Enzyme defect associated with a sex-linked human neurological disorder and excessive purine synthesis

A sex-linked familial neurological disease consisting of cerebral palsy, mental retardation, choreoathetosis, and compulsive aggressive behavior is associated with a loss of an enzyme that participates in purine metabolism, namely, hypoxanthine-guanine phosphoribosyltransferase. The production of excessive uric acid in this disorder implies that the enzyme is involved in the normal regulation of purine biosynthesis. This is the first example of a relation between a specific enzyme defect and abnormal compulsive behavior. It is also the first enzyme defect in purine metabolism demonstrated in a neurological disease.

Synthesis of guanosine tetra- and pentaphosphate requires the presence of a codon-specific, uncharged transfer ribonucleic acid in the acceptor site of ribosomes

The nucleotides ppGpp and pppGpp accumulate when wild-type, but not rel(-), strains of Escherichia coli are starved for required amino acids. These compounds are synthesized on ribosomes, in the presence of the product of the rel gene, from GDP and GTP; ATP is used as the phosphate donor. The signal for making these compounds is the presence of an uncharged tRNA in the ribosomal acceptor site. These compounds are not accumulated if the ribosomes are actively engaged in protein synthesis.

The inhibition of nucleic acid synthesis by mycophenolic acid

1. Mycophenolic acid, an antibiotic of some antiquity that more recently has been found to have marked activity against a range of tumours in mice and rats, strongly inhibits DNA synthesis in the L strain of fibroblasts in vitro. 2. The extent of the inhibition of DNA synthesis is markedly increased by preincubation of the cells with mycophenolic acid before the addition of [(14)C]thymidine. 3. The inhibition of DNA synthesis by mycophenolic acid in L cells in vitro is reversed by guanine in a non-competitive manner, but not by hypoxanthine, xanthine or adenine. 4. The reversal of inhibition by guanine can be suppressed by hypoxanthine, 6-mercaptopurine and adenine. 5. Mycophenolic acid does not inhibit the incorporation of [(14)C]thymidine into DNA in suspensions of Landschütz and Yoshida ascites cells in vitro. 6. Mycophenolic acid inhibits the conversion of [(14)C]hypoxanthine into cold-acid-soluble and -insoluble guanine nucleotides in Landschütz and Yoshida ascites cells and also in L cells in vitro. There is some increase in the radioactivity of the adenine fraction in the presence of the antibiotic. 7. Mycophenolic acid inhibits the conversion of [(14)C]hypoxanthine into xanthine and guanine fractions in a cell-free system from Landschütz cells capable of converting hypoxanthine into IMP, XMP and GMP. 8. Preparations of IMP dehydrogenase from Landschütz ascites cells, calf thymus and LS cells are strongly inhibited by mycophenolic acid. The inhibition showed mixed type kinetics with K(i) values of between 3.03x10(-8) and 4.5x10(-8)m. 9. Evidence was also obtained for a partial, possibly indirect, inhibition by mycophenolic acid of an early stage of biosynthesis of purine nucleotides as indicated by a decrease in the accumulation of formylglycine amide ribonucleotide induced by the antibiotic azaserine in suspensions of Landschütz and Yoshida ascites cells and L cells in vitro.

Acyl carrier protein/SpoT interaction, the switch linking SpoT-dependent stress response to fatty acid metabolism

Bacteria respond to nutritional stresses by producing an intracellular alarmone, guanosine 5'-(tri)diphosphate, 3'-diphosphate [(p)ppGpp], which triggers the stringent response resulting in growth arrest and expression of resistance genes. In Escherichia coli, upon fatty acid or carbon starvation, SpoT enzyme activity switches from (p)ppGpp degradation to (p)ppGpp synthesis, but the signal and mechanism for this response remain totally unknown. Here, we characterize for the first time a physical interaction between SpoT and acyl carrier protein (ACP) using affinity co-purifications and two-hybrid in E. coli. ACP, as a central cofactor in fatty acid synthesis, may be an ideal candidate as a mediator signalling starvation to SpoT. Accordingly, we show that the ACP/SpoT interaction is specific of SpoT and ACP functions because ACP does not interact with the homologous RelA protein and because SpoT does not interact with a non-functional ACP. Using truncated SpoT fusion proteins, we demonstrate further that ACP binds the central TGS domain of SpoT, consistent with a role in regulation. The behaviours of SpoT point mutants that do not interact with ACP reveal modifications of the balance between the two opposite SpoT catalytic activities thereby changing (p)ppGpp levels. More importantly, these mutants fail to trigger (p)ppGpp accumulation in response to fatty acid synthesis inhibition, supporting the hypothesis that the ACP/SpoT interaction may be involved in SpoT-dependent stress response. This leads us to propose a model in which ACP carries information describing the status of cellular fatty acid metabolism, which in turn can trigger the conformational switch in SpoT leading to (p)ppGpp accumulation.

Pyrimidine starvation induced by adenosine in fibroblasts and lymphoid cells: role of adenosine deaminase

In the presence of 10(-4) to 10(-5) molar adenosine, established cell lines of fibroblastic or lymphoid origin die of pyrimidine starvation. Less than lethal concentrations inhibit cell growth. Over a broad concentration range, the effects of adenosine are prevented by providing a suitable pyrimidine source. We suggest that the recently described immune deficiency disease associated with absence of adenosine deaminase may be the result of pyrimidine starvation induced by adenosine nucleotides in cells of the lymphoid system.

The structure of inosine 5'-monophosphate dehydrogenase and the design of novel inhibitors

The enzyme IMPDH is a homotetramer of approximately 55 kDa subunits and consists of a (beta/alpha)(8) barrel core domain and a smaller subdomain. The active site has binding pockets for the two substrates IMP and NAD. The enzymatic reaction of oxidation of IMP to XMP proceeds through a covalent mechanism involving an active site cysteine residue. This enzyme is a target for immunosuppressive agents because it catalyzes a key step in purine nucleotide biosynthesis which is important for the proliferation of lymphocytes. Several X-ray structures of inhibitors bound to IMPDH have been published. The uncompetitive IMPDH inhibitor MPA is the active metabolite of the immunosuppressive agent mycophenolate mofetil (CellCept(R)) which is approved for the prevention of acute rejection after kidney and heart transplantation. The bicyclic ring system of MPA packs underneath the hypoxanthine ring of XMP*, thereby trapping this covalent intermediate of the enzymatic reaction. Ribavirin monophosphate, the active metabolite of the antiviral agent ribavirin, is a substrate mimic of IMP. The structure of the two inhibitors 6-Cl-IMP and SAD binding in the IMP and NAD pockets of IMPDH, respectively, gives information for the binding mode of the di-nucleotide cofactor to the enzyme. At Vertex Pharmaceuticals a structure-based drug design program for the design of IMPDH inhibitors was initiated. Several new lead compound classes unrelated to other IMPDH inhibitors were found. Integrating structural information into an iterative drug-design process led to the design of VX-497. VX-497 is a potent uncompetitive enzyme inhibitor of IMPDH. The phenyl-oxazole moiety of the molecule packs underneath XMP*, analogous to MPA. VX-497 also makes several new interactions that are not observed in the binding of MPA. VX-497 is a potent immunosuppressive agent in vitro and in vivo. A Phase I clinical trial has been successfully concluded and the compound is currently in Phase II trials in psoriasis and hepatitis C. The rapid progress from initiation of the drug design program to a compound entering clinical trials illustrates the power of structure-based drug design to accelerate the drug discovery process. The structural information on IMPDH has also significantly increased our knowledge about the mechanistic details of this fascinating enzyme.

Non-enzymatic template-directed synthesis on RNA random copolymers. Poly(C, U) templates

Poly(C, U) random copolymer templates direct the oligomerization of 2-MeImpG and 2-MeImpA, resulting in the production of a variety of oligo/(G,A)s. The efficiency of monomer incorporation into newly synthesized oligomers is greater for 2-MeImpG than for 2-MeImpA, and decreases for both monomers as the uracil content of the template increases. The relatively poor incorporation of adenine is partly due to an intrinsically less efficient incorporation reaction, and partly due to the masking of uracil sites by G X U non-complementary pairing. The efficiency of adenine incorporation can be improved by decreasing the concentration of 2-MeImpG and increasing the concentration of 2-MeImpA in the reaction mixture. The oligomeric product distribution can be characterized in detail using high-pressure liquid chromatography on an RPC-5 column. Oligomers are separated on the basis of chain length, base composition, and phospho-diester-linkage isomerism. The 3'----5' regiospecificity of monomer addition to template-bound oligomers is lower for 2-MeImpA than for 2-MeImpG. The presence of an adenine residue at the 2'(3') terminus of the acceptor strand lowers the regiospecificity of 2-MeImpA addition even further.

X-linked hypoxanthine-guanine phosphoribosyl transferase deficiency: heterozygote has two clonal populations

Clones of skin fibroblasts cultured from the mother of two sons with X-linked hypoxanthine-guanine phosphoribosyl transferase deficiency (Lesch-Nyhan syndrome) were assayed for activity of this enzyme by measurement of the incorporation of (3)H-guanine into guanylic acid as counts per minute per microgram of protein and by autoradiography. The demonstration of two populations of clones, wild-type clones with normal enzyme activity and mutant clones unable to incorporate (3)H-guanine, is evidence that the locus for hypoxanthineguanine phosphoribosyl transferase on one of the X chromosomes is inactive.

Biosynthetic regulation of individual proteins in relA+ and relA strains of Escherichia coli during amino acid starvation

An isogenic pair of Escherichia coli mutants (relA+ tufB valSts and relA1 tufB valSts) has been cultured at several temperatures to establish various degrees of limitation for valyl-tRNA synthetase. The biosynthetic rate of 16 identifiable proteins, most of which are components of the transcription and translation apparatus, was measured by pulse-labelling with [35S]-methionine, followed by protein separation using two-dimensional gel electrophoresis (O'Farrell, 1975). No single pattern of response to amino acid starvation of biosynthetic rate was observed. EF-Ts, L12 and S6 were found to be under strong stringent and relaxed regulation; EF-G, EF-Tu-A and S1 are under strong stringent, but weak relaxed regulation; EF-Tu-B, alpha, VRS, IRS and ARS are under week stringent and weak relaxed regulation; beta is under weak stringent regulation and does not respond at all to relaxed conditions; the biosynthetic rate of a protein called stringent starvation protein is strongly stimulated, relative to other proteins, in the starved stringent strain.

Identification of the synthesis of guanosine tetraphosphate (MS I) as insertion of a pyrophosphoryl group into the 3'-position in guanosine 5'-diphosphate

The phosphate transfer system of Haseltine et al., consisting of a ribosomal wash obtained from a stringent strain of Escherichia coli, washed ribosomes, GTP, and ATP, was used to prepare large quantities of guanosine tetra- and pentaphosphates, the magic spot compounds MS I and MS II of Cashel and Gallant. In our hands, the Haseltine et al. system yielded predominantly guanosine tetraphosphate, ppGpp. This system was used exclusively in the described experiments, with ATP labeled with (32)P in the beta- and gamma-positions as donor. The beta-label was found to produce a ppG[unk]p and the gamma-label a ppGp 0001000 1101011 0011100 0011100 1101011 0001000 0000000 0101100 1110010 0100001 0100001 0100001 0110010 0101100 0100000 0100000 1110000 . Furthermore, [(3)H]GDP + [gamma-(32)P]ATP yielded ppGpp in a (3)H:(32)P ratio of 1:1. The results indicate a transfer of the terminal pyrophosphoryl group of ATP as a unit. The position of the transferred pyrophosphoryl was assayed for by preparation of pG from ppGp with Zn(++)-activated inorganic pyrophosphatase from yeast. The pG was then assayed with 3'-nucleotidase, which liberated practically all the labeled phosphate. This result indicate that the phosphate transfer from ATP to GDP yields guanosine 5'-diphosphate-3'-diphosphate.

A new enzymatic assay for guanosine 3':5'-cyclic monophosphate and its application to the ductus deferens of the rat

A sensitive enzymatic procedure has been developed for the determination of guanosine 3':5'-cyclic monophosphate (cyclic GMP). It is based on the conversion of cyclic GMP to GMP by cyclic nucleotide phosphodiesterase and on the transfer of (32)P from [gamma-(32)P]ATP to GMP by the action of a specific ATP:GMP phosphotransferase (EC 2.7.4.8). The [(32)P]GDP is separated from the remaining [(32)P]ATP by enzymatic degradation of ATP by myosin and by precipitation of the (32)P(i) formed. The reaction blank, which is mostly caused by the nucleotide content of the enzymes, is doubled by about 0.1 pmol of cyclic GMP. The procedure has advantages in speed and/or accuracy over other methods in current use. Cyclic nucleotide concentrations were studied in the ductus deferens of the rat; two agents were used, carbachol and norepinephrine, which cause contraction. Incubation with 0.1 mM carbachol caused a 3-fold increase in cyclic GMP content, which was maximal about 2 min after carbachol addition. Cyclic AMP concentrations were not significantly changed. Addition of 0.01 mM norepinephrine increased cyclic GMP content by about 25% within 1 min and by 40% within 3 min; cyclic AMP concentrations were only slightly increased. A 3-min incubation with the phosphodiesterase inhibitor 1-methyl-3-isobutylxanthine (0.1 mM) doubled the cyclic GMP content and increased cyclic AMP concentration by 50%.

Stringent control of ribosomal protein gene expression in Escherichia coli

The regulation of the expression of ribosomal protein genes was examined during partial inhibition of tRNA aminoacylation in isogenic rel(+) and rel(-) strains of E. coli B carrying a temperature-sensitive mutation in valyl-tRNA synthetase (EC 6.1.1.9) gene. Measurements of the differential synthesis rate of ribosomal protein, alpha(r) (ribosomal protein synthesis rate/total protein synthesis rate), indicate that ribosomal protein synthesis is regulated directly or indirectly by availability of charged tRNA, and that the synthesis of ribosomal protein, like the synthesis of rRNA, is subject to the influence of the rel gene control system.

Interaction of alleles of the relA, relC and spoT genes in Escherichia coli: analysis of the interconversion of GTP, ppGpp and pppGpp

Mutants in the spo T gene have been isolated as stringent second site revertants of the relC mutation. These show varying degrees of the characteristics associated with the spoT1 gene, viz relative amount and absolute levels of both pppGpp and ppGpp and the decay rate of the latter. The entry of 3H-guanosine into GTP and ppGpr pools in spoT+ and spoT1 cells either growing exponentially or during amino acid starvation was determined, and the rate of ppGpp synthesis and its decay constant calculated. During exponential growth the ppGpp pool is 2-fold higher, its decay constant 10-fold lower, and its synthesis rate 5-fold lower in spoT- than in spoT+ cells; during amino acid starvation the ppGpp pool is 2-fold higher, its decay constant 20-fold lower, and its synthesis rate 10-fold lower in spoT than in spoT+ cells. In one of the "intermediate" spoT mutants the rate of entry of 3H-guanosine into GTP, ppGpp and pppGpp was measured during amino acid starvation. The data form the basis of a model for the interconversion of the guanosine nucleotides in which the flow is: GDP leads to GTP leads to pppGpp leads to ppGpp leads to Y. Calculations of the rates of synthesis and conversion of pppGpp and ppGpp under various conditions in various spoT+ and spoT- strains indicate that the ppGpp concentration indirectly controls the rate of pppGpp synthesis.

A rifampicin resistance mutation in the rpoB gene confers ppGpp-independent antibiotic production in Streptomyces coelicolor A3(2)

In Streptomyces coelicolor A3(2), deletion of relA or a specific mutation in rplK ( relC) results in an inability to synthesize ppGpp (guanosine 5'-diphosphate 3'-diphosphate) and impairs production of actinorhodin. We have found that certain rifampicin-resistant ( rif) mutants isolated from either relA or relC strains regain the ability to produce actinorhodin at the same level as the wild-type strain, although their capacity to synthesize ppGpp is unchanged. These rif mutants were found to have a missense mutation in the rpoB gene that encodes the RNA polymerase beta-subunit. This rpoB mutation was shown to be responsible for the observed changes in phenotype, as demonstrated by gene replacement experiments. Gene expression analysis revealed that the restoration of actinorhodin production in both relA and relC strains is accompanied by increased expression of the pathway-specific regulator gene actII-ORF4, which is normally decreased in the rel mutants. In addition to the restoration of antibiotic production, the rif mutants also exhibited a lower rate of RNA synthesis compared to the parental strain when grown in a rich medium, suggesting that these mutant RNA polymerases behave like "stringent" RNA polymerases. These results indicate that rif mutations can alter gene expression patterns independently of ppGpp. We propose that RNA polymerases carrying particular rif mutations in the beta-subunit can functionally mimic the modification induced by binding of ppGpp.

Anomalous synthesis of ppGpp in growing cells

In E. coli cells, accumulation of ppGpp is normally triggered by conditions that restrict the aminoacylation of tRNA or interfere with carbon/energy source metabolism; in both cases, the nucleotide's accumulation is associated with control of stable RNA synthesis and is generally believed to bring it about. We have found an anomalous situation wherein vigorously growing cells accumulate a high level of ppGpp and there is no restriction of stable RNA synthesis. This occurs when wild-type cells are shifted up from an abnormally low growth temperature to one in the optimal range (35 degrees C-40 degrees C). The effect is partly, but not entirely, dependent upon the presence of a functional relA gene product. These results appear to call into question the simpler interpretations of the role of ppGpp in the control of stable RNA synthesis.

Near-UV stress in Salmonella typhimurium: 4-thiouridine in tRNA, ppGpp, and ApppGpp as components of an adaptive response

We have examined the role of 4-thiouridine in the responses of Salmonella typhimurium to near-UV irradiation. Mutants lacking 4-thiouridine (nuv) and mutants defective in the synthesis of ppGpp (guanosine 5'-diphosphate-3'-diphosphate) (relA) were found to be sensitive to killing by near-UV. Near-UV induced the synthesis of a set of proteins that were not induced in the nuv mutant. Some of these proteins were identified as oxidative defense proteins, and others were identified as ppGpp-inducible proteins. Over 100-fold increases in ApppGpp (adenosine 5', 5"'-triphosphoguanosine-3"'-diphosphate, the adenylylated form of ppGpp) were observed in wild-type cells after near-UV irradiation but not in the 4-thiouridine-deficient mutant. These data support a model in which ppGpp and ApppGpp, a dinucleotide proposed to be synthesized by tRNA-aminoacyl synthetases as a response to the cross-linking of 4-thiouridine in tRNA by near-UV, induce the synthesis of proteins necessary for resistance to near-UV irradiation.

On the molecular pathology of neurodegeneration in IMPDH1-based retinitis pigmentosa

Retinitis pigmentosa (RP), the hereditary degenerative disease of the photoreceptor neurons of the retina, probably represents the most prevalent cause of registered blindness amongst those of working age in developed countries. Mutations within the gene encoding inosine monophosphate dehydrogenase 1 (IMPDH1), the widely expressed rate-limiting enzyme of the de novo pathway of guanine nucleotide biosynthesis, have recently been shown to cause the RP10 form of autosomal dominant RP. We examined the expression of IMPDH1, IMPDH2 and HPRT transcripts, encoding enzymes of the de novo and salvage pathways of guanine nucleotide biosynthesis, respectively, in retinal sections of mice, the data indicating that the bulk of GTP within photoreceptors is generated by IMPDH1. Impdh1(-/-) null mice are shown here to display a slowly progressive form of retinal degeneration in which visual transduction, analysed by electroretinographic wave functions, becomes gradually compromised, although at 12 months of age most photoreceptors remain structurally intact. In contrast, the human form of RP caused by mutations within the IMPDH1 gene is a severe autosomal dominant degenerative retinopathy in those families that have been examined to date. Expression of mutant IMPDH1 proteins in bacterial and mammalian cells, together with computational simulations, indicate that protein misfolding and aggregation, rather than reduced IMPDH1 enzyme activity, is the likely cause of the severe phenotype experienced by human subjects. Taken together, these findings suggest that RP10 may represent an attractive target for therapeutic intervention, based upon a strategy combining simultaneous suppression of transcripts from normal and mutant IMPDH1 alleles with supplementation of GTP within retinal tissues.

Radioisotope uptake as a measure of synthesis of messenger RNA

Exogenously supplied radioactive uracil (or guanine) enters the intracellular pools of RNA precursors in Escherichia coli only as nucleotides are removed from these pools by net synthesis of RNA. Consequently uptake of uracil over a short period does not measure the sum of the synthesis of all forms of RNA, unstable and stable, as is often supposed. Uptake of uracil during changing conditions of growth may be influenced by changes in types of RNA's being made; under such conditions that no stable RNA is being made, the synthesis of unstable forms may be greatly underestimated.