Xanthosine and xanthine. Substrate properties with purine nucleoside phosphorylases, and relevance to other enzyme systems

Nucleoside Phosphorylases make N7-xanthosine

Modern, highly evolved nucleoside-processing enzymes are known to exhibit perfect regioselectivity over the glycosylation of purine nucleobases at N9. We herein report an exception to this paradigm. Wild-type nucleoside phosphorylases also furnish N7-xanthosine, a "non-native" ribosylation regioisomer of xanthosine. This unusual nucleoside possesses several atypical physicochemical properties such as redshifted absorption spectra, a high equilibrium constant of phosphorolysis and low acidity. Ultimately, the biosynthesis of this previously unknown natural product illustrates how even highly evolved, essential enzymes from primary metabolism are imperfect catalysts.

Scaling Catalytic Contributions of Small Self-Cleaving Ribozymes

Nucleolytic ribozymes utilize general acid-base catalysis to perform phosphodiester cleavage. In most ribozyme classes, a conserved active site guanosine is positioned to act as general base, thereby activating the 2'-OH group to attack the scissile phosphate (γ-catalysis). Here, we present an atomic mutagenesis study for the pistol ribozyme class. Strikingly, "general base knockout" by replacement of the guanine N1 atom by carbon results in only 2.7-fold decreased rate. Therefore, the common view that γ-catalysis critically depends on the N1 moiety becomes challenged. For pistol ribozymes we found that γ-catalysis is subordinate in overall catalysis, made up by two other catalytic factors (α and δ). Our approach allows scaling of the different catalytic contributions (α, β, γ, δ) with unprecedented precision and paves the way for a thorough mechanistic understanding of nucleolytic ribozymes with active site guanines.

Scaling Catalytic Contributions of Small Self-Cleaving Ribozymes

Nucleolytic ribozymes utilize general acid-base catalysis to perform phosphodiester cleavage. In most ribozyme classes, a conserved active site guanosine is positioned to act as general base, thereby activating the 2'-OH group to attack the scissile phosphate (γ-catalysis). Here, we present an atomic mutagenesis study for the pistol ribozyme class. Strikingly, "general base knockout" by replacement of the guanine N1 atom by carbon results in only 2.7-fold decreased rate. Therefore, the common view that γ-catalysis critically depends on the N1 moiety becomes challenged. For pistol ribozymes we found that γ-catalysis is subordinate in overall catalysis, made up by two other catalytic factors (α and δ). Our approach allows scaling of the different catalytic contributions (α, β, γ, δ) with unprecedented precision and paves the way for a thorough mechanistic understanding of nucleolytic ribozymes with active site guanines.

Nucleotides, Nucleosides, and Nucleobases

We review literature on the metabolism of ribo- and deoxyribonucleotides, nucleosides, and nucleobases in Escherichia coli and Salmonella,including biosynthesis, degradation, interconversion, and transport. Emphasis is placed on enzymology and regulation of the pathways, at both the level of gene expression and the control of enzyme activity. The paper begins with an overview of the reactions that form and break the N-glycosyl bond, which binds the nucleobase to the ribosyl moiety in nucleotides and nucleosides, and the enzymes involved in the interconversion of the different phosphorylated states of the nucleotides. Next, the de novo pathways for purine and pyrimidine nucleotide biosynthesis are discussed in detail.Finally, the conversion of nucleosides and nucleobases to nucleotides, i.e.,the salvage reactions, are described. The formation of deoxyribonucleotides is discussed, with emphasis on ribonucleotidereductase and pathways involved in fomation of dUMP. At the end, we discuss transport systems for nucleosides and nucleobases and also pathways for breakdown of the nucleobases.

Dynamic simulation and metabolome analysis of long-term erythrocyte storage in adenine-guanosine solution

Although intraerythrocytic ATP and 2,3-bisphophoglycerate (2,3-BPG) are known as direct indicators of the viability of preserved red blood cells and the efficiency of post-transfusion oxygen delivery, no current blood storage method in practical use has succeeded in maintaining both these metabolites at high levels for long periods. In this study, we constructed a mathematical kinetic model of comprehensive metabolism in red blood cells stored in a recently developed blood storage solution containing adenine and guanosine, which can maintain both ATP and 2,3-BPG. The predicted dynamics of metabolic intermediates in glycolysis, the pentose phosphate pathway, and purine salvage pathway were consistent with time-series metabolome data measured with capillary electrophoresis time-of-flight mass spectrometry over 5 weeks of storage. From the analysis of the simulation model, the metabolic roles and fates of the 2 major additives were illustrated: (1) adenine could enlarge the adenylate pool, which maintains constant ATP levels throughout the storage period and leads to production of metabolic waste, including hypoxanthine; (2) adenine also induces the consumption of ribose phosphates, which results in 2,3-BPG reduction, while (3) guanosine is converted to ribose phosphates, which can boost the activity of upper glycolysis and result in the efficient production of ATP and 2,3-BPG. This is the first attempt to clarify the underlying metabolic mechanism for maintaining levels of both ATP and 2,3-BPG in stored red blood cells with in silico analysis, as well as to analyze the trade-off and the interlock phenomena between the benefits and possible side effects of the storage-solution additives.

Comparative kinetic analysis of AzgA and Fcy21p, prototypes of the two major fungal hypoxanthine-adenine-guanine transporter families

In fungi, uptake of salvageable purines is carried out by members of two evolutionarily distinct protein families, the Purine-Related Transporters (PRT/NCS1) and the AzgA-like Transporters. We carried out a comparative kinetic analysis of two prototypes of these transporter families. The first was Fcy21p, a herein characterized protein of Candida albicans, and the second was AzgA, a transporter of Aspergillus nidulans. Our results showed that: (i) AzgA and Fcy21p are equally efficient high-affinity, high-capacity, purine transporters, (ii) Fcy21p, but not AzgA, is an efficient cytosine and 5-fluorocytosine transporter, interacting with =O2 and C4-NH2 of the pyrimidine ring, (iii) the major interactions of AzgA and Fcy21p with the purine ring are similar, but not identical, involving in all cases positions 6 and 7, and for some substrates, positions 1 and 9 as well, and (iv) in AzgA, bulky groups at position N3 have a detrimental steric effect on substrate binding, while similar substitutions at C2 or N9 are fully or partially tolerated. In contrast, in Fcy21p, C2 and N9 bulky substitutions abolish substrate binding, while similar substitutions in N3 are fully tolerated. These results suggest that all fungal purine transporters might have evolved from a single ancestral protein, and show that fungal transporters use different substrate interactions compared to the analogous protozoan or mammalian proteins. Finally, results are also discussed in respect of the possibility of using fungal purine transporters as specific gateways for the development of targeted antifungal pharmacological therapies.

An ab initio and AIM investigation into the hydration of 2-thioxanthine

BACKGROUND

Hydration is a universal phenomenon in nature. The interactions between biomolecules and water of hydration play a pivotal role in molecular biology. 2-Thioxanthine (2TX), a thio-modified nucleic acid base, is of significant interest as a DNA inhibitor yet its interactions with hydration water have not been investigated either computationally or experimentally. Here in, we reported an ab initio study of the hydration of 2TX, revealing water can form seven hydrated complexes.

RESULTS

Hydrogen-bond (H-bond) interactions in 1:1 complexes of 2TX with water are studied at the MP2/6-311G(d, p) and B3LYP/6-311G(d, p) levels. Seven 2TX...H2O hydrogen bonded complexes have been theoretically identified and reported for the first time. The proton affinities (PAs) of the O, S, and N atoms and deprotonantion enthalpies (DPEs) of different N-H bonds in 2TX are calculated, factors surrounding why the seven complexes have different hydrogen bond energies are discussed. The theoretical infrared and NMR spectra of hydrated 2TX complexes are reported to probe the characteristics of the proposed H-bonds. An improper blue-shifting H-bond with a shortened C-H bond was found in one case. NBO and AIM analysis were carried out to explain the formation of improper blue-shifting H-bonds, and the H-bonding characteristics are discussed.

CONCLUSION

2TX can interact with water by five different H-bonding regimes, N-H...O, O-H...N, O-H...O, O-H...S and C-H...O, all of which are medium strength hydrogen bonds. The most stable H-bond complex has a closed structure with two hydrogen bonds (N(7)-H...O and O-H...O), whereas the least stable one has an open structure with one H-bond. The interaction energies of the studied complexes are correlated to the PA and DPE involved in H-bond formation. After formation of H-bonds, the calculated IR and NMR spectra of the 2TX-water complexes change greatly, which serves to identify the hydration of 2TX.

Steric and electrostatic effects at the C2 atom substituent influence replication and miscoding of the DNA deamination product deoxyxanthosine and analogs by DNA polymerases

Deoxyinosine (dI) and deoxyxanthosine (dX) are both formed in DNA at appreciable levels in vivo by deamination of deoxyadenosine (dA) and deoxyguanosine (dG), respectively, and can miscode. Structure-activity relationships for dA pairing have been examined extensively using analogs but relatively few studies have probed the roles of the individual hydrogen-bonding atoms of dG in DNA replication. The replicative bacteriophage T7 DNA polymerase/exonuclease and the translesion DNA polymerase Sulfolobus solfataricus pol IV were used as models to discern the mechanisms of miscoding by DNA polymerases. Removal of the 2-amino group from the template dG (i.e., dI) had little impact on the catalytic efficiency of either polymerase, as judged by either steady-state or pre-steady-state kinetic analysis, although the misincorporation frequency was increased by an order of magnitude. dX was highly miscoding with both polymerases, and incorporation of several bases was observed. The addition of an electronegative fluorine atom at the 2-position of dI lowered the oligonucleotide T(m) and strongly inhibited incorporation of dCTP. The addition of bromine or oxygen (dX) at C2 lowered the T(m) further, strongly inhibited both polymerases, and increased the frequency of misincorporation. Linear activity models show the effects of oxygen (dX) and the halogens at C2 on both DNA polymerases as mainly due to a combination of both steric and electrostatic factors, producing a clash with the paired cytosine O2 atom, as opposed to either bulk or perturbation of purine ring electron density alone.

Comparative substrate recognition by bacterial and fungal purine transporters of the NAT/NCS2 family

We compared the interactions of purines and purine analogues with representative fungal and bacterial members of the widespread Nucleobase-Ascorbate Transporter (NAT) family. These are: UapA, a well-studied xanthine-uric acid transporter of A. nidulans, Xut1, a novel transporter from C. albicans, described for the first time in this work, and YgfO, a recently characterized xanthine transporter from E. coli. Using transport inhibition experiments with 64 different purines and purine-related analogues, we describe a kinetic approach to build models on how NAT proteins interact with their substrates. UapA, Xut1 and YgfO appear to bind several substrates via interactions with both the pyrimidine and imidazol rings. Fungal homologues interact with the pyrimidine ring of xanthine and xanthine analogues via H-bonds, principally with N1-H and =O6, and to a lower extent with =O2. The E. coli homologue interacts principally with N3-H and =O2, and less strongly with N1-H and =O6. The basic interaction with the imidazol ring appears to be via a H-bond with N9. Interestingly, while all three homologues recognize xanthines with similar high affinities, interaction with uric acid or/and oxypurinol is transporter-specific. UapA recognizes uric acid with high affinity, principally via three H-bonds with =O2, =O6 and =O8. Xut1 has a 13-fold reduced affinity for uric acid, based on a different set of interactions involving =O8, and probably H atoms from positions N1, N3, N7 or N9. YgfO does not recognize uric acid at all. Both Xut1 and UapA recognize oxypurinol, but use different interactions reflected in a nearly 26-fold difference in their affinities for this drug, while YgfO interacts with this analogue very inefficiently.

Mechanistic studies in the synthesis of a series of thieno-expanded xanthosine and guanosine nucleosides

During the synthetic pursuit of guanosine (triG) and xanthosine (triX) tricyclic nucleosides analogues, an interesting side product was discovered. In an effort to uncover the mechanistic factors leading to this result, a series of reaction conditions were investigated. It was found that by varying the conditions, the appearance of the side product could be controlled. In addition, the yield of the desired products could be manipulated to afford either a 50:50 mix of both triG and triX, or a majority of one or the other. To demonstrate the broad utility of the method, it was also adapted to the synthesis of guanosine and xanthosine from 5-amino-1-beta-D-ribofuranosyl-4-imidazolecarboxyamide (AICAR). The mechanistic details surrounding the synthetic efforts are reported herein.

Structural requirements for the activation of Escherichia coli CTP synthase by the allosteric effector GTP are stringent, but requirements for inhibition are lax

Cytidine 5'-triphosphate synthase catalyzes the ATP-dependent formation of CTP from UTP using either NH(3) or l-glutamine (Gln) as the source of nitrogen. GTP acts as an allosteric effector promoting Gln hydrolysis but inhibiting Gln-dependent CTP formation at concentrations of >0.15 mM and NH(3)-dependent CTP formation at all concentrations. A structure-activity study using a variety of GTP and guanosine analogues revealed that only a few GTP analogues were capable of activating Gln-dependent CTP formation to varying degrees: GTP approximately 6-thio-GTP > ITP approximately guanosine 5'-tetraphosphate > O(6)-methyl-GTP > 2'-deoxy-GTP. No activation was observed with guanosine, GMP, GDP, 2',3'-dideoxy-GTP, acycloguanosine, and acycloguanosine monophosphate, indicating that the 5'-triphosphate, 2'-OH, and 3'-OH are required for full activation. The 2-NH(2) group plays an important role in binding recognition, whereas substituents at the 6-position play an important role in activation. The presence of a 6-NH(2) group obviates activation, consistent with the inability of ATP to substitute for GTP. Nucleotide and nucleoside analogues of GTP and guanosine, respectively, all inhibited NH(3)- and Gln-dependent CTP formation (often in a cooperative manner) to a similar extent (IC(50) approximately 0.2-0.5 mM). This inhibition appeared to be due solely to the purine base and was relatively insensitive to the identity of the purine with the exception of inosine, ITP, and adenosine (IC(50) approximately 4-12 mM). 8-Oxoguanosine was the best inhibitor identified (IC(50) = 80 microM). Our findings suggest that modifying 2-aminopurine or 2-aminopurine riboside may serve as an effective strategy for developing cytidine 5'-triphosphate synthase inhibitors.

Role of ionization of the phosphate cosubstrate on phosphorolysis by purine nucleoside phosphorylase (PNP) of bacterial (E. coli) and mammalian (human) origin

Kinetics of the reactions of purine nucleoside phosphorylases (PNP) from E. coli (PNP-I, the product of the deoD gene) and human erythrocytes with their natural substrates guanosine (Guo), inosine (Ino), a substrate analogue N(7)-methylguanosine (m(7)Guo), and orthophosphate (P(i), natural cosubstrate) and its thiophosphate analogue (SP(i)), found to be a weak cosubstrate, have been studied in the pH range 5-8. In this pH range Guo and Ino exist predominantly in the neutral forms (pK(a) 9.2 and 8.8); m(7)Guo consists of an equilibrium mixture of the cationic and zwitterionic forms (pK(a) 7.0); and P(i) and SP(i) exhibit equilibria between monoanionic and dianionic forms (pK(a) 6.7 and 5.4, respectively). The phosphorolysis of m(7)Guo (at saturated concentration) with both enzymes exhibits Michaelis kinetics with SP(i), independently of pH. With P(i), the human enzyme shows Michaelis kinetics only at pH approximately 5. However, in the pH range 5-8 for the bacterial enzyme, and 6-8 for the human enzyme, enzyme kinetics with P(i) are best described by a model with high- and low-affinity states of the enzymes, denoted as enzyme-substrate complexes with one or two active sites occupied by P(i), characterized by two sets of enzyme-substrate dissociation constants (apparent Michaelis constants, K (m1) and K (m2)) and apparent maximal velocities (V (max1) and V (max2)). Their values, obtained from non-linear least-squares fittings of the Adair equation, were typical for negative cooperativity of both substrate binding (K (m1) < K (m2)) and enzyme kinetics (V (max1)/K (m1) > V (max2)/K (m2)). Comparison of the pH-dependence of the substrate properties of P(i) versus SP(i) points to both monoanionic and dianionic forms of P(i) as substrates, with a marked preference for the dianionic species in the pH range 5-8, where the population of the P(i) dianion varies from 2 to 95%, reflected by enzyme efficiency three orders of magnitude higher at pH 8 than that at pH 5. This is accompanied by an increase in negative cooperativity, characterized by a decrease in the Hill coefficient from n (H) approximately 1 to n (H) approximately 0.7 for Guo with the human enzyme, and to n (H) approximately 0.7 and 0.5 for m(7)Guo with the E. coli and human enzymes, respectively. Possible mechanisms of cooperativity are proposed. Attention is drawn to the substrate properties of SP(i) in relation to its structure.

Multiple transients in the pre-steady-state of nucleoside hydrolase reveal complex substrate binding, product base release, and two apparent rates of chemistry

We have investigated the transient kinetics of the nucleoside hydrolase from Trypanosoma vivax (TvNH) at low temperatures (5 degrees C). Three novel absorbance transients (termed tau1, tau3, and tau4) were detected during multiple-guanosine turnover stopped-flow absorbance spectroscopy, in addition to a transient (tau2) that had been observed previously at 35 degrees C. At 5 degrees C, TvNH displays full-sites activity and not half-of-the-sites activity as is apparent at 35 degrees C. Both tau1 and tau2 are assigned to chemistry based on rapid-quench results. For tau1, the rate of chemistry is ca. 3000-fold faster than kcat (1-2 orders of magnitude greater than previous estimates). The pH dependencies of the burst amplitudes for tau1 and tau2 indicate that these transients arise from the formation of two different dimeric TvNH.substrate complexes and not from TvNH that contains kinetically asymmetric monomers. The saturating burst rates for tau1 and tau2 are surprisingly pH-independent, given the prominent role of acid-base chemistry in the proposed mechanism for TvNH. tau3 and tau4 are assigned to the substrate binding and base release processes, respectively, and shown to be equivalent to two fluorescence transients (tau3 and tau4, respectively) observed previously by stopped-flow methods at 35 degrees C. The rate of base release is shown to be an apparent rate. Together with steady-state product inhibition results, the data indicate that TvNH follows an ordered uni-bi kinetic mechanism with a TvNH.base dead-end complex, and not the rapid equilibrium random uni-bi mechanism proposed for other NHs. Two applicable kinetic models are presented and their implications for future mechanistic studies discussed.

The extraordinary specificity of xanthine phosphoribosyltransferase from Bacillus subtilis elucidated by reaction kinetics, ligand binding, and crystallography

Xanthine phosphoribosyltransferase (XPRTase) from Bacillus subtilis is a representative of the highly xanthine specific XPRTases found in Gram-positive bacteria. These XPRTases constitute a distinct subclass of 6-oxopurine PRTases, which deviate strongly from the major class of H(X)GPRTases with respect to sequence, PRPP binding motif, and oligomeric structure. They are more related with the PurR repressor of Gram-positive bacteria, the adenine PRTase, and orotate PRTase. The catalytic function and high specificity for xanthine of B. subtilis XPRTase were investigated by ligand binding studies and reaction kinetics as a function of pH with xanthine, hypoxanthine, and guanine as substrates. The crystal structure of the dimeric XPRTase-GMP complex was determined to 2.05 A resolution. In a sequential reaction mechanism XPRTase binds first PRPP, stabilizing its active dimeric form, and subsequently xanthine. The XPRTase is able also to react with guanine and hypoxanthine albeit at much lower (10(-)(4)-fold) catalytic efficiency. Different pK(a) values for the bases and variations in their electrostatic potential can account for these catalytic differences. The unique base specificity of XPRTase has been related to a few key residues in the active site. Asn27 can in different orientations form hydrogen bonds to an amino group or an oxo group at the 2-position of the purine base, and Lys156 is positioned to make a hydrogen bond with N7. This and the absence of a catalytic carboxylate group near the N7-position require the purine base to dissociate a proton spontaneously in order to undergo catalysis.

Escherichia coli Purine Nucleoside Phosphorylase II, the Product of the xapA Gene

Purine nucleoside phosphorylases (PNPs, E. C. 2.4.2.1) use orthophosphate to cleave the N-glycosidic bond of beta-(deoxy)ribonucleosides to yield alpha-(deoxy)ribose 1-phosphate and the free purine base. Escherichia coli PNP-II, the product of the xapA gene, is similar to trimeric PNPs in sequence, but has been reported to migrate as a hexamer and to accept xanthosine with comparable efficiency to guanosine and inosine, the usual physiological substrates for trimeric PNPs. Here, we present a detailed biochemical characterization and the crystal structure of E.coli PNP-II. In three different crystal forms, PNP-II trimers dimerize, leading to a subunit arrangement that is qualitatively different from the "trimer of dimers" arrangement of conventional high molecular mass PNPs. Crystal structures are compatible with similar binding modes for guanine and xanthine, with a preference for the neutral over the monoanionic form of xanthine. A single amino acid exchange, tyrosine 191 to leucine, is sufficient to convert E.coli PNP-II into an enzyme with the specificity of conventional trimeric PNPs, but the reciprocal mutation in human PNP, valine 195 to tyrosine, does not elicit xanthosine phosphorylase activity in the human enzyme.

Purification and characterization of RihC, a xanthosine-inosine-uridine-adenosine-preferring hydrolase from Salmonella enterica serovar Typhimurium

Salmonella enterica serovar Typhimurium normally salvage nucleobases and nucleosides by the action of nucleoside phosphorylases and phosphoribosyltransferases. In contrast to Escherichia coli, which catabolizes xanthosine by xanthosine phosphorylase (xapA), Salmonella cannot grow on xanthosine as the sole carbon and energy source. By functional complementation, we have isolated a nucleoside hydrolase (rihC) that can complement a xapA deletion in E. coli and we have overexpressed, purified and characterized this hydrolase. RihC is a heat stable homotetrameric enzyme with a molecular weight of 135 kDa that can hydrolyze xanthosine, inosine, adenosine and uridine with similar catalytic efficiency (k(cat)/Km=1 to 4 x 10(4) M(-1)s(-1)). Cytidine and guanosine is hydrolyzed with approximately 10-fold lower efficiency (k(cat)/Km=0.7 to 1.2 x 10(3) M(-1)s(-1)) while RihC is unable to hydrolyze the deoxyribonucleosides thymidine and deoxyinosine. The Km for all nucleosides except adenosine is in the mM range. The pH optimum is different for inosine and xanthosine and the hydrolytic capacity (k(cat)/Km) is 5-fold higher for xanthosine than for inosine at pH 6.0 while they are similar at pH 7.2, indicating that RihC most likely prefers the neutral form of xanthosine.

A Combined Computational and Experimental Study of the Hydrogen-Bonded Dimers of Xanthine and Hypoxanthine

In addition to uracil, the noncanonical nucleobases xanthine and hypoxanthine are important lesions that are formed from the canonical bases when a cell is under oxidative stress. It is known that they lead to point mutations; however, more detailed information about their ability to form hydrogen-bonded complexes is not available. In the present paper such information is obtained by a combined experimental and theoretical approach. Accurate association constants of xanthosine and inosine dimers are determined by concentration dependent 1H NMR experiments, and a structural characterization of individual complexes formed in solution is performed through measurements under slow exchange conditions at very low temperatures. An interpretation of the experimental data concerning complex geometries becomes possible through a comparison of measured and computed NMR chemical shifts. Further qualitative insights into the hydrogen bonding abilities of xanthine and hypoxanthine are obtained by a theoretical characterization of all possible pairing modes of xanthine and hypoxanthine dimers and by a comparison with simplified model systems. The influence of a polar medium on the bonding properties is also estimated and the importance of the various effects is discussed. Our analysis shows to what extent secondary electronic and electrostatic effects influence the hydrogen bonding properties of xanthine and hypoxanthine in the gas phase and in polar solvents.

Evidence for the involvement of cytosolic 5'-nucleotidase (cN-II) in the synthesis of guanine nucleotides from xanthosine

In this paper, we show that in vitro xanthosine does not enter any of the pathways known to salvage the other three main natural purine nucleosides: guanosine; inosine; and adenosine. In rat brain extracts and in intact LoVo cells, xanthosine is salvaged to XMP via the phosphotransferase activity of cytosolic 5'-nucleotidase. IMP is the preferred phosphate donor (IMP + xanthosine --> XMP + inosine). XMP is not further phosphorylated. However, in the presence of glutamine, it is readily converted to guanyl compounds. Thus, phosphorylation of xanthosine by cytosolic 5'-nucleotidase circumvents the activity of IMP dehydrogenase, a rate-limiting enzyme, catalyzing the NAD(+)-dependent conversion of IMP to XMP at the branch point of de novo nucleotide synthesis, thus leading to the generation of guanine nucleotides. Mycophenolic acid, an inhibitor of IMP dehydrogenase, inhibits the guanyl compound synthesis via the IMP dehydrogenase pathway but has no effect on the cytosolic 5'-nucleotidase pathway of guanine nucleotides synthesis. We propose that the latter pathway might contribute to the reversal of the in vitro antiproliferative effect exerted by IMP dehydrogenase inhibitors routinely seen with repletion of the guanine nucleotide pools.

Structural properties of the neutral and monoanionic forms of xanthosine, highly relevant to their substrate properties with various enzyme systems

The monoanions of the 6-oxopurines guanine (Gua) and hypoxanthine (Hx), and their nucleosides, pKa approximately 9 due to dissociation of the N(1)-H, are predominantly in their neutral forms at physiological pH. By contrast, the monoanions of the 6-oxopurine xanthine (Xan) and xanthosine (Xao), were long ago proposed to involve dissociation of the N(3)-H, with pKa values of 7.5 and 5.7, respectively, so that, at physiological pH, the former is mixture of the neutral and monoanionic species, and the latter predominantly the monoanion. We have employed multi-dimensional heteronuclear NMR spectroscopy, which fully confirms the proposed mode of monoanion formation in Xao (and, by implication, in Xan), further supported by the results of ab initio quantum mechanical calculations, and additionally extended to determination of the preferred conformational parameters in solution for the neutral and monoanionic species. These findings are highly relevant to the modes of binding, and to the substrate properties, of Xan, Xao and its 5'-phosphate (XMP) in numerous enzyme systems, hitherto virtually ignored, and illustrated by several concrete examples.