Differential cytostatic effects of NO donors and NO producing cells

A 3-h exposure to NO donors (spermine-NO, DETA-NO, or SNAP), or to NOS II-expressing cells (activated macrophages or EMT6 cells) reversibly inhibited DNA synthesis in K562 tumor cells. In GSH-depleted K562 cells, cytostasis remained reversible when induced by DETA-NO or NOS II activity, but became irreversible after exposure to spermine-NO or SNAP. Only SNAP and spermine-NO efficiently inhibited GAPDH, an enzyme with a critical thiol, in GSH-depleted cells. Thus, the irreversible cytostasis induced in GSH-depleted cells by spermine-NO or SNAP can be tentatively attributed to S-nitrosating or oxidizing species derived from NO. However, these species did not contribute significantly to the early antiproliferative effects of macrophages. Ribonucleotide reductase, a key enzyme in DNA synthesis. has been shown to be inhibited by NO. Supplementation of the medium with deoxyribonucleosides to bypass RNR inhibition restored DNA synthesis in target cells exposed to DETA-NO and NO-producing cells, but was inefficient for GSH-depleted cells previously submitted to spermine-NO or SNAP. These cells also exhibited a persistent depletion of the dATP pool. In conclusion, GSH depletion reveals striking qualitative differences in the nature of the toxic effectors released by various NO sources, questioning the significance of S-nitrosating or oxidizing nitrogen oxides in NOS II-dependent cytostasis.

Antitumor efficacy of combination chemotherapy with UFT and cyclophosphamide against human breast cancer xenografts in nude mice

The combination of cyclophosphamide (CPA) and 5-fluorouracil (5-FU) is currently regarded as the most effective therapy for the treatment of patients with advanced and recurrent breast cancer. We evaluated the augmentation of antitumor activity and toxicity by coadministration of CPA and UFT (1M tegafur--4M uracil) instead of intravenous 5-FU on H-31 human breast cancer xenografts in nude mice. The maximum tolerable dose (MTD) of UFT alone (24 mg/kg) and CPA alone (85 mg/kg) had a significant effect on H-31 tumors in mice with 86.6% and 83.0% inhibition rates of tumor growth, respectively, and without loss of body weight, diarrhea or myelosuppression. The combined administration with full and 83.3% MTD of UFT and CPA augmented the antitumor activity compared to that of UFT alone and CPA alone. The relative tumor volume of the UFT plus CPA-treated group to the UFT- and CPA-treated groups was 0.28 and 0.36 for the full MTD, and 0.51 and 0.67 for 83.3% MTD, respectively. When CPA was consecutively administered to the tumor-bearing mice for 14 days, there were no decreases in the activities of enzymes related to 5-FU metabolism, but there was an significant increase in the activity of ribonucleotide reductase, suggesting that anabolism of 5-FU derived from tegafur is accelerated to some extent by coadministration of CPA. In conclusion, these results suggest that combination therapy with oral UFT and CPA may be useful for the long-term treatment of cancer patients with advanced and recurrent breast cancers.

Vaccine potential of a herpes simplex virus type 2 mutant deleted in the PK domain of the large subunit of ribonucleotide reductase (ICP10)

A herpes simplex virus type 2 (HSV-2) mutant deleted in the PK domain of the large subunit of ribonucleotide reductase (ICP10) was evaluated as a potential vaccine for the prevention of HSV-2 infection and disease. This virus, designated ICP10 deltaPK, expressed a 95 kDa ICP10 protein that lacked PK activity and transforming potential. ICP10 deltaPK was growth compromised in dividing and nondividing cells in culture. In dividing cells, onset of virus growth was delayed, with replication initiating at 10-15 h p.i. depending on the multiplicity of infection. In addition to the delayed growth onset, virus replication was significantly impaired (1000-fold lower titers) in nondividing cells. A revertant virus (HSV-2(R)) expressed ICP10, regained transforming activity and had wild type growth properties. ICP10 deltaPK was growth compromised also in infected animals. It was isolated from the site of infection on day 2, but not day 7 p.i. and its titers at this time (2 x 10(2) pfu/ml) were significantly lower than those of HSV-2 (5 x 10(4) pfu/ml). Mice given high titers of ICP10 deltaPK (5 x 10(7) pfu/footpad) remained free of clinical symptoms and survived infection during a 21-day follow-up period and virus was not isolated from latently infected ganglia at 30 days p.i. ICP10 deltaPK immunized animals developed HSV-specific humoral and T-cell responses and evidenced absolute protection from HSV-2 infection and virus-induced disease.

The activation of ribonucleotide reductase in animal organs as the cellular response against the treatment with DNA-damaging factors and the influence of radioprotectors on this effect

Cellular requirements for deoxyribonucleotide (dNTP) pools during DNA synthesis are related to ensuring of the accuracy of DNA copying during replication and repair. This paper covers some problems on the reactions of dNTP synthesis system in organs of animals against the treatment with DNA-damaging agents. Ribonucleoside diphosphate reductase (NDPR) is the key enzyme for the synthesis of dNTP, since it catalyses the reductive conversion of ribonucleotides to deoxyribonucleotides. The results obtained show that the rapid and transient increase in NDPR activity in animal organs occurs as cellular response against the treatment with DNA-damaging agents (SOS-type activation). We have also found the intensive radioprotector-stimulated activation of deoxyribonucleotide synthesis as well as DNA and protein synthesis in mice organs within 3 days after the administration of two radioprotectors, indralin and indometaphen, that provide the high animal survival. Our studies suggest that these effects are the most important steps in the protective mechanism of the radioprotectors and are responsible for the high animal survival.

Metabolic fate of oxidized guanine ribonucleotides in mammalian cells

8-Oxo-7,8-dihydroguanine- (8-oxoguanine-) containing nucleotides are generated in the cellular nucleotide pool by the action of oxygen radicals produced during normal cellular metabolism. We examined the interconversion and metabolic fate of 8-oxoguanine-containing ribonucleotides in mammalian cells. (1) 8-OxoGTP can be generated not only by direct oxidation of GTP but also by phosphorylation of 8-oxoGDP by nucleotide diphosphate kinase, and the 8-oxoGTP thus formed can serve as a substrate for RNA polymerase II to induce transcription errors. (2) MTH1 protein carrying intrinsic 8-oxo-dGTPase activity has the potential to hydrolyze 8-oxoGTP to 8-oxoGMP, thus preventing misincorporation of 8-oxoguanine into RNA. 8-OxoGMP, the degradation product, cannot be reutilized, since guanylate kinase, which has the potential to phosphorylate both GMP and dGMP, is inactive on 8-oxoGMP. (3) Ribonucleotide reductase, which catalyzes reduction of four naturally occurring ribonucleoside diphosphates, cannot convert 8-oxoguanine-containing ribonucleotide to the deoxyribonucleotide. This step appears to serve as a gatekeeper to prevent formation of mutagenic substrates for DNA synthesis from oxidized ribonucleotides.

Allosteric control of three B12-dependent (class II) ribonucleotide reductases. Implications for the evolution of ribonucleotide reduction

Three separate classes of ribonucleotide reductases are known, each with a distinct protein structure. One common feature of all enzymes is that a single protein generates each of the four deoxyribonucleotides. Class I and III enzymes contain an allosteric substrate specificity site capable of binding effectors (ATP or various deoxyribonucleoside triphosphates) that direct enzyme specificity. Some (but not all) enzymes contain a second allosteric site that binds only ATP or dATP. Binding of dATP to this site inhibits the activity of these enzymes. X-ray crystallography has localized the two sites within the structure of the Escherichia coli class I enzyme and identified effector-binding amino acids. Here, we have studied the regulation of three class II enzymes, one from the archaebacterium Thermoplasma acidophilum and two from eubacteria (Lactobacillus leichmannii and Thermotoga maritima). Each enzyme has an allosteric site that binds ATP or various deoxyribonucleoside triphosphates and that regulates its substrate specificity according to the same rules as for class I and III enzymes. dATP does not inhibit enzyme activity, suggesting the absence of a second active allosteric site. For the L. leichmannii and T. maritima enzymes, binding experiments also indicate the presence of only one allosteric site. Their primary sequences suggest that these enzymes lack the structural requirements for a second site. In contrast, the T. acidophilum enzyme binds dATP at two separate sites, and its sequence contains putative effector-binding amino acids for a second site. The presence of a second site without apparent physiological function leads to the hypothesis that a functional site was present early during the evolution of ribonucleotide reductases, but that its function was lost from the T. acidophilum enzyme. The other two B12 enzymes lost not only the function, but also the structural basis for the site. Also a large subgroup (Ib) of class I enzymes, but none of the investigated class III enzymes, has lost this site. This is further indirect evidence that class II and I enzymes may have arisen by divergent evolution from class III enzymes.

Binding of Cob(II)alamin to the adenosylcobalamin-dependent ribonucleotide reductase from Lactobacillus leichmannii. Identification of dimethylbenzimidazole as the axial ligand

The ribonucleoside triphosphate reductase (RTPR) from Lactobacillus leichmannii catalyzes the reduction of nucleoside 5'-triphosphates to 2'-deoxynucleoside 5'-triphosphates and uses coenzyme B12, adenosylcobalamin (AdoCbl), as a cofactor. Use of a mechanism-based inhibitor, 2'-deoxy-2'-methylenecytidine 5'-triphosphate, and isotopically labeled RTPR and AdoCbl in conjunction with EPR spectroscopy has allowed identification of the lower axial ligand of cob(II)alamin when bound to RTPR. In common with the AdoCbl-dependent enzymes catalyzing irreversible heteroatom migrations and in contrast to the enzymes catalyzing reversible carbon skeleton rearrangements, the dimethylbenzimidazole moiety of the cofactor is not displaced by a protein histidine upon binding to RTPR.

A glycyl radical site in the crystal structure of a class III ribonucleotide reductase

Ribonucleotide reductases catalyze the reduction of ribonucleotides to deoxyribonucleotides. Three classes have been identified, all using free-radical chemistry but based on different cofactors. Classes I and II have been shown to be evolutionarily related, whereas the origin of anaerobic class III has remained elusive. The structure of a class III enzyme suggests a common origin for the three classes but shows differences in the active site that can be understood on the basis of the radical-initiation system and source of reductive electrons, as well as a unique protein glycyl radical site. A possible evolutionary relationship between early deoxyribonucleotide metabolism and primary anaerobic metabolism is suggested.

NMR structure of Escherichia coli glutaredoxin 3-glutathione mixed disulfide complex: implications for the enzymatic mechanism

Glutaredoxins (Grxs) catalyze reversible oxidation/reduction of protein disulfide groups and glutathione-containing mixed disulfide groups via an active site Grx-glutathione mixed disulfide (Grx-SG) intermediate. The NMR solution structure of the Escherichia coli Grx3 mixed disulfide with glutathione (Grx3-SG) was determined using a C14S mutant which traps this intermediate in the redox reaction. The structure contains a thioredoxin fold, with a well-defined binding site for glutathione which involves two intermolecular backbone-backbone hydrogen bonds forming an antiparallel intermolecular beta-bridge between the protein and glutathione. The solution structure of E. coli Grx3-SG also suggests a binding site for a second glutathione in the reduction of the Grx3-SG intermediate, which is consistent with the specificity of reduction observed in Grxs. Molecular details of the structure in relation to the stability of the intermediate and the activity of Grx3 as a reductant of glutathione mixed disulfide groups are discussed. A comparison of glutathione binding in Grx3-SG and ligand binding in other members of the thioredoxin superfamily is presented, which illustrates the highly conserved intermolecular interactions in this protein family.

Thermodynamic and kinetic studies on carbon-cobalt bond homolysis by ribonucleoside triphosphate reductase: the importance of entropy in catalysis

In the catalytic mechanism of nucleotide reduction, ribonucleoside triphosphate reductase (RTPR) from Lactobacillus leichmannii catalyzes the homolytic cleavage of the carbon-cobalt bond of adenosylcobalamin (AdoCbl) at a rate approximately 10(11)-fold faster than the uncatalyzed reaction. Model systems have suggested hypotheses for the thermodynamic basis of this reaction, but relevant measurements of the enzymatic reaction have been lacking. To address this question in a system for which the microscopic rate constants can be measured as a function of temperature, we examined the RTPR-catalyzed exchange reaction. RTPR, in the presence of allosteric effector dGTP and in the absence of substrate, catalyzes carbon-cobalt bond homolysis and formation of a thiyl radical from an active-site cysteine in a concerted fashion [Licht, S., Booker, S. , Stubbe, J. (1999) Biochemistry 38, 1221-1233]. Both the kinetics of cob(II)alamin formation and the amounts of cob(II)alamin formed have been studied as a function of AdoCbl concentration and temperature. Analysis of these data has allowed calculation of a DeltaH of 20 kcal/mol, a DeltaS of 70 cal mol-1 K-1, a DeltaH of 46 kcal/mol, and a DeltaS of 96 cal mol-1 K-1 for carbon-cobalt bond homolysis/thiyl radical formation. The results further show that the enzyme perturbs the equilibrium between the reactant (AdoCbl-bound) state and the product (cob(II)alamin/5'-deoxyadenosine (5'-dA)/thiyl radical state, making them approximately equal in energy. The thermodynamic perturbation, in addition to transition-state stabilization, is required for the large rate acceleration observed. Entropic, rather than enthalpic, factors make the largest contribution in both cases.

Studies on the catalysis of carbon-cobalt bond homolysis by ribonucleoside triphosphate reductase: evidence for concerted carbon-cobalt bond homolysis and thiyl radical formation

Ribonucleotide reductases (RNRs) catalyze the rate-determining step in DNA biosynthesis: conversion of nucleotides to deoxynucleotides. The RNR from Lactobacillus leichmannii utilizes adenosylcobalamin (AdoCbl) as a cofactor and, in addition to nucleotide reduction, catalyzes the exchange of tritium from [5'-3H]-AdoCbl with solvent. Examination of this exchange reaction offers a unique opportunity to investigate the early stages in the nucleotide reduction process [Licht S. S., Gerfen, G. J., and Stubbe, J. (1996) Science 271, 477-481]. The kinetics of and requirements for this exchange reaction have been examined in detail. The turnover number for 3H washout is 0.3 s-1, and it requires an allosteric effector dGTP (Km = 17 +/- 3 microM), AdoCbl (Km = 60 +/- 9 microM) and no external reductant. The effects of active-site mutants of RTPR (C119S, C419S, C731S, C736S, and C408S) on the rate of the exchange reaction have been determined, and only C408 is essential for this process. The exchange reaction has previously been monitored by stopped-flow UV-vis spectroscopy, and cob(II)alamin was shown to be formed with a rate constant of 40 s-1 [Tamao, Y., and Blakley, R. L. (1973) Biochemistry 12, 24-34]. This rate constant has now been measured in D2O, with [5'-2H2]-AdoCbl in H2O, and with [5'-2H2]-AdoCbl in D2O. A comparison of these results with those for AdoCbl in H2O revealed kH/kD of 1.6, 1.7, and 2.7, respectively. The absolute amounts of cob(II)alamin generated with [5'-2H2]-AdoCbl in D2O in comparison with AdoCbl in H2O reveal twice as much cob(II)alamin in the former case. Similar transient kinetic studies with C408S RTPR reveal no cob(II)alamin formation. These experiments allow proposal of a minimal mechanism for this exchange reaction in which RNR catalyzes homolysis of the carbon-cobalt bond in a concerted fashion, to generate a thiyl radical on C408, cob(II)alamin, and 5'-deoxyadenosine.

A 1.4-Mb high-resolution physical map and contig of chromosome segment 11p15.5 and genes in the LOH11A metastasis suppressor region

The centromeric part of chromosome segment 11p15.5 contains a region of frequent allele loss in many adult solid malignancies. This region, called LOH11A, is lost in 75% of lung cancers and is thought to contain a gene that may function as a metastasis suppressor. Genetic complementation studies have shown suppression of the malignant phenotype including reduction of metastasis formation. We constructed a high-resolution physical map and contig over 1.4 Mb that includes the beta-hemoglobin gene cluster and the gene for the large subunit of ribonucleotide reductase (RRM1). Through sequencing and computerized analysis, we determined that this region contains an unusually large number of transposable elements, which suggests that double-stranded DNA breaks occur frequently here. Twenty-two putative genes were identified. Because of its location at the site of maximal allele loss in the 650-kb LOH11A region and previous functional studies, RRM1 is the most likely candidate gene with metastasis suppressor function. The malignant phenotype, in this case, results from a relative loss of function rather than a complete loss.

Ribonucleotide reductase R2 protein is phosphorylated at serine-20 by P34cdc2 kinase

Ribonucleotide reductase is a rate-limiting enzyme in DNA synthesis and is composed of two different proteins, R1 and R2. The R2 protein appears to be rate-limiting for enzyme activity in proliferating cells, and it is phosphorylated by p34cdc2 and CDK2, mediators of cell cycle transition events. A sequence in the R2 protein at serine-20 matches a consensus sequence for p34cdc2 and CDK2 kinases. We tested the hypothesis that the serine-20 residue was the major p34cdc2 kinase site of phosphorylation. Three peptides were synthesized (from Asp-13 to Ala-28) that contained either the wild type amino acid sequence (Asp-Gln-Gln-Gln-Leu-Gln-Leu-Ser-Pro-Leu-Lys-Arg-Leu-Thr-Leu-Ala, serine peptide) or a mutation, in which the serine residue was replaced with an alanine residue (alanine peptide) or a threonine residue (threonine peptide). Only the serine peptide and threonine peptide were phosphorylated by p34cdc2 kinase. In two-dimensional phosphopeptide mapping experiments of serine peptide and Asp-N endoproteinase digested R2 protein, peptide co-migration patterns suggested that the synthetic phosphopeptide containing serine-20 was identical to the major Asp-N digested R2 phosphopeptide. To further test the hypothesis that serine-20 is the primary phosphorylated residue on R2 protein, three recombinant R2 proteins (R2-Thr, R2-Asp and R2-Ala) were generated by site-directed mutagenesis, in which the serine-20 residue was replaced with threonine, aspartic acid or alanine residues. Wild type R2 and threonine-substituted R2 proteins (R2-Thr) were phosphorylated by p34cdc2 kinase, whereas under the same experimental conditions, R2-Asp and R2-Ala phosphorylation was not detected. Furthermore, the phosphorylated amino acid residue in the R2-Thr protein was determined to be phosphothreonine. Therefore, by replacing a serine-20 residue with a threonine, the phosphorylated amino acid in R2 protein was changed to a phosphothreonine. In total, these results firmly establish that a major p34cdc2 phosphorylation site on the ribonucleotide reductase R2 protein occurs near the N-terminal end at serine-20, which is found within the sequence Ser-Pro-Leu-Lys-Arg-Leu. Comparison of ribonucleotide reductase activities between wild type and mutated forms of the R2 proteins suggested that mutation at serine-20 did not significantly affect enzyme activity.

Dual roles of p82, the clam CPEB homolog, in cytoplasmic polyadenylation and translational masking

In the transcriptionally inert maturing oocyte and early embryo, control of gene expression is largely mediated by regulated changes in translational activity of maternal mRNAs. Some mRNAs are activated in response to poly(A) tail lengthening; in other cases activation results from de-repression of the inactive or masked mRNA. The 3' UTR cis-acting elements that direct these changes are defined, principally in Xenopus and mouse, and the study of their trans-acting binding factors is just beginning to shed light on the mechanism and regulation of cytoplasmic polyadenylation and translational masking. In the marine invertebrate, Spisula solidissima, the timing of activation of three abundant mRNAs (encoding cyclin A and B and the small subunit of ribonucleotide reductase, RR) in fertilized oocytes correlates with their cytoplasmic polyadenylation. However, in vitro, mRNA-specific unmasking occurs in the absence of polyadenylation. In Walker et al. (in this issue) we showed that p82, a protein defined as selectively binding the 3' UTR masking elements, is a homolog of Xenopus CPEB (cytoplasmic polyadenylation element binding protein). In functional studies reported here, the elements that support polyadenylation in clam egg lysates include multiple U-rich CPE-like motifs as well as the nuclear polyadenylation signal AAUAAA. This represents the first detailed analysis of invertebrate cis-acting cytoplasmic polyadenylation signals. Polyadenylation activity correlates with p82 binding in wild-type and CPE-mutant RR 3' UTR RNAs. Moreover, since anti-p82 antibodies specifically neutralize polyadenylation in egg lysates, we conclude that clam p82 is a functional homolog of Xenopus CPEB, and plays a positive role in polyadenylation. Anti-p82 antibodies also result in specific translational activation of masked mRNAs in oocyte lysates, lending support to our original model of clam p82 as a translational repressor. We propose therefore that clam p82/CPEB has dual functions in masking and cytoplasmic polyadenylation.

Allosteric regulation of Trypanosoma brucei ribonucleotide reductase studied in vitro and in vivo

Trypanosoma brucei is the causative agent for African sleeping sickness. We have made in vitro and in vivo studies on the allosteric regulation of the trypanosome ribonucleotide reductase, a key enzyme in the production of dNTPs needed for DNA synthesis. Results with the isolated recombinant trypanosome ribonucleotide reductase showed that dATP specifically directs pyrimidine ribonucleotide reduction instead of being a general negative effector as in other related ribonucleotide reductases, whereas dTTP and dGTP directed GDP and ADP reduction, respectively. Pool measurements of NDPs, NTPs, and dNTPs in the cultivated bloodstream form of trypanosomes exposed to deoxyribonucleosides or inhibited by hydroxyurea confirmed our in vitro allosteric regulation model of ribonucleotide reductase. Interestingly, the trypanosomes had extremely low CDP and CTP pools, whereas the dCTP pool was comparable with that of other dNTPs. The trypanosome ribonucleotide reductase seems adapted to this situation by having a high affinity for the CDP/UDP-specific effector dATP and a high catalytic efficiency, Kcat/Km, for CDP reduction. Thymidine and deoxyadenosine were readily taken up and phosphorylated to dTTP and dATP, respectively, the latter in a nonsaturating manner. This uncontrolled uptake of deoxyadenosine strongly inhibited trypanosome proliferation, a valuable observation in the search for new trypanocidal nucleoside analogues.

Cysteinyl and substrate radical formation in active site mutant E441Q of Escherichia coli class I ribonucleotide reductase

All classes of ribonucleotide reductase are proposed to have a common reaction mechanism involving a transient cysteine thiyl radical that initiates catalysis by abstracting the 3'-hydrogen atom of the substrate nucleotide. In the class Ia ribonucleotide reductase system of Escherichia coli, we recently trapped two kinetically coupled transient radicals in a reaction involving the engineered E441Q R1 protein, wild-type R2 protein, and substrate (Persson, A. L., Eriksson, M., Katterle, B., Pötsch, S., Sahlin, M., and Sjöberg, B.-M. (1997) J. Biol. Chem. 272, 31533-31541). Using isotopically labeled R1 protein or substrate, we now demonstrate that the early radical intermediate is a cysteinyl radical, possibly in weak magnetic interaction with the diiron site of protein R2, and that the second radical intermediate is a carbon-centered substrate radical with hyperfine coupling to two almost identical protons. This is the first report of a cysteinyl free radical in ribonucleotide reductase that is a kinetically coupled precursor of an identified substrate radical. We suggest that the cysteinyl radical is localized to the active site residue, Cys439, which is conserved in all classes of ribonucleotide reductase, and which, in the three-dimensional structure of protein R1, is positioned to abstract the 3'-hydrogen atom of the substrate. We also suggest that the substrate radical is localized to the 3'-position of the ribose moiety, the first substrate radical intermediate in the postulated reaction mechanism.

Allosteric regulation of vaccinia virus ribonucleotide reductase, analyzed by simultaneous monitoring of its four activities

As determined by simultaneous monitoring of its four activities, vaccinia virus-coded ribonucleoside diphosphate (rNDP) reductase shows responses to individual nucleoside triphosphate effectors-ATP, dATP, dGTP, and dTTP-similar to those previously reported for rNDP reductase of mouse, which the viral enzyme closely resembles. This investigation uses the vaccinia enzyme as a readily available and convenient model for understanding the cellular enzyme. As previously reported for T4 phage aerobic rNDP reductase, we found the relative activities of ADP, CDP, GDP, and UDP reduction to be reasonably close to the proportions of the four deoxyribonucleotides in the vaccinia virus genome, but only when the four substrates and the four allosteric effectors were all provided at their approximate intracellular concentrations. GDP reductase levels were somewhat higher, proportionately, than the representation of dGMP in vaccinia virus DNA. To understand this behavior and also to evaluate possible relationships between ribonucleotide reductase control and the very low dGTP pools seen in eukaryotic cells, we carried out substrate saturation experiments with a "bioproportional" mixture containing the four rNDP substrates at their relative in vivo concentrations as determined from rNDP pool measurements. Reduction of the two purine substrates was inhibited at high concentrations of this mixture, and data suggest that ADP acts as a specific inhibitor of its own reduction and that of GDP. Use of the four-substrate assay revealed also that a mixture of vaccinia virus R1 protein and mouse R2 protein is catalytically active, making this the first reported chimeric rNDP reductase to show biological activity.

Differential effects of hydroxyurea upon deoxyribonucleoside triphosphate pools, analyzed with vaccinia virus ribonucleotide reductase

Hydroxyurea inhibits DNA synthesis by destroying the catalytically essential free radical of class I ribonucleoside diphosphate (rNDP) reductase, thereby blocking the de novo synthesis of deoxyribonucleotides. In mammalian cells, including those infected by vaccinia virus, hydroxyurea treatment causes a differential depletion of the four deoxyribonucleoside triphosphate pools, suggesting that the activities of rNDP reductase are differentially sensitive to hydroxyurea. In the presence of different substrates and allosteric modifiers, we measured rates of free radical destruction in the vaccinia virus-coded rNDP reductase, by following absorbance at 417 nm as a function of time after hydroxyurea addition. Also, we followed enzyme activity directly, by using a recently developed assay that allows simultaneous monitoring of the four activities, in the presence of substrates and effectors at concentrations that approximate the intracellular environment. We found the primary determinant of radical loss to be not the ensemble of allosteric ligands bound but the activity of the enzyme. Nucleoside triphosphate effectors accelerated radical decay, compared with rates seen with the free enzyme. Adding substrate to the holoenzyme, under conditions where the enzymatic reaction is proceeding, further accelerated radical decay. Alternative models are discussed, to account for selective depletion of purine nucleotide pools by hydroxyurea.

The PK domain of the large subunit of herpes simplex virus type 2 ribonucleotide reductase (ICP10) is required for immediate-early gene expression and virus growth

The large subunit of herpes simplex virus (HSV) ribonucleotide reductase (RR), RR1, contains a unique amino-terminal domain which has serine/threonine protein kinase (PK) activity. To examine the role of the PK activity in virus replication, we studied an HSV type 2 (HSV-2) mutant with a deletion in the RR1 PK domain (ICP10DeltaPK). ICP10DeltaPK expressed a 95-kDa RR1 protein (p95) which was PK negative but retained the ability to complex with the small RR subunit, RR2. Its RR activity was similar to that of HSV-2. In dividing cells, onset of virus growth was delayed, with replication initiating at 10 to 15 h postinfection, depending on the multiplicity of infection. In addition to the delayed growth onset, virus replication was significantly impaired (1,000-fold lower titers) in nondividing cells, and plaque-forming ability was severely compromised. The RR1 protein expressed by a revertant virus [HSV-2(R)] was structurally and functionally similar to the wild-type protein, and the virus had wild-type growth and plaque-forming properties. The growth of the ICP10DeltaPK virus and its plaque-forming potential were restored to wild-type levels in cells that constitutively express ICP10. Immediate-early (IE) genes for ICP4, ICP27, and ICP22 were not expressed in Vero cells infected with ICP10DeltaPK early in infection or in the presence of cycloheximide, and the levels of ICP0 and p95 were significantly (three- to sevenfold) lower than those in HSV-2- or HSV-2(R)-infected cells. IE gene expression was similar to that of the wild-type virus in cells that constitutively express ICP10. The data indicate that ICP10 PK is required for early expression of the viral regulatory IE genes and, consequently, for timely initiation of the protein cascade and HSV-2 growth in cultured cells.

Harnessing free radicals: formation and function of the tyrosyl radical in ribonucleotide reductase

Ribonucleotide reductases (RNRs) are uniquely responsible for converting nucleotides to deoxynucleotides in all organisms. The cofactor of class-I RNRs comprises a di-iron cluster and a tyrosyl radical, and is essential for initiation of radical-dependent nucleotide reduction. Recently, much progress has been made in understanding the mechanism by which this cofactor is generated in vitro and in vivo, as well as the function of the tyrosyl radical in nucleotide reduction. The Escherichia coli RNR cofactor provides a paradigm for cofactors in other di-iron requiring or tyrosyl-radical-requiring proteins.

O2 activation by non-heme diiron proteins: identification of a symmetric mu-1,2-peroxide in a mutant of ribonucleotide reductase

Non-heme diiron clusters occur in a number of enzymes (e.g., ribonucleotide reductase, methane monooxygenase, and Delta9-stearoyl-ACP desaturase) that activate O2 for chemically difficult oxidation reactions. In each case, a kinetically labile peroxo intermediate is believed to form when O2 reacts with the diferrous enzyme, followed by O-O bond cleavage and the formation of high-valent iron intermediates [formally Fe(IV)] that are thought to be the reactive oxidants. Greater kinetic stability of a peroxodiiron(III) intermediate in protein R2 of ribonucleotide reductase was achieved by the iron-ligand mutation Asp84 --> Glu and the surface mutation Trp48 --> Phe. Here, we present the first definitive evidence for a bridging, symmetrical peroxo adduct from vibrational spectroscopic studies of the freeze-trapped intermediate of this mutant R2. Isotope-sensitive bands are observed at 870, 499, and 458 cm-1 that are assigned to the intraligand peroxo stretching frequency and the asymmetric and symmetric Fe-O2-Fe stretching frequencies, respectively. Similar results have been obtained in the resonance Raman spectroscopic study of a peroxodiferric species of Delta9-stearoyl-ACP desaturase [Broadwater, J. A., Ai, J., Loehr, T. M., Sanders-Loehr, J., and Fox, B. G. (1998) Biochemistry 37, 14664-14671]. Similarities among these adducts and transient species detected during O2 activation by methane monooxygenase hydroxylase, ferritin, and wild-type protein R2 suggest the symmetrical peroxo adduct as a common intermediate in the diverse oxidation reactions mediated by members of this class.

Ribonucleotide reductases

Ribonucleotide reductases provide the building blocks for DNA replication in all living cells. Three different classes of enzymes use protein free radicals to activate the substrate. Aerobic class I enzymes generate a tyrosyl radical with an iron-oxygen center and dioxygen, class II enzymes employ adenosylcobalamin, and the anaerobic class III enzymes generate a glycyl radical from S-adenosylmethionine and an iron-sulfur cluster. The X-ray structure of the class I Escherichia coli enzyme, including forms that bind substrate and allosteric effectors, confirms previous models of catalytic and allosteric mechanisms. This structure suggests considerable mobility of the protein during catalysis and, together with experiments involving site-directed mutants, suggests a mechanism for radical transfer from one subunit to the other. Despite large differences between the classes, common catalytic and allosteric mechanisms, as well as retention of critical residues in the protein sequence, suggest a similar tertiary structure and a common origin during evolution. One puzzling aspect is that some organisms contain the genes for several different reductases.

The function of adenosylcobalamin in the mechanism of ribonucleoside triphosphate reductase from Lactobacillus leichmannii

Ribonucleoside triphosphate reductase from Lactobacillus leichmannii catalyzes the reduction of nucleotides to deoxynucleotides and uses adenosylcobalamin as a cofactor. A transient protein-based thiyl radical is essential for catalysis. Studies directed toward the elucidation of the function of adenosylcobalamin during catalysis have shown that formation of the thiyl radical and 5'-deoxyadenosine occurs in a concerted fashion with C-Co bond homolysis, that the homolysis is entropically and not enthalpically driven, that the dimethylbenzimidazole moiety of adenosylcobalamin is the axial ligand during catalysis, and that the C-Co bond is reformed after every turnover.

Structure of Salmonella typhimurium nrdF ribonucleotide reductase in its oxidized and reduced forms

The first class Ib ribonucleotide reductase R2 structure, from Salmonella typhimurium, has been determined at 2.0 A resolution. The overall structure is similar to the Escherichia coli class Ia enzyme despite only 23% sequence identity. The most spectacular difference is the absence of the pleated sheet and adjacent parts present in the E. coli R2 structure; the heart-shaped structure loses its tip. From sequence comparisons, it appears that this feature is shared with all other class Ib enzymes and, in this respect, is more like the mammalian class Ia enzymes. Both the oxidized and reduced iron forms have been investigated. In the ferric iron center, both iron ions are octahedrally coordinated and bridged by one carboxylate and one oxide ion. The ferrous form has lost the bridging oxide ion but is bridged by two carboxylates. Accompanying the change in redox state, helix E changes its conformation from one covering the metal center in the oxidized form to a more open reduced form. A narrow channel is opened which may permit easier access of oxygen to the ferrous iron site and to efficiently generate the tyrosyl radical.