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PubMed1997

Expansion of the cellular content of ribonucleoside triphosphates induces cell shrinkage and KCl loss in Ehrlich ascites tumor cells and in Chinese hamster ovary cells.

The conversion to corresponding triphosphate derivatives of various ribonucleosides has been studied in Ehrlich ascites tumor cells and in Chinese hamster ovary cells under conditions that are optimal for cellular uptake of orthophosphate. The initial cellular uptake of orthophosphate is followed by a cellular loss of Cl- which might be consistent with a H2PO4-/Cl- exchange mechanism. Subsequent addition of ribonucleosides to the medium leads to cellular accumulation of the corresponding triphosphate and to a concomitant loss of KCl and to sustained cell volume reduction. The latter two events are quite unspecific with regard to the nucleobase moiety of the ribonucleoside triphosphate accumulated (adenine, guanine and purine being almost equally effective) and they depend in a rather simple way on the increase of the cellular content of these compounds. The KCl loss seems to depend on opening of the separate K+ and Cl- channels. The pharmacological profile of the putative ion channels could not be identified in spite of experiments with conventional blockers. In the case of purine riboside the accumulation of the corresponding triphosphate and concomitant loss of KCl and cell water may be followed by a regain of cell volume due to a continued purine riboside triphosphate accumulation, which apparently depends on the uptake of orthophosphate by cotransport with Na+ and which for osmotic reasons is accompanied by the uptake of water and hence volume increase. The possibility that the nucleoside triphosphate induced opening of a putative Cl- channel may be due to a direct effect of triphosphate on a channel protein is discussed.

Adenine↗
PubMed1975

Ribonucleoside 3'-di- and -triphosphates. Synthesis of guanosine tetraphosphate (ppGpp).

A procedure has been outlined for the synthesis of ribonucleoside 3'-di- and -triphosphates. The synthetic scheme involves the conversion of a ribonucleoside 3'-monophosphate to its 2'-(5'-di)-O-(1-methoxyethyl) derivative, followed by successive treatments of the blocked ribonucleotide with 1,1'-carbonyldiimidazole and mono(tri-n-butylammonium) phosphate or pyrophosphate. The resulting ribonucleoside 3'-di- and -triphosphate derivatives are then deblocked by treatment with dilute aqueous acetic acid, pH 3.0. The use of this procedure is illustrated for adenosine 3'-monophosphate, which has been converted to its corresponding 3'-di- and -triphosphates in 61% overall yield. The decomposition of adenosine 3'-di- and -triphosphates to adenosine 2'-monophosphate, adenosine 3'-monophosphate, and adenosine cyclic 2',3'-monophosphate as a function of pH at 100 degrees has been studied as has the attempted polymerization of adenosine 3'-diphosphate with polynucleotide phosphorylase. Also prepared was guanosine 5'-diphosphate 3'-diphosphate (guanosine tetraphosphate; ppGpp), which was accessible via treatment of 2'-O-(1-methoxyethyl)guanosine 5'-monophosphate 3'-monophosphate with the phosphorimidazolidate of mono(tri-n-butyl ammonium) phosphate. The resulting blocked tetraphosphate was deblocked in dilute aqueous acetic acid to afford ppGpp in an overall yield of 18%.

Adenine Nucleotides↗
PubMed2003

2'-chloro-2',3'-dideoxy-3'-fluoro-d-ribonucleosides: synthesis, stereospecificity, some chemical transformations, and conformational analysis.

The synthesis of methyl 5-O-benzoyl-2-chloro-2,3-dideoxy-3-fluoro-beta-d-ribofuranoside (5) and its use as a glycosylating agent for persilylated thymine, N(6)-benzoyladenine, and N(4)-benzoylcytosine are described (Scheme 1). The 2'-chloro-2',3'-dideoxy-3'-fluoro-d-ribonucleosides 10-12 synthesized were transformed to 2',3'-dideoxy-3'-fluoro-alpha- and -beta-d-erythro-pentofuranoside nucleosides of thymine (13a,b), adenine (14a,b), and cytidine (15a,b) by treatment with tributyltin hydride in the presence of alpha,alpha'-azobisisobutyronitrile (Scheme 2). Treatment of 2'-chloro-2',3'-dideoxy-3'-fluoro-d-ribonucleosides with 1 M MeONa/MeOH under reflux for 1-5 h afforded 2',3'-didehydro-2',3'-dideoxy-2'-chloro-d-pentofuranosyl nucleosides as the principal products (47-81%) of the reaction, along with recovered starting nucleoside (11-33%) (Scheme 3). Easy HF elimination was also observed in the case of the 2'-azido-2',3'-dideoxy-3'-fluoro-beta-d-ribofuranosides of thymine (17) and adenine (20) (Scheme 3). The role of conformational peculiarities of 2'-chloro-2',3'-dideoxy-3'-fluoro-d-ribonucleosides as well as of 17 and 20 in the observed exclusive elimination of HF is discussed. The conformational analysis of a rather broad palette of 2,3-dideoxy-3-fluoro-2-(X-substituted)-d-ribofuranosides was performed with the aid of the PSEUROT (version 6.3) program, using (i) the recently reparametrized Karplus-type relation (Chattopadhyaya and co-workers. J. Org. Chem. 1998, 63, 4967) and (ii) empirical bond angle correction terms suggested by us. The predictive power of the Brunck and Weinhold model (J. Am. Chem. Soc. 1979, 101, 1700) of the gauche effect between atoms and groups as a conformational driving force acting upon the pentofuranose ring is explored. Their model invokes maximum antiperiplanar sigma <--> sigma stabilization when the donating bond is the least polar one and the acceptor orbital is at the most polarized bond and is found at least as satisfactory, and in various specific cases more so than, as rationalizations on the basis of the preference of the gauche vs the trans conformation of two vicinal electronegative substituents (Wolfe. Acc. Chem. Res. 1972, 5, 102).

Chlorine↗
PubMed1984

Intracellular lymphocyte activation and carrier-mediated transport of C8-substituted guanine ribonucleosides.

The studies described in this report constitute evidence substantiating that certain exogenous nucleoside derivatives can activate lymphocytes by acting intracellularly. These molecules, the C8-substituted guanine ribonucleosides, have recently been demonstrated to exert potent immunostimulatory and immunoregulatory activities both in vitro and in vivo. The current studies were undertaken to investigate whether the site of induction of mitogenesis in murine B lymphocytes by these compounds was intracellular or at the plasma membrane. Uptake of 8-bromoguanosine was found to proceed by carrier-mediated transport. Like that described for adenosine, the uptake system for 8-bromoguanosine could be resolved into high-affinity and low-affinity components. The hypothesis that the C8-substituted guanine ribonucleosides act intracellularly was tested in several ways. Immobilization of these substituted nucleosides, either on Sepharose beads or in the form of high molecular weight polymers, resulted in total loss of their mitogenicity. In addition, maneuvers designed to diminish plasma membrane fluidity interfered with transmembrane signaling by surface membrane-directed mitogens far more than they did with activation by the substituted nucleosides. Furthermore, modulation of surface membrane protein (IgM) with anti-IgM antibodies similarly resulted in differential inhibition of transmembrane signals with relatively little effect on activation by 8-mercaptoguanosine. Taken together, these data are consistent with the hypothesis that the C8-substituted guanine ribonucleosides trigger the cell at an intracellular site.

Animals↗
PubMed2002

Permeability of Coxiella burnetii to ribonucleosides.

Knowledge about transport in Coxiella burnetii, an obligate phagolysosomal parasite, is incomplete. The authors investigated the capability of isolated, intact, host-free Coxiella to transport ribonucleosides while incubated at a pH value typical of lysosomes. Because of the low activities and limitations of obtaining experimental quantities of isolated, purified Coxiella, incorporation of substrate into nucleic acid was used as a trap for determination of uptake abilities. Virulent wild-type (phase I) organisms possessed uptake capability for all ribonucleosides. Both phase I and phase II (avirulent) organisms incorporated the purine nucleosides guanosine, adenosine and inosine, and showed a more limited uptake of thymidine and uridine. Both phases were poorly active in cytidine uptake. Neither phase of the organism was capable of transport and incorporation of NTPs, CMP, cytosine or uracil. Water space experiments confirmed that the uptake process concentrated the purine nucleosides within the cytoplasm of both wild-type and phase II Coxiella via a low-pH-dependent mechanism. Comparison of uptake rates in Escherichia coli versus Coxiella verified that the incorporation of ribonucleosides by Coxiella is a slow process. It is concluded that Coxiella possesses some transport pathways consistent with utilization of pools of nucleosides found within its host cell lysosomal pathway.

Animals↗
PubMed2004

5'-aminoimidazole-4-carboxyamide-ribonucleoside-activated glucose transport is not prevented by nitric oxide synthase inhibition in rat isolated skeletal muscle.

1. The nucleoside intermediate 5'-aminoimidazole-4-carboxyamide-ribonucleoside (AICAR) activates skeletal muscle AMP-activated protein kinase (AMPK) and increases glucose uptake. The AMPK phosphorylates neuronal nitric oxide synthase (nNOS)mu in skeletal muscle fibres. There is evidence that both AMPK and nNOSmu may be involved in the regulation of contraction-stimulated glucose uptake. 2. We examined whether both AICAR- and contraction-stimulated glucose uptake were mediated by NOS in rat skeletal muscle. 3. Rat isolated epitrochlearis muscles were subjected in vitro to electrically stimulated contractions for 10 min and/or incubated in the presence or absence of AICAR (2 mmol/L) or the NOS inhibitor NG-monomethyl-L-arginine (L-NMMA; 100 micromol/L). 4. Muscle contraction significantly (P < 0.05) altered the metabolic profile of the muscle. In contrast, AICAR and L-NMMA had no effect on the metabolic profile of the muscle, except that AICAR increased muscle 5'-aminoimidazole-4-carboxyamide-ribonucleotide (ZMP) and AICAR content. Nitric oxide synthase inhibition caused a small but significant (P < 0.05) reduction in basal 3-O-methylglucose transport, which was observed in all treatments. 5'-Aminoimidazole-4-carboxyamide-ribonucleoside significantly increased (P < 0.05) glucose transport above basal, with NOS inhibition decreasing this slightly (increased by 209% above basal compared with 184% above basal with NOS inhibition). Contraction significantly increased glucose transport above basal, with NOS inhibition substantially reducing this (107% increase vs 31% increase). 5'-Aminoimidazole-4-carboxyamide-ribonucleoside plus contraction in combination were not additive on glucose transport. 5. These results suggest that NO plays a role in basal glucose uptake and may regulate contraction-stimulated glucose uptake. However, NOS/nitric oxide do not appear to be signalling intermediates in AICAR-stimulated skeletal muscle glucose uptake.

3-O-Methylglucose↗
PubMed1986

Inhibition of herpes simplex virus DNA polymerase by purine ribonucleoside monophosphates.

Purine ribonucleoside monophosphates were found to inhibit chain elongation catalyzed by herpes simplex virus (HSV) DNA polymerase when DNA template-primer concentrations were rate-limiting. Inhibition was fully competitive with DNA template-primer during chain elongation; however, DNA polymerase-associated exonuclease activity was inhibited noncompetitively with respect to DNA. Combinations of 5'-GMP and phosphonoformate were kinetically mutually exclusive in dual inhibitor studies. Pyrimidine nucleoside monophosphates and deoxynucleoside monophosphates were less inhibitory than purine riboside monophosphates. The monophosphates of 9-beta-D-arabinofuranosyladenine, Virazole (1-beta-D-ribofuranosyl-1,2,4-triazole-3-carboxamide), 9-(2-hydroxyethoxymethyl)guanine, and 9-(1,3-dihydroxy-2-propoxymethyl)guanine exerted little or no inhibition. In contrast to HSV DNA polymerase, human DNA polymerase alpha was not inhibited by purine ribonucleoside monophosphates. These studies suggest the possibility of a physiological role of purine ribonucleoside monophosphates as regulators of herpesvirus DNA synthesis and a new approach to developing selective anti-herpesvirus compounds.

Antiviral Agents↗
PubMed1979

Evaluation of ribonucleoside and deoxyribonucleoside triphosphate pools in cultured leukemia cells during exposure to methotrexate or methotrexate plus thymidine.

Continuous exposure to inhibitory concentrations of methotrexate produces distinct rates of steady-state growth of murine leukemia L1210 and human leukemia CCRF-CEM cells in culture. Addition of thymidine to the medium produces reversal (6 to 40%) of this steady-state growth rate inhibition. This study utilized combinations of methotrexate and thymidine for an evaluation of the accompanying relationship between steady-state growth rate and changes in the ribo- and deoxyribonucleoside triphosphate pools. In L1210 cells exposed to methotrexate alone, the deoxythymidine 5'-phosphate (dTTP) pools decreased, whereas deoxyadenosine 5'-triphosphate, deoxyguanosine 5'-triphosphate, and deoxycytidine 5'-triphosphate (dCTP) remained relatively constant up to 70% inhibition of growth rate, with dCTP at a constant 112% of controls. The corresponding ribonucleoside triphosphates decreased only slightly. With the combination of methotrexate and thymidine resulting in up to 40% inhibition of growth rate, there was also a decrease in the dTTP pool while the other deoxyribonucleoside triphosphates remained relatively constant, and the corresponding ribonucleoside triphosphates again decreased only slightly. The dCTP pool was reduced to a constant 42% of control comparable to that produced by thymidine alone. With greater than 40% (with thymidine) or 70% (without thymidine) inhibition of growth rate, all pools decreased, but only dTTP was substantially reduced in proportion to the growth rate inhibition caused by methotrexate. The dTTP pool became depleted in spite of the presence of exogenous thymidine. Evaluation of CCRF-CEM cells indicated that inhibition of growth rate and nucleotide pool perturbations by methotrexate were similar to those observed in L1210 cells. However, in the presence of thymidine, inhibition of growth rate appeared related to decreased pools of dCTP, deoxyadenosine 5'-triphosphate, and deoxyguanosine 5'-triphosphate, rather than dTTP as was observed for L1210 cells. Hence, mammalian cells were capable of responding in a differential fashion to pharmacological perturbations, and this capacity may play a role in determining therapeutic selectivity. Since the ribonucleoside triphosphate decreases were slight and relatively uniform during methotrexate-induced perturbations, the deoxyribonucleoside triphosphate pools appear to be more directly related to inhibition of growth rate. The results are consistent with the concept that slight imbalances in the deoxyribonucleoside triphosphate pools dramatically inhibit DNA synthesis, as mediated through their interaction with DNA polymerase.

Animals↗
PubMed1996

Degradation of pyrimidine ribonucleosides by Pseudomonas aeruginosa.

Pyrimidine ribonucleoside degradation in the human pathogen Pseudomonas aeruginosa ATCC 15692 was investigated. Either uracil, cytosine, 5-methylcytosine, thymine, uridine or cytidine supported P. aeruginosa growth as a nitrogen source when glucose served as the carbon source. Using thin-layer chromatographic analysis, the enzymes nucleoside hydrolase and cytosine deaminase were shown to be active in ATCC 15692. Compared to (NH4)2SO4-grown cells, nucleoside hydrolase activity in ATCC 15692 approximately doubled after growth on 5-methylcytosine as a nitrogen source while its cytosine deaminase activity increased several-fold after growth on the pyrimidine bases and ribonucleosides examined as nitrogen sources. Regulation at the level of protein synthesis by 5-methylcytosine was indicated for nucleoside hydrolase and cytosine deaminase in P. aeruginosa.

Ammonium Sulfate↗
PubMed1980

Conformational analysis of ribonucleosides from proton-proton coupling constants.

A graphical method is described which permits the determination in solution of conformational equilibria and parameters of ribonucleosides from proton-proton coupling constants. The method is based on the pseudorotational concept and the use of the Karplus equation. Using a large number of 1H-NMR data on ribonucleosides from the literature, on optimal set of Karplus constants, A = 10.0, B = -1.2, was obtained. It is shown that this set of Karplus constants, in connection with the pseudorotational treatment, describes satisfactorily the nucleoside conformations, especially the pucker amplitude. The solution results are compared with X-ray data and the differences are discussed. The method can be used either graphically or by direct computer treatment of the data.

Magnetic Resonance Spectroscopy↗
PubMed1986

Selective inhibition of herpes simplex virus ribonucleoside diphosphate reductase by derivatives of 2-acetylpyridine thiosemicarbazone.

The effects of thiosemicarbazone derivatives of 2-acetylpyridine on mammalian and viral ribonucleoside diphosphate reductases were investigated. The enzymes were partially purified from uninfected and herpes simplex virus type-1 (HSV-1)-infected KB cells by sequential salt fractionation with streptomycin sulfate and ammonium sulfate and by affinity chromatography on ATP-agarose. The five thiosemicarbazone derivatives investigated were all potent inhibitors of the virus-induced reductase. Fifty percent inhibitory concentrations (IC50 values) range from 2 to 13 microM. Four of the five derivatives also were inhibitors of the host cell reductase (IC50 values = 7-34 microM). A semicarbazone was inactive against the cellular enzyme and relatively weak as an inhibitor of the viral enzyme (IC50 = 340 microM). Four of six compounds were preferential inhibitors of the viral reductase based on a comparison of IC50 values (5- to greater than 85-fold difference). Kinetic experiments revealed that inhibition of the HSV-1 reductase by the thiosemicarbazones was noncompetitive with respect to CDP and dithiothreitol. A comparison of the inhibitory effects of 2-acetylpyridine thiosemicarbazone itself on viral reductase and on virus replication in vitro demonstrated a similarity in the dose-response relationships for the two parameters. This observation supports the hypothesis that the HSV-induced ribonucleoside diphosphate reductase is an important target for the design of antiviral drugs.

Antiviral Agents↗
PubMed1985

The effect of allopurinol ribonucleoside and formycin B on Trypanosoma cruzi infections in mice.

The anti-Trypanosoma cruzi effect of allopurinol ribonucleoside and formycin B was examined against infections of the sensitive Y and Peru strains in inbred mice, strain DBA/1. Allopurinol ribonucleoside given in the drinking water at doses calculated to be 239, 511 and 929 mg/kg/day for 28 days, prevented the death of the mice but did not eradicate the infection. Formycin B given orally at 100 and 10 mg/kg/day X 5 days, showed a similar effect.

Allopurinol↗
PubMed1995

NMR studies of binding of 5-FdUDP and dCDP to ribonucleoside-diphosphate reductase from Escherichia coli.

5-Fluoro-2'-deoxyuridine-5'-diphosphate (5-FdUDP) has been synthesised using an original route, previously applied to the synthesis of natural nucleoside diphosphates. The interaction between 5-FdUDP and the enzyme ribonucleoside-diphosphate reductase (EC 1.17.4.1) has been studied with 19F-NMR. The product analogue is shown to be in fast exchange with substrate binding sites on protein subunit 1 (R1) of ribonucleoside-diphosphate (NDP) reductase. The number of binding sites is reduced to half when the complete holoenzyme R1R2 is formed. The temperature dependence of the line broadening of 5-FdUDP was studied using 19F-NMR, and of dCDP and dUDP using 1H-NMR. The temperature dependences are complex and a molecular model in which R1 is in a temperature dependent equilibrium between at least two conformations is suggested in order to explain the observed behaviour. Binding of a ligand to the substrate binding sites affects the conformational equilibrium in a ligand specific way. Formation of the holoenzyme R1R2 also affects the equilibrium.

Binding Sites↗
PubMed1982

Modified ribonucleosides as biological markers for patients with small cell carcinoma of the lung.

A variety of individual modified ribonucleosides may be elevated in the urine of cancer patients. They can be readily measured quantitatively in a single reversed-phase high-performance liquid chromatographic run. A total of 41 patients with small cell carcinoma of the lung were studied. For 5-ribonucleosides determined in the pretreatment urine of 28 patients, the respective frequency of elevation was directly related to stage of disease. One or more nucleosides were evaluated in the pretreatment urine of 27 out of 28 patients (96%). Included were 11 patients with limited disease and 10 (91%) had 2 or less than 2 nucleosides elevated, whereas 16 out of 17 (94%) with extensive disease had 3 or more elevated. Based on this same discriminant, median survival was significantly extended for patients with 2 or less nucleosides elevated (24 months) in contrast to 3 or more (10 months). Using a single number to represent the summation of equally weighted individual nucleoside values as a composite score, a direct relationship was found between increasing extent of disease or tumor burden. This was in contrast to more variable results for carcinoembryonic antigen analyzed in plasma samples obtained at the same time. When determined serially the composite score paralleled in general the clinical response categories for individual patients.

Carcinoembryonic Antigen↗
PubMed2005

The yicM (nepI) gene of Escherichia coli encodes a major facilitator superfamily protein involved in efflux of purine ribonucleosides.

The yicM gene of Escherichia coli was found by selection for resistance to 6-mercaptopurine. Translation and transcription initiation sites of yicM were determined. Overexpression of yicM increased resistance of sensitive cells to inosine and guanosine, decreased E. coli growth rate in medium containing these ribonucleosides as the sole carbon source, led to inosine accumulation by the E. coli strain deficient in purine nucleoside phosphorylase and enhanced the rate of inosine excretion by an inosine-producing strain. These results suggest that yicM encodes a purine ribonucleoside exporter and we have accordingly renamed it nepI (for 'nucleoside efflux permease-inosine').

Base Sequence↗
PubMed2001

Synthesis of isotopically labeled D-[1'-13C]ribonucleoside phosphoramidites.

The preparation of fully protected labeled diisopropylamino-beta-cyanoethyl-[1'-13C]ribonucleoside phosphoramidites with regioisomeric purity is described. We demonstrated in this paper that a regioselective 2'-O-silylation, through a 3',5'-O-di-tert-butylsilanediyl protection, has been applied for the synthesis of [1'-13C]ribonucleoside phosphoramidite units. This method allowed us to obtain only the desired 2'-O-silyl-3'-O-phosphoramidites avoiding the undesired 3'-O-silyl-2'-O-phosphoramidite nucleosides isolated by standard procedures. This is a suitable procedure to RNA precursors with respect to the isotope-containing precursors.

Carbon Isotopes↗
PubMed1989

Ribonucleoside analysis by reversed-phase high-performance liquid chromatography.

Over the past fifteen years we have developed and refined the analytical chromatographic methodologies using reversed-phase high-performance liquid chromatography and UV-photodiode array detection (RPLC-UV) for the detection and measurement of the major and modified nucleosides in nucleic acids and biological fluids. RPLC-UV nucleoside analysis as it has now evolved is a powerful new research tool to aid investigators in the fields of biochemical and biomedical research. This RPLC-UV nucleoside method can resolve more than 65 nucleosides in a single analysis with "run-to-run" peak retention variations of less than 1%. A complete nucleoside composition can be obtained from as little as 0.5 micrograms RNA. Identification and confirmation of nucleosides can be made from the highly reproducible retention times and from the characteristic UV spectrum from a few picomoles (ca. 1 ng) of nucleoside. In this paper we introduce standard RPLC-UV methodologies for the analysis of nucleosides and nucleoside composition of RNAs. The chromatographic protocols and standard nucleoside columns are presented and the essential requirements necessary in the HPLC instrumentation are described. Three optimized RPLC systems were developed, each with particular emphasis placed on resolution, speed, or sensitivity. In addition, three unfractionated tRNAs were selected as sources of reference nucleosides and for assessment of the performance of the chromatography. From these tRNAs, a large array of nucleosides were characterized which are used in standardization and calibration of the method. Also discussed is the use of a diode-array detector for enhancement of the reliability of nucleoside identification and accuracy of measurement. An extended enzymatic hydrolysis protocol for the liberation of exotically modified nucleosides in tRNAs is also described. Chromatographic retention times and UV spectra for a large number of ribonucleosides are tabulated. The RPLC-UV ribonucleoside analytical protocols are capable of quantifying 31 nucleosides. Approximately 1 microgram of an isoaccepting tRNA, or 20 micrograms of unfractionated tRNA are needed for quantitative analysis. With this amount of tRNA, the percent relative error of measurement of the four major nucleosides is less than 2%, and for the modified nucleosides about 5%. As little as 0.2 micrograms of pure isoaccepting tRNA can be analyzed, but at the expense of precision as at this low sample size a 20-30% relative error for modified nucleosides is to be expected. For quantitation of the modified nucleosides in rRNA, which contains much less modification than tRNAs, 10-100 micrograms of sample are needed per injection.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Synthesis of 3'-C-methyladenosine and 3'-C-methyluridine diphosphates and their interaction with the ribonucleoside diphosphate reductase from Corynebacterium nephridii.

Two nucleoside diphosphate analogs, 3'-C-methyl-ADP and 3'-C-methyl-UDP, have been tested as substrate and/or allosteric effectors using the adenosylcobalamin-dependent ribonucleoside diphosphate reductase of Corynebacterium nephridii. Neither analog was a substrate for the reductase. However, they did function as allosteric effectors and as inhibitors of the reduction of ADP and UDP, respectively. The nucleotide analogs did not stimulate the hydrogen exchange reaction between [5'-3H2]adenosylcobalamin and the solvent, indicating that the cleavage of the 3'-carbon-hydrogen bond is a prerequisite for the exchange reaction. A reinvestigation of the requirements for the exchange reaction revealed that the deoxyribonucleoside diphosphate products are very effective promoters of this reaction. Indeed, the deoxyribonucleoside diphosphates were found to be more effective in promoting the exchange reaction than the ribonucleoside diphosphate substrates. In contrast, the deoxyribonucleoside triphosphate effectors, dATP, dUTP, and dTTP, were only marginally effective as promoters of this reaction.

Adenosine Diphosphate↗