Regulation of de novo purine biosynthesis in human lymphoblasts. Coordinate control of proximal (rate-determining) steps and the inosinic acid branch point.
Purine nucleotide synthesis de novo has been studied in a permanent tissue culture line of human splenic lymphoblasts with particular attention to coordination of control of the proximal (rate-determining) steps with the distal branch point of the pathway. An assay was used which permits simultaneous determination of the overall rate of labeling of all intracellular purines with sodium [14C]formate, as well as the distribution of isotope into all intracellular guanine- and adenine-containing compounds. The guanine to adenine labeling ratio was used as an index of IMP branch point regulation. It was found that exogenous adenine and guanine produce feedback-controlling effects not only on the first step in the de novo pathway, but also on the IMP branch point. Concentrations of adenine which produce less than 40% inhibition of the overall rate of de novo purine synthesis do so by selectively inhibiting adenine nucleotide synthesis de novo by 50 to 70% while stimulating guanine nucleotide synthesis de novo by up to 20%. A reciprocal effect is seen with exogenous guanine. The adenosine analog 6-methylmercaptopurine ribonucleoside selectivity inhibits adenine nucleotide synthesis via the de novo pathway but not from exogenous hypoxanthine. Thus, the reactions of purine nucleotide interconversion, in particular adenylosuccinate synthetase, may be regulated differently in cells deriving their purine nucleotides solely from de novo synthesis than when deriving them via "salvage" of preformed hypoxanthine. (+info
Base pairing of anhydrohexitol nucleosides with 2,6-diaminopurine, 5-methylcytosine and uracil asbase moiety.
Hexitol nucleic acids (HNAs) with modified bases (5-methylcytosine, 2,6-diaminopurine or uracil) were synthesized. The introduction of the 5-methylcytosine base demonstrates that N -benzoylated 5-methylcytosyl-hexitol occurs as the imino tautomer. The base pairing systems (G:CMe, U:D, T:D and U:A) obey Watson-Crick rules. Substituting hT for hU, hCMefor hC and hD for hA generally leads to increased duplex stability. In a single case, replacement of hC by hCMedid not result in duplex stabilization. This sequence-specific effect could be explained by the geometry of the model duplex used for carrying out the thermal stability study. Generally, polypurine HNA sequences give more stable duplexes with their RNA complement than polypyrimidine HNA sequences. This observation supports the hypothesis that, besides changes in stacking pattern, the difference in conformational stress between purine and pyrimidine nucleosides may contribute to duplex stability. Introduction of hCMeand hD in HNA sequences further increases the potential of HNA to function as a steric blocking agent. (+info
Isolation and characterization of mutations in the Escherichia coli regulatory protein XapR.
In this work, the LysR-type protein XapR has been subjected to a mutational analysis. XapR regulates the expression of xanthosine phosphorylase (XapA), a purine nucleoside phosphorylase in Escherichia coli. In the wild type, full expression of XapA requires both a functional XapR protein and the inducer xanthosine. Here we show that deoxyinosine can also function as an inducer in the wild type, although not to the same extent as xanthosine. We have isolated and characterized in detail the mutants that can be induced by other nucleosides as well as xanthosine. Sequencing of the mutants has revealed that two regions in XapR are important for correct interactions between the inducer and XapR. One region is defined by amino acids 104 and 132, and the other region, containing most of the isolated mutations, is found between amino acids 203 and 210. These regions, when modelled into the three-dimensional structure of CysB from Klebsiella aerogenes, are placed close together and are most probably directly involved in binding the inducer xanthosine. (+info
Bystander effect of purine nucleoside analogues in HSV-1 tk suicide gene therapy is superior to that of pyrimidine nucleoside analogues.
Introduction of the herpes simplex virus type 1 thymidine kinase gene into tumor cells, followed by the administration of the antiherpes nucleoside analogue ganciclovir has been demonstrated to be effective in eliminating solid tumors in animals. The success of this combination treatment largely depends on the bystander effect, i.e. the killing of nontransfected tumor cells by activated drug carried over from the nearby herpes thymidine kinase (tk) gene-transfected cells. We evaluated the in vitro bystander effect of several antiherpes purine and pyrimidine nucleoside analogues, using a colorimetric assay. All pyrimidine nucleoside analogues, including (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU), showed low, if any, bystander killing effect. In contrast, purine nucleoside analogues, such as ganciclovir, were endowed with a pronounced bystander killer effect. Lobucavir (Cyclobut-G), a ganciclovir analogue, displayed a two- to three-fold more pronounced bystander killer effect than ganciclovir, eliminating, at a concentration of 10 microM, 75% and 90% of a cell population that contained 5% and 10% tk gene-transfected cells, respectively. These findings were corroborated by autoradiographic analysis that showed that 2'-3H-BVDU metabolites formed in the herpes tk gene-transfected tumor cells were much less efficiently incorporated in the DNA of bystander cells than 8-3H-GCV. This indicates that, under the same experimental conditions, BVDU metabolites are less prone to pass the gap junctions than GCV metabolites. (+info
Identification of amino acid residues responsible for the pyrimidine and purine nucleoside specificities of human concentrative Na(+) nucleoside cotransporters hCNT1 and hCNT2.
hCNT1 and hCNT2 mediate concentrative (Na(+)-linked) cellular uptake of nucleosides and nucleoside drugs by human cells and tissues. The two proteins (650 and 658 residues, 71 kDa) are 72% identical in sequence and contain 13 putative transmembrane helices (TMs). When produced in Xenopus oocytes, recombinant hCNT1 is selective for pyrimidine nucleosides (system cit), whereas hCNT2 is selective for purine nucleosides (system cif). Both transport uridine. We have used (i) chimeric constructs between hCNT1 and hCNT2, (ii) sequence comparisons with a newly identified broad specificity concentrative nucleoside transporter (system cib) from Eptatretus stouti, the Pacific hagfish (hfCNT), and (iii) site-directed mutagenesis of hCNT1 to identify two sets of adjacent residues in TMs 7 and 8 of hCNT1 (Ser(319)/Gln(320) and Ser(353)/Leu(354)) that, when converted to the corresponding residues in hCNT2 (Gly(313)/Met(314) and Thr(347)/Val(348)), changed the specificity of the transporter from cit to cif. Mutation of Ser(319) in TM 7 of hCNT1 to Gly enabled transport of purine nucleosides, whereas concurrent mutation of Gln(320) to Met (which had no effect on its own) augmented this transport. The additional mutation of Ser(353) to Thr in TM 8 converted hCNT1/S319G/Q320M, from cib to cif, but with relatively low adenosine transport activity. Additional mutation of Leu(354) to Val (which had no effect on its own) increased the adenosine transport capability of hCNT1/S319G/Q320M/S353T, producing a full cif-type transporter phenotype. On its own, the S353T mutation converted hCNT1 into a transporter with novel uridine-selective transport properties. Helix modeling of hCNT1 placed Ser(319) (TM 7) and Ser(353) (TM 8) within the putative substrate translocation channel, whereas Gln(320) (TM 7) and Leu(354) (TM 8) may exert their effects through altered helix packing. (+info
Nucleosides as a carbon source in Bacillus subtilis: characterization of the drm-pupG operon.
In Bacillus subtilis, nucleosides are readily taken up from the growth medium and metabolized. The key enzymes in nucleoside catabolism are nucleoside phosphorylases, phosphopentomutase, and deoxyriboaldolase. The characterization of two closely linked loci, drm and pupG, which encode phosphopentomutase (Drm) and guanosine (inosine) phosphorylase (PupG), respectively, is reported here. When expressed in Escherichia coli mutant backgrounds, drm and pupG confer phosphopentomutase and purine-nucleoside phosphorylase activity. Northern blot and enzyme analyses showed that drm and pupG form a dicistronic operon. Both enzymes are induced when nucleosides are present in the growth medium. Using mutants deficient in nucleoside catabolism, it was demonstrated that the low-molecular-mass effectors of this induction most likely were deoxyribose 5-phosphate and ribose 5-phosphate. Both Drm and PupG activity levels were higher when succinate rather than glucose served as the carbon source, indicating that the expression of the operon is subject to catabolite repression. Primer extension analysis identified two transcription initiation signals upstream of drm; both were utilized in induced and non-induced cells. The nucleoside-catabolizing system in B. subtilis serves to utilize the base for nucleotide synthesis while the pentose moiety serves as the carbon source. When added alone, inosine barely supports growth of B. subtilis. This slow nucleoside catabolism contrasts with that of E. coli, which grows rapidly on a nucleoside as a carbon source. When inosine was added with succinate or deoxyribose, however, a significant increase in growth was observed in B. subtilis. The findings of this study therefore indicate that the B. subtilis system for nucleoside catabolism differs greatly from the well-studied system in E. coli. (+info
Hydroxyurea potentiates the antiherpesvirus activities of purine and pyrimidine nucleoside and nucleoside phosphonate analogs.
Hydroxyurea has been shown to potentiate the anti-human immunodeficiency virus activities of 2',3'-dideoxynucleoside analogs such as didanosine. We have now evaluated in vitro the effect of hydroxyurea on the antiherpesvirus activities of several nucleoside analogs (acyclovir [ACV], ganciclovir [GCV], penciclovir [PCV], lobucavir [LBV], (R)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine [H2G], and brivudin and nucleoside phosphonate analogs (cidofovir [CDV] and adefovir [ADV]). When evaluated in cytopathic effect (CPE) reduction assays, hydroxyurea by itself had little effect on CPE progression and potentiated in a subsynergistic (herpes simplex virus type 1 [HSV-1]) to synergistic (HSV-2) fashion the antiviral activities of ACV, GCV, PCV, LBV, H2G, ADV, and CDV. Hydroxyurea also caused marked increases in the activities of ACV, GCV, PCV, LBV, and H2G (compounds that depend for their activation on a virus-encoded thymidine kinase [TK]) against TK-deficient (TK(-)) HSV-1. In fact, in combination with hydroxyurea the 50% effective concentrations of these compounds for inhibition of TK(-) HSV-1-induced CPE decreased from values of 20 to > or = 100 microg/ml (in the absence of hydroxyurea) to values of 1 to 5 microg/ml (in the presence of hydroxyurea at 25 to 100 microg/ml). When evaluated in a single-cycle virus yield reduction assay, hydroxyurea at a concentration of 100 microg/ml inhibited progeny virus production by 60 to 90% but had little effect on virus yield at a concentration of 25 microg/ml. Under these assay conditions hydroxyurea still elicited a marked potentiating effect on the antiherpesvirus activities of GCV and CDV, but this effect was less pronounced than that in the CPE reduction assay. It is conceivable that the potentiating effect of hydroxyurea stems from a depletion of the intracellular deoxynucleoside triphosphate pools, thus favoring the triphosphates of the nucleoside analogues (or the diphosphates of the nucleoside phosphonate analogues) in their competition with the natural nucleotides at the viral DNA polymerase level. The possible clinical implications of these findings are discussed. (+info
Nucleotide requirements for the in vitro activation of the apoptosis protein-activating factor-1-mediated caspase pathway.
Adenine deoxynucleosides, such as 2-chlorodeoxyadenosine (2CdA) and fludarabine, induce apoptosis in quiescent lymphocytes, and are thus useful drugs for the treatment of indolent lymphoproliferative diseases. We previously demonstrated that that the 5'-triphosphate metabolite of 2CdA (2CdATP), similar to dATP, can cooperate with cytochrome c and apoptosis protein-activating factor-1 (APAF-1) to trigger a caspase pathway in a HeLa cell-free system. We used a fluorometry-based assay of caspase activation to extend the analysis to several other clinically relevant adenine deoxynucleotides in B-chronic lymphocytic leukemia extracts. The nucleotide-induced caspase activation displayed typical Michaelis-Menten kinetics. As estimated by the V(max)/K(m) ratios, the relative efficiencies of different nucleotides were Ara-ATP > 9-fluoro-9-beta-D-arabinofuranosyladenine 5'-triphosphate > dATP > 2CdATP > 9-beta-D-arabinofuranosylguanine 5'-triphosphate > dADP > ATP. In contrast to dADP, both ADP and its nonhydrolyzable alpha, beta-methylphosphonate analog were strong inhibitors of APAF-1-dependent caspase activation. The hierarchy of nucleotide activation was confirmed in a fully reconstituted system using recombinant APAF-1 and recombinant procaspase-9. These results suggest that the potency of adenine deoxynucleotides as co-factors for APAF-1-dependent caspase activation is due both to stimulation by the 5'-triphosphates and lack of inhibition by the 5'-diphosphates. The capacity of adenine deoxynucleoside metabolites to activate the apoptosome pathway may be an additional biochemical mechanism that plays a role in the chemotherapy of indolent lymphoproliferative diseases. (+info