to gentamicin. The crystal structure of apoenzyme C. jejuni MTAN was solved at 1.25 Å, and five CjMTAN complexes with transition-state analogues were solved at 1.42 to 1.95 Å resolution. Inhibitor binding induces a loop movement to create a closed catalytic site with Asp196 and Ile152 providing purine leaving group activation and Arg192 and Glu12 activating the water nucleophile. With inhibitors bound, the interactions of the 4'-alkylthio or 4'-alkyl groups of this inhibitor family differ from the Escherichia coli MTAN structure by altered protein interactions near the hydrophobic pocket that stabilizes 4'-substituents of transition-state analogues. These CjMTAN inhibitors have potential as specific antibiotic candidates against C. jejuni.

The essential role of methylthioadenosine phosphorylase in prostate cancer

Prostatic epithelial cells secrete high levels of acetylated polyamines into the prostatic lumen. This distinctive characteristic places added strain on the connected pathways, which are forced to increase metabolite production to maintain pools. The methionine salvage pathway recycles the one-carbon unit lost to polyamine biosynthesis back to the methionine cycle, allowing for replenishment of SAM pools providing a mechanism to help mitigate metabolic stress associated with high flux through these pathways. The rate-limiting enzyme involved in this process is methylthioadenosine phosphorylase (MTAP), which, although commonly deleted in many cancers, is protected in prostate cancer. We report near universal retention of MTAP expression in a panel of human prostate cancer cell lines as well as patient samples. Upon metabolic perturbation, prostate cancer cell lines upregulate MTAP and this correlates with recovery of SAM levels. Furthermore, in a mouse model of prostate cancer we find that both normal prostate and diseased prostate maintain higher SAM levels than other tissues, even under increased metabolic stress. Finally, we show that knockdown of MTAP, both genetically and pharmacologically, blocks androgen sensitive prostate cancer growth in vivo. Our findings strongly suggest that the methionine salvage pathway is a major player in homeostatic regulation of metabolite pools in prostate cancer due to their high level of flux through the polyamine biosynthetic pathway. Therefore, this pathway, and specifically the MTAP enzyme, is an attractive therapeutic target for prostate cancer.

Methylthioinosine phosphorylase from Pseudomonas aeruginosa. Structure and annotation of a novel enzyme in quorum sensing

The PA3004 gene of Pseudomonas aeruginosa PAO1 was originally annotated as a 5'-methylthioadenosine phosphorylase (MTAP). However, the PA3004 encoded protein uses 5'-methylthioinosine (MTI) as a preferred substrate and represents the only known example of a specific MTI phosphorylase (MTIP). MTIP does not utilize 5'-methylthioadenosine (MTA). Inosine is a weak substrate with a k(cat)/K(m) value 290-fold less than MTI and is the second best substrate identified. The crystal structure of P. aeruginosa MTIP (PaMTIP) in complex with hypoxanthine was determined to 2.8 Å resolution and revealed a 3-fold symmetric homotrimer. The methylthioribose and phosphate binding regions of PaMTIP are similar to MTAPs, and the purine binding region is similar to that of purine nucleoside phosphorylases (PNPs). The catabolism of MTA in P. aeruginosa involves deamination to MTI and phosphorolysis to hypoxanthine (MTA → MTI → hypoxanthine). This pathway also exists in Plasmodium falciparum, where the purine nucleoside phosphorylase (PfPNP) acts on both inosine and MTI. Three tight-binding transition state analogue inhibitors of PaMTIP are identified with dissociation constants in the picomolar range. Inhibitor specificity suggests an early dissociative transition state for PaMTIP. Quorum sensing molecules are associated with MTA metabolism in bacterial pathogens suggesting PaMTIP as a potential therapeutic target.

Bacterial variations on the methionine salvage pathway

BACKGROUND

The thiomethyl group of S-adenosylmethionine is often recycled as methionine from methylthioadenosine. The corresponding pathway has been unravelled in Bacillus subtilis. However methylthioadenosine is subjected to alternative degradative pathways depending on the organism.

RESULTS

This work uses genome in silico analysis to propose methionine salvage pathways for Klebsiella pneumoniae, Leptospira interrogans, Thermoanaerobacter tengcongensis and Xylella fastidiosa. Experiments performed with mutants of B. subtilis and Pseudomonas aeruginosa substantiate the hypotheses proposed. The enzymes that catalyze the reactions are recruited from a variety of origins. The first, ubiquitous, enzyme of the pathway, MtnA (methylthioribose-1-phosphate isomerase), belongs to a family of proteins related to eukaryotic intiation factor 2B alpha. mtnB codes for a methylthioribulose-1-phosphate dehydratase. Two reactions follow, that of an enolase and that of a phosphatase. While in B. subtilis this is performed by two distinct polypeptides, in the other organisms analyzed here an enolase-phosphatase yields 1,2-dihydroxy-3-keto-5-methylthiopentene. In the presence of dioxygen an aci-reductone dioxygenase yields the immediate precursor of methionine, ketomethylthiobutyrate. Under some conditions this enzyme produces carbon monoxide in B. subtilis, suggesting a route for a new gaseous mediator in bacteria. Ketomethylthiobutyrate is finally transaminated by an aminotransferase that exists usually as a broad specificity enzyme (often able to transaminate aromatic aminoacid keto-acid precursors or histidinol-phosphate).

CONCLUSION

A functional methionine salvage pathway was experimentally demonstrated, for the first time, in P. aeruginosa. Apparently, methionine salvage pathways are frequent in Bacteria (and in Eukarya), with recruitment of different polypeptides to perform the needed reactions (an ancestor of a translation initiation factor and RuBisCO, as an enolase, in some Firmicutes). Many are highly dependent on the presence of oxygen, suggesting that the ecological niche may play an important role for the existence and/or metabolic steps of the pathway, even in phylogenetically related bacteria. Further work is needed to uncover the corresponding steps when dioxygen is scarce or absent (this is important to explore the presence of the pathway in Archaea). The thermophile T. tengcongensis, that thrives in the absence of oxygen, appears to possess the pathway. It will be an interesting link to uncover the missing reactions in anaerobic environments.

Structural analyses reveal two distinct families of nucleoside phosphorylases

The reversible phosphorolysis of purine and pyrimidine nucleosides is an important biochemical reaction in the salvage pathway, which provides an alternative to the de novo purine and pyrimidine biosynthetic pathways. Structural studies in our laboratory and by others have revealed that only two folds exist that catalyse the phosphorolysis of all nucleosides, and provide the basis for defining two families of nucleoside phosphorylases. The first family (nucleoside phosphorylase-I) includes enzymes that share a common single-domain subunit, with either a trimeric or a hexameric quaternary structure, and accept a range of both purine and pyrimidine nucleoside substrates. Despite differences in substrate specificity, amino acid sequence and quaternary structure, all members of this family share a characteristic subunit topology. We have also carried out a sequence motif study that identified regions of the common subunit fold that are functionally significant in differentiating the various members of the nucleoside phosphorylase-I family. Although the substrate-binding sites are arranged similarly for all members of the nucleoside phosphorylase-I family, a comparison of the active sites from the known structures of this family indicates significant differences between the trimeric and hexameric family members. Sequence comparisons also suggest structural identity between the nucleoside phosphorylase-I family and both 5'-methylthioadenosine/S-adenosylhomocysteine nucleosidase and AMP nucleosidase. Members of the second family of nucleoside phosphorylases (nucleoside phosphorylase-II) share a common two-domain subunit fold and a dimeric quaternary structure, share a significant level of sequence identity (>30%) and are specific for pyrimidine nucleosides. Members of this second family accept both thymidine and uridine substrates in lower organisms, but are specific for thymidine in mammals and other higher organisms. A possible relationship between nucleoside phosphorylase-II and anthranilate phosphoribosyltransferase has been identified through sequence comparisons. Initial studies in our laboratory suggested that members of the nucleoside phosphorylase-II family require significant domain movements in order for catalysis to proceed. A series of recent structures has confirmed our hypothesis and provided details of these conformational changes. Structural studies of the nucleoside phosphorylases have resulted in a wealth of information that begins to address fundamental biological questions, such as how Nature makes use of the intricate relationships between structure and function, and how biological processes have evolved over time. In addition, the therapeutic potential of suppressing the nucleoside phosphorylase activity in either family of enzymes has motivated efforts to design potent inhibitors. Several research groups have synthesized a variety of nucleoside phosphorylase inhibitors that are at various stages of preclinical and clinical evaluation.

Purification and characterization of Plasmodium falciparum hypoxanthine-guanine-xanthine phosphoribosyltransferase and comparison with the human enzyme

The human malaria parasite Plasmodium falciparum is auxotrophic for purines and relies on the purine salvage pathway for the synthesis of its purine nucleotides. Hypoxanthine-guanine-xanthine phosphoribosyltransferase (HGXPRT) is a key purine salvage enzyme in P. falciparum, making it a potential target for chemotherapy. Previous attempts to purify this enzyme have been unsuccessful because of the difficulty in obtaining cultured parasite material and because of the inherent instability of the enzyme during purification and storage. Other groups have tried to express recombinant P. falciparum HGXPRT but only small amounts of activity were obtained. The successful expression of recombinant P. falciparum HGXPRT in Escherichia coli has now been achieved and the enzyme purified to homogeneity in mg quantities. The measured molecular mass of 26 229+/-2 Da is in excellent agreement with the calculated value of 26232 Da. A method to stabilise the activity and to reactivate inactive samples has been developed. The subunit structure of P. Jilciparum HGXPRT has been determined by ultracentrifugation in the absence (tetramer) and presence (dimer) of KC1. Kinetic constants were determined for 5-phospho-alpha-D-ribosyl-1-pyrophosphate, for the three naturally-occurring 6-oxopurine bases guanine, hypoxanthine, and xanthine and for the base analogue, allopurinol. Differences in specificity between the purified P. falciparum HGXPRT and human hypoxanthine guanine phosphoribosyltransferase enzymes were detected which may be able to be exploited in rational drug design.

Deficiency of 5'-deoxy-5'-methylthioadenosine phosphorylase activity in malignancy. Absence of the protein in human enzyme-deficient cell lines

The absence of 5'-deoxy-5'-methylthioadenosine phosphorylase (MTAase) activity in malignant cells, and the putative localization of its gene, suggest that this enzyme deficiency might be due to a genomic alteration also involving a tumour-suppressor gene. We studied the possible occurrence of inactive forms of the protein in two MTAase-negative cell lines, namely K562 and Jurkat, by immunochemical methods. Two highly specific antisera, directed against different epitopes of the phosphorylase [Della Ragione, Oliva, Gragnaniello, Russo, Palumbo & Zappia (1990) J. Biol. Chem. 265, 6241-6246], were used to carry out immunotitration and immunoblotting analyses, as well as to investigate the biosynthesis of the enzyme. No MTAase protein was detected by Western-blotting technique performed under conditions where all the phosphorylase-positive samples gave a clear band at the MTAase subunit molecular mass. No cross-reacting material was observed by a sensitive immunotitration method which permitted the detection of as low as 0.5 ng of protein. Moreover, the results obtained by [35S]methionine-labelling experiments ruled out phosphorylase biosynthesis in the negative cell lines. Altogether, these data suggest that an alteration at the gene level hampering the specific mRNA biosynthesis or resulting in an untranslatable mRNA is the cause of the enzyme deficiency in the MTAase-negative cell lines studied.

Purification and characterization of 5'-deoxy-5'-methylthioadenosine (MTA) phosphorylase from human liver

5'-Methylthioadenosine phosphorylase was purified 8000-fold from human liver using a combination of affinity chromatography, chromatofocusing and gel filtration. A 25% overall yield was obtained. The specific activity of the final preparation was 40 mumol of 5'-methylthioadenosine cleaved per hr per mg of protein. The enzyme had an apparent molecular weight of 55,000 daltons, as determined by gel filtration of Superose 12 and Sephadex G-150, with a subunit molecular weight of 30,000 daltons, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The substrate specificity of the purified enzyme was studied in both the direction of nucleoside cleavage and nucleoside synthesis.

Chromatographic and radioimmunological methods for the determination of 5'-deoxy-5'-methylthioadenosine in biological fluids

Two specific methods for the determination of 5'-deoxy-5'-methylthioadenosine (MTA) in biological samples have been developed. The chromatographic procedure requires a preliminary step on a phenylboronate column to remove non-cis-diol compounds. The sample is then analysed using a high-performance liquid chromatography system equipped with a reversed-phase column. 5'-Deoxy-5'-methyl-thio[2-3H]adenosine with high specific activity was synthesized and employed as an internal standard. An alternative radioimmunoassay (RIA) procedure has also been developed. The RIA method is based on competition between the unlabelled thio-ether and 3H-labelled MTA for the binding to a specific antiserum. Anti-MTA antibodies were obtained from rabbits immunized with the nucleoside covalently linked to carrier proteins. Both the chromatographic and RIA procedures gave identical results when employed to determine MTA in human urine.

5'-deoxy-5'-methylthioadenosine phosphorylase--IV. Biological activity of 2-fluoroadenine-substituted 5'-deoxy-5'-methylthioadenosine analogs

5'-Deoxy-5'-methylthioadenosine phosphorylase (MTAPase) phosphorolyzes 5'-deoxy-5'-methylthioadenosine (MTA) generated during polyamine biosynthesis to adenine and 5-methylthioribose-1-phosphate. Two doubly-substituted, 2-fluoroadenine-containing analogs of MTA, 5'-deoxy-2-fluoroadenosine (5'-dFAdo) and 5'-deoxy-5'-iodo-2-fluoroadenosine (5'-IFAdo), were synthesized and studied as substrates of MTAPase: their reaction with this enzyme resulted in the liberation of the cytotoxic base, 2-fluoroadenine, as well as potentially cytotoxic analogs of 5-methylribose-1-phosphate. The activities of these MTA analogs were compared to that of the singly-substituted analog, 5'-deoxy-5'-methylthio-2-fluoroadenosine (5'-MTFAdo). The cytotoxic action of these MTA analogs depended primarily on their conversion to 2-fluoroadenine-containing nucleotides, as a cell line that contains both MTAPase and adenine phosphoribosyltransferase (APRT) activity (HL-60 human promyelocytic leukemia) readily converted these MTA analogs to 2-fluoroadenine-containing nucleotides (especially 2-fluoroadenosine triphosphate) and was highly sensitive to the growth-inhibitory effects of all three compounds (IC50 values in the 10(-8) M range), whereas cell lines lacking MTAPase (CCRF-CEM human T-cell leukemia) or APRT (HL-60/aprt1 cells) did not form analog nucleotides and were relatively insensitive to these compounds (IC50 values in the 10(-5) M range). The doubly-substituted analogs were not more growth inhibitory than 5'-MTFAdo in wild type HL-60 cells as the potent effects of 2-fluoroadenine may mask the activity of the 5-methylthioribose-1-phosphate analogs generated in the reaction of these compounds with MTAPase. 5'-dFAdo and 5'-IFAdo also were irreversible inhibitors of S-adenosylhomocysteine hydrolase, which may explain in part the weak but observable growth inhibitory action of these compounds against MTAPase-deficient cell lines.

Methylthioadenosine phosphorylase in human breast cancer

Methylthioadenosine (MTA) phosphorylase activity was measured in 47 biopsies from primary breast cancers (n = 34) and metastatic tumors (n = 13). Most specimens were also evaluated by DNA flow cytometry and determination of estrogen and progesterone receptor contents. Median MTA phosphorylase activity was 317 pmol/mg protein/min (range 50-1312 pmol/mg protein/min), but great variations were observed. Samples from four individuals had very low MTA phosphorylase activity (less than or equal to 70 pmol/mg protein/min). No correlation with aneuploidy, receptor status, or the presence of metastases in the lymph nodes could be demonstrated. However, MTA phosphorylase activity showed a significant (p = 0.009) negative correlation with the fraction of cells in the S-phase of the cell cycle.

Purification and characterization of propylamine transferase from Sulfolobus solfataricus, an extreme thermophilic archaebacterium

The enzyme propylamine transferase, catalyzing the transfer of the propylamine moiety from S-adenosyl(5')-3-methylthiopropylamine to several amine acceptors, has been purified 643-fold in 20% yield from Sulfolobus solfataricus, an extreme thermophilic archaebacterium optimally growing at 87 degrees C. The purified enzyme (specific activity 2.05 units/mg protein), is homogeneous by criteria of gel electrophoresis, gel filtration, isoelectric focusing and ultracentrifugation analysis. The molecular mass of the native enzyme was estimated to be about 110 kDa by gel permeation and ultracentrifugation analysis. The protein migrates on SDS/polyacrylamide gel electrophoresis as a single band of 35 kDa, suggesting that the enzyme is a trimer composed by identical subunits. An optimum pH of 7.5 and an acidic isoelectric point of 5.3 have been calculated. The optimum temperature was 90 degrees C and no loss of activity is observable even after exposure of the purified enzyme to 100 degrees C for 1 h. No reducing agents are required for enzymatic activity. Substrate specificity towards the amine acceptors is rather broad in that 1,3-diaminopropane (Km = 1675 microM), putrescine (Km = 3850 microM), sym-norspermidine (Km = 954 microM) and spermidine (Km = 1539 microM) are recognized as substrates. Conversely S-adenosyl(5')-3-methylthiopropylamine is the only propylamine donor (Km = 7.9 microM) and the deamination of the sulfonium compound prevents the recognition by the enzyme. The reaction is irreversible and initial-rate kinetic studies indicate that the propylamine transfer is operated through a sequential mechanism. 5'-Methylthioadenosine, a product of the reaction, acts as a powerful competitive inhibitor with a Ki of 3.7 microM. Enzyme-substrate binding sites have been investigated with the aid of several substrate analogs and products. Among the compounds assayed, 5'-methylthiotubercidin, S-adenosyl(5')-3-thiopropylamine and S-adenosyl-3-thio-1,8-diaminooctane are the most active inhibitors.

Regional pattern and heat-resistance of brain 5'-deoxy-5'-methylthioadenosine phosphorylase

The distribution of 5'-deoxy-5'-methylthioadenosine (MTA) phosphorylase in 10 pig brain areas was determined. The observed regional differences of the enzymatic activity seem to reflect more the pattern of brain spermine distribution rather than that of spermidine. Moreover, comparative studies on the heat-resistance of MTA phosphorylase extracted from the whole brain of various species suggest structural differences in the enzyme molecules occurring in the brains of different animals.

Escherichia coli S-adenosylhomocysteine/5'-methylthioadenosine nucleosidase. Purification, substrate specificity and mechanism of action

S-Adenosylhomocysteine/5'-methylthioadenosine nucleosidase (EC 3.2.2.9) was purified to homogeneity from Escherichia coli to a final specific activity of 373 mumol of 5'-methylthioadenosine cleaved/min per mg of protein. Affinity chromatography on S-formycinylhomocysteine-Sepharose is the key step of the purification procedure. The enzyme, responsible for the cleavage of the glycosidic bond of both S-adenosylhomocysteine and 5'-methylthioadenosine, was partially characterized. The apparent Km for 5'-methylthioadenosine is 0.4 microM, and that for S-adenosylhomocysteine is 4.3 microM. The maximal rate of cleavage of S-adenosylhomocysteine is approx. 40% of that of 5'-methylthioadenosine. Some 25 analogues of the two naturally occurring thioethers were studied as potential substrates or inhibitors of the enzyme. Except for the analogues modified in the 5'-position of the ribose moiety or the 2-position of the purine ring, none of the compounds tested was effective as a substrate. Moreover, 5'-methylthioformycin, 5'-chloroformycin, S-formycinylhomocysteine, 5'-methylthiotubercidin and S-tubercidinylhomocysteine were powerful inhibitors of the enzyme activity. The results obtained allow the hypothesis of a mechanism of enzymic catalysis requiring as a key step the protonation of N-7 of the purine ring.

5'-Deoxy-5'-methylthioadenosine phosphorylase--III. Role of the enzyme in the metabolism and action of 5'-halogenated adenosine analogs

5'-Deoxy-5'-halogenated adenosines are alternative substrates for 5'-deoxy-5'-methylthioadenosine phosphorylase (MTAPase), an enzyme responsible for the metabolism of 5'-deoxy-5'-methylthioadenosine (MTA), a by-product of polyamine biosynthesis. The relative reactivity of these nucleosides with MTAPase from HL-60 human promyelocytic leukemia cells is MTA greater than 5'-deoxy-5'-fluoroadenosine (5'-FlAdo) greater than 5'-chloro-5'-deoxyadenosine (5'-ClAdo) greter than 5'-bromo-5'-deoxyadenosine (5'-BrAdo) greater than 5'-deoxy-5'-iodoadenosine (5'-IAdo). In MTAPase-containing cells, the adenine released from the 5'-halogenated adenosine was incorporated into adenine nucleotide pools; cleavage by (MTAPase appeared to be the rate-limiting step in this process. 5'-BrAdo and 5'-IAdo were growth inhibitors (EC50 values less than 10 microM) of MTAPase-containing cell lines (HL-60 human promyelocytic leukemia and the L5178Y murine lymphoblastic leukemia) but were much less active (EC50 values greater than 65 microM) against MTAPase-deficient cell lines (the CCRF-CEM human T cell leukemia and the L1210 murine leukemia). The full cytotoxicity of these compounds, therefore, appeared to be related to their phosphorolysis by MTAPase. Indirect evidence suggests that 5-halogenated ribose-1-phosphate derivatives of 5'-BrAdo or 5'-IAdo produced by the MTAPase reaction were the active metabolites of these 5'-halogenated adenosines.

Kinetic properties of spermine synthase from bovine brain

A kinetic analysis including initial-velocity and product-inhibition studies were performed with spermine synthase purified from bovine brain. The enzyme activity was assayed in the presence of 5'-methylthioadenosine phosphorylase as an auxiliary enzyme to prevent the accumulation of the inhibitory product, 5'-methylthioadenosine, and thus to obtain linearity of the reaction with time. Initial-velocity studies gave intersecting or converging linear double-reciprocal plots. No substrate inhibition by decarboxylated S-adenosylmethionine was observed at concentrations up to 0.4 mM. Apparent Michaelis constants were 60 microM for spermidine and 0.1 microM for decarboxylated S-adenosylmethionine. Spermine was a competitive product inhibitor with respect to decarboxylated S-adenosylmethionine, but a mixed one with respect to the other substrate, spermidine. 5'-Methylthioadenosine showed a mixed inhibition with both substrates, predominantly competitive with respect to decarboxylated S-adenosylmethionine and predominantly uncompetitive with respect to spermidine. The observed kinetic and inhibition patterns are consistent with a compulsory-order mechanism, where both substrates add to the enzyme before products can be released.

Comparison of inhibitors of S-adenosylmethionine decarboxylase from different species

S-Adenosyl-L-methionine decarboxylases were purified from rat ventral prostate, yeast (Saccharomyces cerevisiae), slime mould (Physarum polycephalum) and bacteria (Escherichia coli) and tested for inhibition by a variety of nucleosides related to S-adenosylmethionine and by methyl- and ethyl-glyoxal bis(guanylhydrazone). Although the enzymes from these different sources are markedly different with respect to activation by cations, the inhibition by nucleosides was quite similar. Very little inhibition was seen when analogues of S-adenosylmethionine with a different base were tested or when the ribose ring was opened or the positive charge on the sulphur atom was not present. Some derivatives in which the amino acid portion of the molecule was altered were more potent inhibitors, but again there was little difference between the enzymes from different sources. 5'-(Dimethylsulphonio)-5'-deoxyadenosine and S-adenosyl-3-methylthiopropylamine were the most inhibitory substances and had similar Ki values, suggesting that the aminopropyl group does not contribute significantly to the binding. All of the S-adenosylmethionine decarboxylases were strongly competitively inhibited by methylglyoxal bis(guanylhydrazone) and even more powerfully by its ethyl analogue, although the putrescine-activated enzymes from prostate and yeast were more sensitive than the bacterial and slime-mould enzymes. All of the S-adenosylmethionine decarboxylases tested bound to a column of methylglyoxal bis(guanylhydrazone) linked to Sepharose and were not eluted by 0.5 M-NaCl, but could be released by 1 mM concentrations of the drug, providing a rapid and efficient method for their purification.

5'-deoxy-5'-methylthioadenosine phosphorylase--II. Role of the enzyme in the metabolism and antineoplastic action of adenine-substituted analogs of 5'-deoxy-5'-methylthioadenosine

The biological activities of several previously synthesized [J. A. Montgomery et al., J. med. Chem. 17, 1197 (1974)] adenine-substituted analogs of 5'-deoxy-5'-methylthio- or 5'-deoxy-5'-ethyl-thioadenosine, including the 2-fluoroadenine, 2-chloroadenine, 2,6-diaminopurine, 8-azaadenine, and 4-aminopyrazolo [3,4-d]pyrimidine-containing derivatives, have been reexamined. It is demonstrated that many of these analogs are cleaved to their respective free base analogs by 5'-deoxy-5'-methyl-thioadenosine phosphorylase (MTAPase), an enzyme associated with polyamine biosynthesis, and that this reaction is necessary for the cytotoxic action of these MTA analogs to be fully expressed. Evidence to support this includes: (1) the growth of two MTAPase-containing human colon carcinoma cell lines (the HCT-15 and DLD-1 lines) was inhibited by these analogs, whereas an MTAPase-deficient cell line, the CCRF-CEM human T-cell leukemia, was relatively insensitive to their cytotoxic action; (2) extracts of the MTAPase-containing colon carcinoma cell lines were able to cleave these analogs to their respective free base analogs; in contrast, extracts of MTAPase-deficient CCRF-CEM cells were unable to cleave these analogs; (3) intact colon carcinoma cells converted these MTA analogs to their corresponding 5'-phosphorylated analog nucleotides, whereas CCRF-CEM cells did not, at least to detectable levels; and (4) the MTA analog, 5'-deoxy-5'-ethylthio-4-aminopyrazolo [3,4-d]pyrimidine ribonucleoside, which is not a substrate of MTAPase, did not form analog nucleotides and was essentially noncytotoxic to all cell lines tested, whereas the corresponding adenine analog, 4-aminopyrazolo [3,4-d]pyrimidine, readily formed analog nucleotides and was highly cytotoxic to all the lines. It is postulated that the corresponding adenine analog 5'-phosphorylated nucleotides are the primary active metabolites of these MTA analogs, having been formed by the cleavage of these nucleosides to free adenine analogs by MTAPase, followed by the conversion of these base analogs to analog nucleotides by adenine phosphoribosyltransferase and the enzymes of adenine nucleotide phosphorylation. This pathway represents a novel drug-activation system for the synthesis of analog nucleotides and has the potential to be exploited chemotherapeutically.

Methylthioadenosine phosphorylase activity in human erythrocytes

An enzyme capable of degrading 5'-methylthioadenosine to adenine was found in the human erythrocyte. A rapid assay for this enzyme, 5'-methylthioadenosine phosphorylase, was developed using high pressure liquid chromatography. The specific activity in 24 normal subjects was 8.9 +/- 2.0 nmol . mg-1 Hb . h-1. Levels within this range were also found in erythrocyte lysates from gouty subjects and patients with a variety of inborn errors of purine metabolism, including patients with a complete deficiency of the adenine salvage enzyme--adenine phosphoribosyltransferase. Erythrocyte lysates from the latter however, were unable to convert the adenine produced to AMP in a linked assay system, in contrast to controls and other patients. These results support the suggestion that adenine, which is excreted in quantity by patients with adenine phosphoribosyltransferase deficiency is derived endogenously from 5'-methylthioadenosine as a by-product of polyamine biosynthesis.

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

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

Transport and metabolism of 5'-methylthioadenosine in human erythrocytes

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

Uptake and utilization of 5'-methylthioadenosine by cultured baby-hamster kidney cells

5'-Methylthioadenosine was taken up and immediately metabolized further by cultured baby-hamster kidney cells during the exponential phase of growth. The adenine moiety supplied the purine-nucleotide pool via the salvage pathway and was efficiently incorporated into nucleic acids. Catabolites of methylthioadenosine excreted by the cells included adenine, purinic compounds and metabolites of the ribose portion. 5'-Methylthiotubercidin had no significant effect on the cellular metabolism of methyl-thioadenosine, but greatly inhibited its uptake. erythro-9-(2-Hydroxy-3-nonyl)adenine had no effect on the uptake, but markedly interfered with the further utilization of methylthioadenosine after cleavage in the cells.

Inhibition of the synthesis of polyamines and macromolecules by 5'-methylthioadenosine and 5'-alkylthiotubercidins in BHK21 cells

5'-Methylthioadenosine and four 5'-alkylthiotubercidins were tested for their ability to inhibit polyamine synthesis in vitro and to decrease polyamine concentration and prevent growth of baby-hamster-kidney (BHK21) cells. 5'-Methylthioadenosine and 5'-methylthiotubercidin decreased the activity of spermidine synthase from brain to roughly the same extent, whereas brain spermine synthase was much more strongly inhibited by 5'-methylthioadenosine compared with 5'-methylthiotubercidin. These nucleoside derivatives also inhibited the growth of BHK21 cells and increased the concentration of putrescine. 5'-Methylthioadenosine decreased cellular spermine concentration, whereas 5'-methylthiotubercidin lowered the concentration of spermidine. The activities of ornithine decarboxylase and S-adenosylmethionine decarboxylase were enhanced in cells grown in the presence of 5'-methylthiotubercidin. The growth inhibition produced by these nucleoside derivatives was not reversed by exogenous spermidine or spermine. 5'-Ethylthiotubercidin, 5'-propylthiotubercidin and 5'-isopropylthiotubercidin did not appreciably inhibit spermidine or spermine synthase in vitro or decrease the cellular polyamine content, but effectively prevented the growth of BHK21 cells. All nucleoside derivatives at concentrations of 0.2-1 mm caused a rapid inhibition of protein synthesis. It is concluded that the growth inhibition produced by 5'-methylthioadenosine and 5'-alkylthiotubercidins was not primarily due to polyamine depletion but other target sites, for instance the cellular nucleotide pool, cell membranes etc. must be considered.

Dependence of adenine production upon polyamine synthesis in cultured human lymphoblasts

The exact source of de novo adenine produced by mammalian cells remain poorly understood, and this prompted the present study. Using a human lymphoblastoid cell line (WI-L2) deficient in adenine phosphoribosyltransferase (EC 2.4.2.7), we have quantitated the rate of adenine synthesis and the relative importance of the phosphorolysis of 5'-methylthioadenosine versus adenosine or 2'-deoxyadenosine in adenine generation. Dividing adenine phosphoribosyltransferase-deficient WI-L2 cells produced adenine at a rate of 0.27 nmol/mg protein/h. This represented approximately 10% of the rate of hypoxanthine production by WI-L2 cells deficient in hypoxanthine phosphoribosyltransferase (EC 2.4.2.8) but was equivalent to the rate of 5'-methylthioadenosine synthesis by human lymphoblastoid CCRF-CEM deficient in 5'-methylthioadenosine, phosphorylase (5'-methylthioadenosine: orthophosphate methylthioribosyltransferase). Up to 97% of adenine, but not hypoxanthine, synthesis was inhibited dose-dependently by the S-adenosylmethionine decarboxylase-inhibitor methylglyoxal bis(guanylhydrazone) and also by spermidine and spermine, but was enhanced by putrescine. The addition of 2-fluoroadenine, a potent competitive inhibitor of methylthioadenosine phosphorylase (Ki = 0.43 microM) to adenine phosphoribosyl-transferase-deficient cells resulted in a progressive accumulation of 5'-methylthioadenosine in the culture medium, and up to an 85% decrease in adenine production at non-toxic concentrations. These results show that de novo adenine synthesis by dividing human cells is considerable, and that 85-97% derives from the cleavage of 5'-methylthioadenosine and hence from polyamine synthesis.

Studies of inhibition of rat spermidine synthase and spermine synthase

1. S-Adenosyl-l-methionine, S-adenosyl-l-homocysteine, 5'-methylthioadenosine and a number of analogues having changes in the base, sugar or amino acid portions of the molecule were tested as potential inhibitors of spermidine synthase and spermine synthase from rat ventral prostate. 2. S-Adenosyl-l-methionine was inhibitory to these reactions, as were other nucleosides containing a sulphonium centre. The most active of these were S-adenosyl-l-ethionine, S-adenosyl-4-methylthiobutyric acid, S-adenosyl-d-methionine and S-tubercidinylmethionine, which were all comparable in activity with S-adenosylmethionine itself, producing 70-98% inhibition at 1mm concentrations. Spermine synthase was somewhat more sensitive than spermidine synthase. 3. 5'-Methylthioadenosine, 5'-ethylthioadenosine and 5'-methylthiotubercidin were all powerful inhibitors of both enzymes, giving 50% inhibition of spermine synthase at 10-15mum and 50% inhibition of spermidine synthase at 30-45mum. 4. S-Adenosyl-l-homocysteine was a weak inhibitor of spermine synthase and practically inactive against spermidine synthase. Analogues of S-adenosylhomocysteine lacking either the carboxy or the amino group of the amino acid portion were somewhat more active, as were derivatives in which the ribose ring had been opened by oxidation. The sulphoxide and sulphone derivatives of decarboxylated S-adenosyl-l-homocysteine and the sulphone of S-adenosyl-l-homocysteine were quite potent inhibitors and were particularly active against spermidine synthase (giving 50% inhibition at 380, 50 and 20mum respectively). 5. These results are discussed in terms of the possible regulation of polyamine synthesis by endogenous nucleosides and the possible value of some of the inhibitory substances in experimental manipulations of polyamine concentrations. It is suggested that 5'-methylthiotubercidin and the sulphone of S-adenosylhomocysteine or of S-adenosyl-3-thiopropylamine may be particularly valuable in this respect.

5'-Methylthioadenosine phosphorylase from Caldariella acidophila. Purification and properties

The occurrence of 5'-methylthioadenosine phosphorylase in Caldariella acidophila, a thermophilic bacterium growing optimally at 87 degrees C, is reported. It represents the first example in prokaryotes of a phosphoryolytic cleavage of the thioether. The reaction products, purified by ion-exchange chromatography, have been identified as 5-methylthioribose-1-phosphate and adenine by several analytical procedures. The enzyme has been purified to homogeneity in 32% yield by using DEAE-cellulose and hydroxyapatite chromatography, gel filtration and isoelectric focusing. The enzyme shows a high degree of thermophilicity, its temperature optimum being at 93 degrees C; furthermore no loss of activity is observable after exposure for 1 h at 100 degrees C. The kinetic data indicate a sequential mechanism of the reaction. The apparent Km values are 0.095 mM for 5'-methylthioadenosine and 6.1 mM for phosphate. The specificity of the reaction is rather strict. Experiments performed with analogues of the substrate, i.e. 5'-methylthioinosine, 5'-dimethylthioadenosine sulfonium salt, 5'-n-butylthioadenosine, 5'-isobutylthioadenosine, 5'-isobutylthioinosine, adenosylhomocysteine, 5'-thioethanoladenosine, adenosine, indicate the relevance of the adenine amino group and the sulfur in thioether form in the binding to the enzyme protein.