IMP dehydrogenase from the intracellular parasitic protozoan Eimeria tenella and its inhibition by mycophenolic acid

Inosine 5'-monophosphate dehydrogenase inhibitors as antimicrobial agents: recent progress and future perspectives

Inosine 5'-monophosphate dehydrogenase (IMPDH), a crucial enzyme required for de novo synthesis of guanine nucleotides, is an important target for cancer, bacterial, parasitic and viral infections and autoimmune disorders. Several classes of IMPDH inhibitors are known in the literature. The current review succinctly summarizes the progress made in the design and development of IMPDH inhibitors as antimicrobial agents in last five years or so. The focus is on the inhibitor and enzyme structural features responsible for imparting selectivity for the microbial over the host enzyme. Future perspectives clearly outline the inhibitor design opportunities available in this area to address the present challenges of drug resistance and re-emergence of newer and deadly strains of microbes, posing a serious threat to public.

Application of prodrugs to inflammatory diseases of the gut

Oral delivery is the most common and preferred route of drug administration although the digestive tract exhibits several obstacles to drug delivery including motility and intraluminal pH profiles. The gut milieu represents the largest mucosal surface exposed to microorganisms with 10^10-12 colony forming bacteria/g of colonic content. Approximately, one third of fecal dry matter is made of bacteria/ bacterial components. Indeed, the normal gut microbiota is responsible for healthy digestion of dietary fibers (polysaccharides) and fermentation of short chain fatty acids such as acetate and butyrate that provide carbon sources (fuel) for these bacteria. Inflammatory bowel disease (IBD) results in breakage of the mucosal barrier, an altered microbiota and dysregulated gut immunity. Prodrugs that are chemically constructed to target colonic release or are degraded specifically by colonic bacteria, can be useful in the treatment of IBD. This review describes the progress in digestive tract prodrug design and delivery in light of gut metabolic activities.

IMP dehydrogenase from the protozoan parasite Toxoplasma gondii

The opportunistic apicomplexan parasite Toxoplasma gondii damages fetuses in utero and threatens immunocompromised individuals. The toxicity associated with standard antitoxoplasmal therapies, which target the folate pathway, underscores the importance of examining alternative pharmacological strategies. Parasitic protozoa cannot synthesize purines de novo; consequently, targeting purine salvage enzymes is a plausible pharmacological strategy. Several enzymes critical to purine metabolism have been studied in T. gondii, but IMP dehydrogenase (IMPDH), which catalyzes the conversion of IMP to XMP, has yet to be characterized. Thus, we have cloned the gene encoding this enzyme in T. gondii. Northern blot analysis shows that two IMPDH transcripts are present in T. gondii tachyzoites. The larger transcript contains an open reading frame of 1,656 nucleotides whose deduced protein sequence consists of 551 amino acids (TgIMPDH). The shorter transcript is an alternative splice product that generates a 371-amino-acid protein lacking the active-site flap (TgIMPDH-S). When TgIMPDH is expressed as a recombinant protein fused to a FLAG tag, the fusion protein localizes to the parasite cytoplasm. Immunoprecipitation with anti-FLAG was employed to purify recombinant TgIMPDH, which converts IMP to XMP as expected. Mycophenolic acid is an uncompetitive inhibitor relative to NAD+, with a intercept inhibition constant (Kii) of 0.03+/-0.004 microM. Tiazofurin and its seleno analog were not inhibitory to the purified enzyme, but adenine dinucleotide analogs such as TAD and the nonhydrolyzable beta-methylene derivatives of TAD or SAD were inhibitory, with Kii values 13- to 60-fold higher than that of mycophenolic acid.

Inhibition of reovirus by mycophenolic acid is associated with the M1 genome segment

Mycophenolic acid (MPA), an inhibitor of IMP dehydrogenase, inhibits reovirus replication and viral RNA and protein production. In mouse L929 cells, antiviral effects were greatest at 30 microg of MPA/ml. At this dosage, MPA inhibited replication of reovirus strain T3D more than 1,000-fold and inhibited replication of reovirus strain T1L nearly 100-fold, compared to non-drug-treated controls. Genetic reassortant analysis indicated the primary determinant of strain-specific differences in sensitivity to MPA mapped to the viral M1 genome segment, which encodes the minor core protein mu2. MPA also inhibited replication of both strains of reovirus in a variety of other cell lines, including Vero monkey kidney and U373 human astrocytoma cells. Addition of exogenous guanosine to MPA-treated reovirus-infected cells restored viral replicative capacity to nearly normal levels. These results suggest the mu2 protein is involved in the uptake and processing of GTP in viral transcription in infected cells and strengthens the evidence that the mu2 protein can function as an NTPase and is likely a transcriptase cofactor.

Erosive enterocolitis in mycophenolate mofetil-treated renal-transplant recipients with persistent afebrile diarrhea

BACKGROUND

Diarrhea is the most frequently reported adverse event in mycophenolate mofetil (MMF)-treated transplant patients. The aim of this study was to explore the gastrointestinal tract in MMF-treated renal transplant recipients with persistent afebrile diarrhea to characterize its nature and etiology.

METHODS

Renal transplant recipients with persistent afebrile diarrhea (daily fecal output >200 g) were prospectively investigated for infections, morphologic, and functional (gastrointestinal motility and intestinal absorptive capacity) integrity of the gastrointestinal tract; 26 patients met the inclusion criteria.

RESULTS

All but one patient had an erosive enterocolitis. Seventy percent of the patients had malabsorption of nutrients, contributing to the diarrhea. In +/-60%, an infectious origin was demonstrated and successfully treated with antimicrobial agents without changes in immunosuppressive regimen. In +/-40%, no infection occurred, but a Crohn's disease-like pattern of inflammation was noted. These patients also had a less pronounced bile-acid malabsorption but a significant faster colonic transit time, correlating with the trough level of mycophenolic acid (MPA). Cessation of MMF, however, was associated with allograft rejection in one third of these patients.

CONCLUSIONS

Persistent afebrile diarrhea in renal transplant recipients is characterized by erosive enterocolitis, which is of infectious origin in +/-60%. In +/-40%, a Crohn's disease-like (entero-)colitis was present. Because reduction or cessation of MMF was the only effective therapy, MPA or one of its metabolites may be suggested as a possible cause. However, reduction or cessation of MMF was associated with an increased risk for rejection.

Crystal structures of Tritrichomonasfoetus inosine monophosphate dehydrogenase in complex with substrate, cofactor and analogs: a structural basis for the random-in ordered-out kinetic mechanism

The enzyme inosine monophosphate dehydrogenase (IMPDH) is responsible for the rate-limiting step in guanine nucleotide biosynthesis. Because it is up-regulated in rapidly proliferating cells, human type II IMPDH is actively targeted for immunosuppressive, anticancer, and antiviral chemotherapy. The enzyme employs a random-in ordered-out kinetic mechanism where substrate or cofactor can bind first but product is only released after the cofactor leaves. Due to structural and kinetic differences between mammalian and microbial enzymes, most drugs that are successful in the inhibition of mammalian IMPDH are far less effective against the microbial forms of the enzyme. It is possible that with greater knowledge of the structural mechanism of the microbial enzymes, an effective and selective inhibitor of microbial IMPDH will be developed for use as a drug against multi-drug resistant bacteria and protists. The high-resolution crystal structures of four different complexes of IMPDH from the protozoan parasite Tritrichomonas foetus have been solved: with its substrate IMP, IMP and the inhibitor mycophenolic acid (MPA), the product XMP with MPA, and XMP with the cofactor NAD(+). In addition, a potassium ion has been located at the dimer interface. A structural model for the kinetic mechanism is proposed.

Neonatal pneumonia caused by Trichomonas vaginalis

The authors present two cases of newborn babies infected by Trichomonas vaginalis (hereafter referred to as T. vaginalis) and suffering from severe congenital breathing difficulties and needing artificial respiration. Microscopic examination of the tracheal discharge revealed characteristically moving, flagellated, pear-shaped unicellular organisms. Cultures on CPLM medium proved the presence of T. vaginalis. During pregnancy the mothers' clinical status was negative and both of them mentioned leukorrhoea of changing intensity. They were regularly involved in antenatal care. The infection caused by T. vaginalis could be detected in the two mothers later by culture procedures.

Molecular cloning and characterization of a cDNA encoding soybean nodule IMP dehydrogenase

Inosine monophosphate dehydrogenase (IMPDH) catalyzes the rate-limiting step in de novo purine biosynthesis and is a postulated key enzyme in nitrogen assimilation in ureide-exporting nodules. A 2016 bp cDNA for IMPDH, designated as IMPDH, was cloned from a soybean nodule cDNA library. IMPDH encodes a polypeptide of 502 amino acids with a predicted molecular weight of 53000 and a pI of 5.54. The deduced IMPDH is 70.5% identical to that in Arabidopsis, with a 100% homology in the putative active-site region. Expressing the cloned cDNA in Escherichia coli mutant strain KLC381 (DeltaguaB) restored IMPDH activity, permitting bacterial growth on minimal medium. Southern blot analysis suggested a single copy of IMPDH gene in the soybean genome. Northern blot analysis showed that the expression of IMPDH gene is apparently nodule-specific.

Inosine-5'-monophosphate dehydrogenase is required for mitogenic competence of transformed pancreatic beta cells

The relation of inosine-5'-monophosphate dehydrogenase (IMPDH; the rate-limiting enzyme in GTP synthesis) to mitogenesis was studied by enzymatic assay, immunoblots, and RT-PCR in several dissimilar transformed pancreatic ss-cell lines, using intact cells. Both of the two isoforms of IMPDH (constitutive type 1 and inducible type 2) were identified using RT-PCR in transformed beta cells or in intact islets. IMPDH 2 messenger RNA (mRNA) and IMPDH protein were both regulated reciprocally by changes in levels of their end-products. Flux through IMPDH was greatest in rapidly growing cells, due mostly to increased uptake of precursor. Glucose (but not 3-0-methylglucose, L-glucose, or fructose) further augmented substrate uptake and also increased IMPDH enzymatic activity after either 4 or 21 h of stimulation. Serum or ketoisocaproate also increased IMPDH activity (but not uptake). Two selective IMPDH inhibitors (mycophenolic acid and mizoribine) reduced IMPDH activity in all cell lines, and, with virtually identical concentration-response curves, inhibited DNA synthesis (assessed as bromodeoxyuridine incorporation) in response to glucose, serum, or ketoisocaproate. Inhibition of DNA synthesis was reversible, completely prevented by repletion of cellular guanine (but not adenine) nucleotides, and could not be attributed to toxic effects. Despite the fact that modulation of IMPDH expression by guanine nucleotides was readily detectable, glucose and/or serum failed to alter IMPDH mRNA or protein, indicating that their effects on IMPDH activity were largely at the enzyme level. Precursors of guanine nucleotides failed, by themselves, to induce mitogenesis. Thus, adequate IMPDH activity (and thereby, availability of GTP) is a critical requirement for beta-cell proliferation. Although it is unlikely that further increases in GTP can, by themselves, initiate DNA synthesis, such increments may be needed to sustain mitogenesis.

Asp338 controls hydride transfer in Escherichia coli IMP dehydrogenase

IMP dehydrogenase (IMPDH) catalyzes the oxidation of IMP to XMP with the concomitant reduction of NAD(+). This reaction involves the formation of a covalent adduct with an active site Cys. This intermediate, E-XMP, hydrolyzes to produce XMP. The mutation of Asp338 to Ala severely impairs the activity of Escherichia coli IMPDH, decreasing the value of k(cat) by 650-fold. No (D)V(m) or (D)V/K(m) isotope effects are observed when 2-(2)H-IMP is the substrate for wild-type IMPDH. Values of (D)V(m) = 2.6 and (D)V/K(m) (IMP) = 3.4 are observed for Asp338Ala. Moreover, while a burst of NADH production is observed for wild-type IMPDH, no burst is observed for Asp338Ala. These observations indicate that the mutation has decreased the rate of hydride transfer by at least 5 x 10(3)-fold. In contrast, k(cat) for the hydrolysis of 2-chloroinosine-5'-monophosphate is decreased by only 8-fold. In addition, the rate constant for inactivation by 6-chloropurine riboside 5'-monophosphate is increased by 3-fold. These observations suggest that the mutation has little effect on the nucleophilicity of the active site Cys residue. These results are consistent with a recent crystal structure that shows a hydrogen bonding network between Asp338, the 2'-OH of IMP, and the amide group of NAD(+) [Colby, T. D., Vanderveen, K., Strickler, M. D., Markham, G. D., and Goldstein, B. M. (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 3531-3536].

Mycophenolate mofetil and its mechanisms of action

Mycophenolate mofetil (MMF, CellCept(R)) is a prodrug of mycophenolic acid (MPA), an inhibitor of inosine monophosphate dehydrogenase (IMPDH). This is the rate-limiting enzyme in de novo synthesis of guanosine nucleotides. T- and B-lymphocytes are more dependent on this pathway than other cell types are. Moreover, MPA is a fivefold more potent inhibitor of the type II isoform of IMPDH, which is expressed in activated lymphocytes, than of the type I isoform of IMPDH, which is expressed in most cell types. MPA has therefore a more potent cytostatic effect on lymphocytes than on other cell types. This is the principal mechanism by which MPA exerts immunosuppressive effects. Three other mechanisms may also contribute to the efficacy of MPA in preventing allograft rejection and other applications. First, MPA can induce apoptosis of activated T-lymphocytes, which may eliminate clones of cells responding to antigenic stimulation. Second, by depleting guanosine nucleotides, MPA suppresses glycosylation and the expression of some adhesion molecules, thereby decreasing the recruitment of lymphocytes and monocytes into sites of inflammation and graft rejection. Third, by depleting guanosine nucleotides MPA also depletes tetrahydrobiopterin, a co-factor for the inducible form of nitric oxide synthase (iNOS). MPA therefore suppresses the production by iNOS of NO, and consequent tissue damage mediated by peroxynitrite. CellCept(R) suppresses T-lymphocytic responses to allogeneic cells and other antigens. The drug also suppresses primary, but not secondary, antibody responses. The efficacy of regimes including CellCept(R) in preventing allograft rejection, and in the treatment of rejection, is now firmly established. CellCept(R) is also efficacious in several experimental animal models of chronic rejection, and it is hoped that the drug will have the same effect in humans.

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

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

Species-specific inhibition of inosine 5'-monophosphate dehydrogenase by mycophenolic acid

IMPDH catalyzes the oxidation of IMP to XMP with the concomitant reduction of NAD(+) to NADH. This reaction is the rate-limiting step in de novo guanine nucleotide biosynthesis. Mycophenolic acid (MPA) is a potent inhibitor of mammalian IMPDHs but a poor inhibitor of microbial IMPDHs. MPA inhibits IMPDH by binding in the nicotinamide half of the dinucleotide site and trapping the covalent intermediate E-XMP. The MPA binding site of resistant IMPDH from the parasite Tritrichomonas foetuscontains two residues that differ from human IMPDH. Lys310 and Glu431 of T. foetus IMPDH are replaced by Arg and Gln, respectively, in the human type 2 enzyme. We characterized three mutants of T. foetusIMPDH: Lys310Arg, Glu431Gln, and Lys310Arg/Glu431Gln in order to determine if these substitutions account for the species selectivity of MPA. The mutation of Lys310Arg causes a 10-fold decrease in the K(i) for MPA inhibition and a 8-13-fold increase in the K(m) values for IMP and NAD(+). The mutation of Glu431Gln causes a 6-fold decrease in the K(i) for MPA. The double mutant displays a 20-fold increase in sensitivity to MPA. Pre-steady-state kinetics were performed to obtain rates of hydride transfer, NADH release, and hydrolysis of E-XMP for the mutant IMPDHs. The Lys310Arg mutation results in a 3-fold increase in the accumulation level of E-XMP, while the Glu431Gln mutation has only a minimal effect on the kinetic mechanism. These experiments show that 20 of the 450-fold difference in sensitivity between the T. foetus and human IMPDHs derive from the residues in the MPA binding site. Of this, 3-fold can be attributed to a change in kinetic mechanism. In addition, we measured MPA binding to enzyme adducts with 6-Cl-IMP and EICARMP. Neither of these adducts proved to be a good model for E-XMP.

Degradation products of mycophenolate mofetil in aqueous solution

The thermal and peroxide-catalyzed degradation products of mycophenolate mofetil (1) were studied in aqueous solution at pH 2.0, 3.5, 6.0, and 8.2. The major thermal degradation product observed was mycophenolic acid (2). At pH 6.0 and 8.2, 2 was the only product observed in the absence of peroxide, while at pH 2.0 and 3.5, the lactone analogue of mycophenolic acid (5), a hydroxylactone due to oxygenation of the double bond (6), and an unidentified product were formed. Compound 6 degraded to 4-hydroxy-6-methoxy-7-methyl-3-oxo-1,3-dihydro-isobenzofuran-5-car baldehyde (9) on prolonged storage and was present in the sample stressed at pH 2. Mycophenolic acid (2), the N-oxide of mycophenolate mofetil (3), the hydroxylactone of mycophenolic acid (6), and the erythro form of 4-methoxy-5-methyl- 2-(2-methyl-5-oxo-tetrahydro-furan-2-yl)-3,6-dihydro-2H-1, 7-dioxa-as-indacen-8-one (8) were observed in the presence of hydrogen peroxide at pH 3.5, 6.0, and 8.2. In addition, at pH 8.2, 4-hydroxy-4-(4-methoxy-5-methyl-8-oxo-2,3,6, 8-tetrahydro-1,7-dioxa-as-indacen-2-yl)-pentanoic acid (7) was seen. Peroxide-stressed samples at pH 2.0 gave no major degradation peaks, but a small amount of the hydroxylactone of mycophenolic acid (6) was formed.

Kinetic mechanism of Tritrichomonas foetus inosine 5'-monophosphate dehydrogenase

IMP dehydrogenase (IMPDH) catalyzes the oxidation of IMP to XMP with conversion of NAD+ to NADH. This reaction is the rate-limiting step in de novo guanine nucleotide biosynthesis. IMPDH is a target for antitumor, antiviral, and immunosuppressive chemotherapy. We have determined the complete kinetic mechanism for IMPDH from Tritrichomonas foetus using ligand binding, isotope effect, pre-steady-state kinetic, and rapid quench kinetic experiments. Both substrates bind to the free enzyme, which suggests a random mechanism. IMP binds to the enzyme in two steps. Two steps are also involved when IMP binds to a mutant IMPDH in which the active site Cys is substituted with a Ser. This observation suggests that this second step may be a conformational change of the enzyme. No Vm isotope effect is observed when [2-2H]IMP is the substrate which indicates that hydride transfer is not rate-limiting. This result is confirmed by the observation of a pre-steady-state burst of NADH production when monitored by absorbance. However, when NADH production was monitored by fluorescence, the rate constant for the exponential phase is 5-10-fold lower than when measured by absorbance. This observation suggests that the fluorescence of enzyme-bound NADH is quenched and that this transient represents NADH release from the enzyme. The time-dependent formation and decay of [14C]E-XMP intermediates was monitored using rapid quench kinetics. These experiments indicate that both NADH release and E-XMP hydrolysis are rate-limiting and suggest that NADH release precedes hydrolysis of E-XMP.

Inosine-5'-monophosphate dehydrogenase: regulation of expression and role in cellular proliferation and T lymphocyte activation

Guanine nucleotide synthesis is essential for the maintenance of normal cell growth and function, as well as for cellular transformation and immune responses. The expression of two genes encoding human inosine-5'-monophosphate dehyrogenase (IMPDH) type I and type II results in the translation of catalytically indistinguishable enzymes that control the rate-limiting step in the de novo synthesis of guanine nucleotides. Cellular IMPDH activity is increased more than 10-fold in activated peripheral blood T lymphocytes and is attributable to the increased expression of both the type I and type II enzymes. In contrast, abrogation of cellular IMPDH activity by selective inhibitors prevents T lymphocyte activation and establishes a requirement for elevated IMPDH activity in T lymphocytic responses. In order to assess the molecular mechanisms governing the expression of the IMPDH type I and type II genes in resting and activated peripheral blood T lymphocytes, we have cloned the human IMPDH type I and type II genes and characterized their genomic organization and their respective 5'-flanking regions. Both genes contain 14 highly conserved exons that vary in size from 49 to 207 base pairs. However, the intron structures are completely divergent, resulting in disparities in gene length (18 kilobases for type I and 5.8 kilobases for type II). In addition, the 5'-regulatory sequences are highly divergent; expression of the IMPDH type I gene is controlled by three distinct promoters in a tissue specific manner while the type II gene is regulated by a single promoter and closely flanked in the 5' region by a gene of unknown function. The conservation of the IMPDH type I and type II coding sequence in the presence of highly divergent 5'-regulatory sequences points to a multifactorial control of enzyme expression and suggests that tissue-specific and/or developmentally specific regulation of expression may be important. Delineation of these regulatory mechanisms will aid in the elucidation of the signaling events that ultimately lead to the synthesis of guanine nucleotides required for cellular entry into S phase and the initiation of DNA replication.

A potent "fat base" nucleotide inhibitor of IMP dehydrogenase

Inosine monophosphate dehydrogenase (IMPDH) is a target for anticancer, antiviral, immunosuppressive, and antimicrobial chemotherapy. Thus, IMPDH inhibitors have great potential as chemotherapeutic agents. Here we show that imidazo[4,5-e][1, 4]diazapine nucleotide (I) is a potent inhibitor of both human type II and Escherichia coli IMPDH. I is a slow-binding inhibitor. The values of Kd are 1.4 nM and 53 nM for human and E. coli IMPDH, respectively. Inhibition is reversible, as demonstrated by the recovery of activity upon denaturation and renaturation of the enzyme.I complex. I is not a substrate for IMPDH. I may form a covalent adduct with the active-site Cys of IMPDH. Such an adduct would serve as an analogue for an intermediate in the IMPDH reaction.

The roles of conserved carboxylate residues in IMP dehydrogenase and identification of a transition state analog

IMP dehydrogenase (IMPDH) catalyzes the oxidation of IMP to XMP with the concomitant reduction of NAD+; the enzyme is activated by K+. This reaction is the rate-limiting step in de novo guanine nucleotide biosynthesis. In order to identify functionally important residues in IMPDH, including those involved in substrate and K+ binding, we have mutated 11 conserved Asp and Glu residues to Ala in Escherichia coli IMPDH. The values of kcat, Km, and Ki for GMP, XMP, mizoribine 5'-monophosphate (MMP), and beta-methylene-tiazofurin adenine dinucleotide (TAD) were determined. Five of these mutations caused a significant change (>/=10-fold) in one of these parameters. The Asp248 --> Ala mutation caused 100-fold decrease in the value of kcat and a 25-fold increase in the value of Kii for TAD; these observations suggest that Asp248 is in the NAD+ binding site. The Asp338 --> Ala mutation caused a 600-fold decrease in the value of kcat, but only a 5-10-fold increase in the values of Km for IMP and Kis for IMP analogs, suggesting that Asp338 may be involved in acid-base catalysis as well as IMP binding. The remaining three residues, Asp13, Asp50, and Glu469, appear to be involved in K+ activation; these residues may be ligands at one or more K+ binding sites. Interestingly, changes in the values of Ki for MMP correlate with changes in kcat/KmKm of IMPDH, while no such correlation is observed for GMP, XMP, and TAD. This observation indicates that MMP is a transition state analog for the IMPDH reaction.

Expression, purification, and characterization of inosine 5'-monophosphate dehydrogenase from Borrelia burgdorferi

Inosine 5'-monophosphate dehydrogenase (IMPDH) is the rate-limiting enzyme in de novo guanine nucleotide biosynthesis. IMPDH converts IMP to xanthosine 5'-monophosphate with concomitant conversion of NAD+ to NADH. All IMPDHs characterized to date contain a 130-residue "subdomain" that extends from an N-terminal loop of the alpha/beta barrel domain. The role of this subdomain is unknown. An IMPDH homolog has been cloned from Borrelia burgdorferi, the causative agent of Lyme disease (Margolis, N., Hogan, D., Tilly, K., and Rosa, P. A. (1994) J. Bacteriol. 176, 6427-6432). This homolog has replaced the subdomain with a 50-residue segment of unrelated sequence. We have expressed and characterized the B. burgdorferi IMPDH homolog. This protein has IMPDH activity, which unequivocally demonstrates that the subdomain is not required for catalytic activity. The monovalent cation and dinucleotide binding sites of B. burgdorferi IMPDH are significantly different from those of human IMPDH. Therefore, these sites are targets for the design of specific inhibitors for B. burgdorferi IMPDH. Such inhibitors might be new treatments for Lyme disease.

Kinetic mechanism of human inosine 5'-monophosphate dehydrogenase type II: random addition of substrates and ordered release of products

IMP dehydrogenase (IMPDH) catalyzes the oxidation of IMP to XMP with the concomitant reduction of NAD to NADH. This reaction is the rate-limiting step of guanine nucleotide biosynthesis. IMPDH is a target of immunosuppressive, antiviral, anticancer, and antiparasitic chemotherapy. We have determined a minimal kinetic mechanism for human IMPDH type II using NAD analogs, isotope effects, hydride exchange, and presteady state kinetics. The values of kcat for the NAD analogs are similar despite a great variation in the structure and reactivity of the compounds. This observation suggests that a common step is rate-limiting, i.e., either hydrolysis of the E-XMP* intermediate or release of XMP. No Vm isotope effect is observed when 2-2H-IMP is the substrate, which indicates that hydride transfer is fast. This conclusion is confirmed by the observation of a burst of NADH production under presteady state conditions. These observations further suggest that either E-XMP* hydrolysis or XMP release is rate-limiting. V/Km deuterium isotope effects are observed for both substrates (1.9 for IMP and 2.5 for NAD), which indicates that substrate association is random. This result contradicts previous conclusions based on product inhibition studies. No NADH consumption is observed in the presence of XMP and IMPDH, which indicates that the overall reaction is irreversible. NADH consumption is observed in the presence of thio-NAD, IMP, and enzyme. These observations indicate that NADH traps the E-XMP* intermediate and demonstrates that hydride transfer is reversible. At infinite NADH, all of E-XMP* is trapped by NADH, as indicated by the equivalence of the rates of consumption of thio-NAD and NADH. Therefore the release of products is ordered, with NADH release preceding hydrolysis of E-XMP*.

Overexpression of a cloned IMP dehydrogenase gene of Candida albicans confers resistance to the specific inhibitor mycophenolic acid

An IMP dehydrogenase gene was isolated from Candida albicans on a approximately 2.9-kb XbaI genomic DNA fragment. The putative Candida IMP dehydrogenase gene (IMH3) encodes a protein of 521 amino acids with extensive sequence similarity to the IMP dehydrogenases of Saccharomyces cerevisiae and various other organisms. Like the S. cerevisiae IMH3 sequence characterized in the genome sequencing project, the open reading frame of the C. albicans IMH3 gene is interrupted by a small intron (248 bp) with typical exon-intron boundaries and a consensus S. cerevisiae branchpoint sequence. IMP dehydrogenase mRNAs are detected in both the yeast and hyphal forms of C. albicans as judged by Northern hybridization. Growth of wild-type (sensitive) C. albicans cells is inhibited at 1 microg of mycophenolic acid (MPA), a specific inhibitor of IMP dehydrogenases, per ml, whereas transformants hosting a plasmid with the IMH3 gene are resistant to MPA levels of up to at least 40 microg/ml. The resistance of cells to MPA is gene dosage dependent and suggests that IMH3 can be used as a dominant selection marker in C. albicans.

Isolation and characterization of mycophenolic acid-resistant mutants of inosine-5'-monophosphate dehydrogenase

Mycophenolic acid (MPA) is a potent and specific inhibitor of mammalian inosine-monophosphate dehydrogenases (IMPDH); most microbial IMPDHs are not sensitive to MPA. MPA-resistant mutants of human IMPDH type II were isolated in order to identify the structural features that determine the species selectivity of MPA. Three mutant IMPDHs were identified with decreased affinity for MPA The mutation of Gln277 --> Arg causes a 9-fold increase in the Ki of MPA, a 5-6-fold increase in the Km values for IMP and NAD, and a 3-fold decrease in kcat relative to wild type. The mutation of Ala462 --> Thr causes a 3-fold increase in the Ki for MPA, a 2.5-fold increase in the Km for NAD, and a 1.5-fold increase in kcat. The combination of these two mutations does not increase the Ki for MPA, but does increase the Km for NAD 3-fold relative to Q277R and restores kcat to wild type levels. Q277R/A462T is the first human IMPDH mutant with increased Ki for MPA and wild type activity. The third mutant IMPDH contains two mutations, Phe465 --> Ser and Asp470 --> Gly. Ki for MPA is increased 3-fold in this mutant enzyme, and Km for IMP is also increased 3-fold, while the Km for NAD and kcat are unchanged. Thus increases in the Ki for MPA do not correlate with changes in Km for either IMP or NAD, nor to changes in kcat. All four of these mutations are in regions of the IMPDH that differ in mammalian and microbial enzymes, and thus can be structural determinants of MPA selectivity.

The conformation of inosine 5'-monophosphate (IMP) bound to IMP dehydrogenase determined by transferred nuclear overhauser effect spectroscopy

IMP dehydrogenase (IMPDH) catalyzes the NAD-dependent synthesis of xanthosine 5'-monophosphate which is the rate-limiting step in guanine nucleotide biosynthesis. Although IMPDH is the target of numerous chemotherapeutic agents, nothing has been known about the conformation of the enzyme-bound substrates. The conformation of IMP bound to human type II IMP dehydrogenase has been determined by two-dimensional transferred nuclear Overhauser effect NMR spectroscopy at 600 MHz. NOE buildup rates were determined by recording NOESY spectra at numerous mixing times. The cross-relaxation rates determined from the initial NOE build-up rates were used to calculate inter-proton distances of bound IMP. The conformation of the enzyme-bound IMP was obtained by molecular modeling with energy minimization using the experimentally determined inter-proton distance constraints. The glycosidic torsion angle of the bound nucleotide is anti and the sugar is in the C2-endo-conformation. This conformation places H2 of IMP, which is transferred to NAD in the reaction, in a position clear of the rest of the molecule in order to facilitate the reaction.

Cloning and characterization of the gene encoding IMP dehydrogenase from Arabidopsis thaliana

We have cloned and characterized the gene encoding inosine monophosphate dehydrogenase (IMPDH) from Arabidopsis thaliana (At). The transcription unit of the At gene spans approximately 1900 bp and specifies a protein of 503 amino acids with a calculated relative molecular mass (M(r)) of 54,190. The gene is comprised of a minimum of four introns and five exons with all donor and acceptor splice sequences conforming to previously proposed consensus sequences. The deduced IMPDH amino-acid sequence from At shows a remarkable similarity to other eukaryotic IMPDH sequences, with a 48% identity to human Type II enzyme. Allowing for conservative substitutions, the enzyme is 69% similar to human Type II IMPDH. The putative active-site sequence of At IMPDH conforms to the IMP dehydrogenase/guanosine monophosphate reductase motif and contains an essential active-site cysteine residue.

Cloning, characterization and sequence comparison of the gene coding for IMP dehydrogenase from Pyrococcus furiosus

We have cloned and characterized the gene encoding inosine monophosphate dehydrogenase (IMPDH) from Pyrococcus furiosus (Pf), a hyperthermophillic archeon. Sequence analysis of the Pf gene indicated an open reading frame specifying a protein of 485 amino acids (aa) with a calculated M(r) of 52900. Canonical Archaea promoter elements, Box A and Box B, are located -49 and -17 nucleotides (nt), respectively, upstream of the putative start codon. The sequence of the putative active-site region conforms to the IMPDH signature motif and contains a putative active-site cysteine. Phylogenetic relationships derived by using all available IMPDH sequences are consistent with trees developed for other molecules; they do not precisely resolve the history of Pf IMPDH but indicate a close similarity to bacterial IMPDH proteins. The phylogenetic analysis indicates that a gene duplication occurred prior to the division between rodents and humans, accounting for the Type I and II isoforms identified in mice and humans.

Monovalent cation activation and kinetic mechanism of inosine 5'-monophosphate dehydrogenase

Human type II inosine 5'-monophosphate dehydrogenase has been purified to homogeneity from an Escherichia coli strain that express large quantities of the enzyme from the cloned gene. Steady state kinetic studies have been used to characterize the activation by monovalent cations, including Li+, Na+, K+, Rb+, Cs+, Tl+, NH4+, and N(CH3)4+. The enzyme has less than 1% of the maximal activity in the absence of an added monovalent cation, such as K+, Na+, Rb+, Tl+, or NH4+. The enzyme is activated by K+ and Tl+ at lower concentrations than those of other monovalent cations. Li+ and N(CH3)4+ do not activate the enzyme, nor do they inhibit the K(+)-activated enzyme, implying that ionic radius is important in binding selectivity. The Km values for both substrates and Vmax differ with different monovalent cations. Initial velocity and product inhibition kinetic data are consistent with an ordered steady state mechanism in which the enzyme binds K+ first, TMP second, and then NAD; the product NADH is released before xanthosine 5'-monophosphate. Substrate and product binding experiments support this mechanism and show the presence of one substrate binding site per subunit. Several rate constants were obtained from a computer simulation of the complete steady state rate equation.

The raspberry locus encodes Drosophila inosine monophosphate dehydrogenase

Investigation of an enhancer-trap line exhibiting testis-specific beta-galactosidase expression led to the isolation of the Drosophila gene encoding inosine monophosphate dehydrogenase (IMPD), the rate-limiting enzyme in guanine nucleotide synthesis, which has been implicated in cell cycle control and malignant transformation. Northern and in situ hybridization analysis demonstrated that the gene has a complex expression pattern involving several independently regulated transcripts. Two ubiquitous, but highly ovary enriched, transcripts of 2.5 and 1.9 kb are expressed in the nurse cells and delivered to the oocyte, whilst a 0.9 kb transcript is found exclusively in the testis. The 2.5 kb transcript encodes a 58 kDa protein, which is highly similar in length and sequence to mouse and human IMPDs and is presumably required for GTP synthesis during early embryogenesis. Over-expression of this cDNA in Escherichia coli yielded a product of the predicted size, which was demonstrated to possess IMPD activity in a spectrophotometric assay. The coding capacity of the other transcripts is currently uncertain. We present evidence that IMPD is the product of the raspberry (ras) locus at 9E and the functions of the gene are discussed in relation to the phenotypes of ras mutants.

The IMP dehydrogenase inhibitor mycophenolic acid antagonizes the CTP synthetase inhibitor 3-deazauridine in MOLT-3 human leukemia cells: a central role for phosphoribosyl pyrophosphate

Mycophenolic acid, an inhibitor of the enzyme IMP dehydrogenase, antagonizes the CTP synthetase inhibitor 3-deazauridine in its anti-proliferative effects on MOLT-3 human T leukemia cells. No depletion of CTP occurred, and decreased amounts of 3-deazuridine-triphosphate were measured in cells incubated with mycophenolic acid and 3-deazuridine. Most probably, these phenomena are related to the increased amounts of PRPP observed, which can result in an increased pyrimidine biosynthesis de novo and, as a consequence, a decreased metabolism of 3-deazauridine via the salvage pathway.

Gene amplification and dual point mutations of mouse IMP dehydrogenase associated with cellular resistance to mycophenolic acid

Mouse neuroblastoma cells (NB) selected for 10,000-fold increased resistance to mycophenolic acid (NB-Myco) showed a 200-500-fold increase in IMP dehydrogenase protein, and the enzyme (IMP: NAD+ oxidoreductase, EC 1.1.1.205) also exhibited a 2400-fold increased ki for mycophenolic acid and reduced catalytic efficiency (Hodges, S.D., Fung, E., McKay, D.J., Renaux, B.S., and Snyder, F.F. (1989) J. Biol. Chem. 264, 18137-18141). The molecular basis of these changes is the subject of the present study. The nucleotide sequence of IMP dehydrogenase cDNA from NB-Myco cells revealed four nucleotide changes. One of these changes did not result in a codon change, and a second one corresponding to methionine-483 was present in the parental NB mouse line. The remaining two nucleotide substitutions and deduced residue changes are: the C to T transition at base 998 relative to initiation of translation, altering threonine-333 to isoleucine; and the C to A transversion at base 1052, altering serine-351 to tyrosine. Evidence was also obtained for IMP dehydrogenase having undergone gene amplification. IMP dehydrogenase mRNA levels were 500-fold increased in NB-Myco cells as compared to parental NB cells. DNA dot blot analysis showed a 25-fold increase in IMP dehydrogenase gene copy number and restriction enzyme analysis revealed similar gene structure for NB and NB-myco cells.

Inhibition of IMP dehydrogenase by mycophenolic acid in Molt F4 human malignant lymphoblasts

The effects of inhibition of inosine monophosphate dehydrogenase (IMPDH), the rate-limiting enzyme in guanine nucleotide de novo synthesis, on cell growth, cell viability, endogenous nucleotide concentrations and concentrations of extracellular nucleosides and bases were studied in Molt F4 human malignant lymphoblasts. Mycophenolic acid (MPA) was used as a specific inhibitor of the enzyme activity. IMPDH activity was maximally inhibited with 0.5 microM MPA. After a 2 h exposure of the cells to 0.5 microM MPA, guanine nucleotides were depleted to approximately 50% of control values, whereas 5-phosphoribosyl-1-pyrophosphate levels increased to approximately 200%. Under these conditions, cytotoxicity became obvious after 24 h. Depletion of guanine nucleotides and cytotoxicity were prevented by addition of guanosine to MPA treatment. Daily supplements of guanosine were required to prevent MPA cytotoxicity during the entire incubation period of 72 h. We conclude that depletion of guanine nucleotides, induced by treatment with MPA, induces a severe and rapid cytotoxicity in Molt F4 cells.

Human IMP dehydrogenase catalyzes the dehalogenation of 2-fluoro- and 2-chloroinosine 5'-monophosphate in the absence of NAD

The ability of human type II inosine monophosphate dehydrogenase (IMPDH, EC 1.1.1.205) to catalyze the formation of xanthosine 5'-monophosphate (XMP) from C2 halogen-substituted analogs of IMP has been investigated. Adenosine deaminase was used to enzymatically synthesize 2-fluoroinosine and 2-chloroinosine from the 2-fluoro- and 2-chloroadenosine nucleoside analogs. Chemical phosphorylation yielded the corresponding 5'-nucleoside monophosphate derivatives. IMPDH catalyzes the conversion of both 2-fluoro- and 2-chloroinosine 5'-monophosphate (2-F- and 2-Cl-IMP) to XMP. The dehalogenation reactions proceed without nicotinamide adenine dinucleotide (NAD), the hydride acceptor required for the oxidation of IMP, the normal substrate of the enzyme. Formation of XMP from the 2-halo-IMPs was verified by UV absorption spectroscopy and by HPLC. Formation of XMP from 2-F-IMP yielded stoichiometric amounts of fluoride anion. IMP and XMP were competitive inhibitors toward 2-Cl-IMP in the dehalogenation reaction. Neither 2-F-IMP nor 2-Cl-IMP irreversibly inactivate IMPDH. Kinetic constants for the dehalogenation reactions have been determined and compared to the dehydrogenation reaction at 25 degrees C. (For 2-F-IMP: kcat = 0.058 s-1, Km = 62 microM. For 2-Cl-IMP: kcat = 0.049 s-1, Km = 48 microM. For the IMP dehydrogenation reaction: kcat = 0.25 s-1, Km [IMP] = 4.1 microM, Km [NAD] = 29 microM). Hydrolytic dehalogenation of 2-halo-IMPs without a requirement for NAD demonstrates the formation of a tetrahedral intermediate in the catalytic mechanism of IMP dehydrogenase.

Inosine-5'-monophosphate dehydrogenase activity is maintained in immortalized murine cells growth-arrested by serum deprivation

We have examined properties of IMPD activity in soluble extracts from immortalized murine epithelial and fibroblastic cells. The absence of significant xanthine oxidase activity in these extracts allowed the use of a spectrophotometric assay to study the enzyme activity. The observed enzymatic activity had subcellular localization and kinetic properties similar to those of previously described mammalian IMPD from other sources. Analysis of IMPD activity in extracts from cells in different states of growth related to serum concentration gave a surprising result. Extracts from exponentially growing cells exhibited a level of IMPD activity similar to that of extracts from quiescent cells arrested by serum-deprivation. In previous studies, the cellular variable designated to account for changes in IMPD activity was proliferative rate. Our findings suggest that either proliferative rate is not the functionally significant variable related to IMPD regulation or that there are other factors that can supersede it in certain contexts. Given the role of the enzyme in regulating the synthesis of guanine nucleotides, which are key regulatory molecules for many cellular processes, this may indeed be the case. Using immortalized cell lines growth-arrested by serum deprivation, we have experimentally isolated the enzyme activity from the previously assigned variable of growth rate. Based on our findings we propose that regulation of IMPD activity is more appropriately related to proliferative capacity as opposed to proliferative rate.

Pharmacodynamics of the inhibition of GTP synthesis in vivo by mycophenolic acid

Mycophenolic acid is effective against a wide range of experimental tumors in rodents when given orally, despite rapid metabolism to the inactive glucuronide derivative and rapid clearance from plasma. In the light of this, the pharmacodynamic action of mycophenolic acid on the radiolabelling of GTP and ATP by [14C]hypoxanthine in spleen and heart has been investigated in vivo in the rat as a preliminary to studies in tumor tissue. The data indicate that inhibition of GTP, and more surprisingly, ATP synthesis in spleen was sustained for at least 24.25 hr after single oral doses of the disodium salt of mycophenolic acid, indicating that the inhibitor is retained in sensitive cells for considerably longer than might be expected from the pharmacokinetic profile in the plasma in this species. GTP and ATP levels became depressed in rat spleen subsequent to the inhibition of nucleotide radiolabelling. The persistence of mycophenolic acid in proliferating cells may account for the effectiveness of once daily dosing against rapidly growing experimental tumors. In contrast with spleen, there was no incorporation of radiolabel from [14C]hypoxanthine into either GTP or ATP in rat heart and mycophenolic acid had no effect on the cardiac content of either nucleotide. The lack of effect of mycophenolic acid on cardiac GTP levels is consistent with the absence of any pharmacological action on cardiac functions associated with receptor-G-protein-GTP interactions. The ability of the morpholinoethyl ester of mycophenolic acid (a clinically effective immunosuppressive agent) to inhibit GTP synthesis and depress GTP levels in rat spleen in vivo was compared with that of mycophenolic free acid and its disodium salt. The ester derivative was clearly more effective than the poorly water-soluble free acid but showed comparable activity with the freely soluble disodium salt.

Effect of diets containing adenosine, guanosine, inosine or xanthosine on the nucleotide content of Artemia. Influence of mycophenolic acid

Artemia uses the stored diguanosine tetraphosphate as a source of adenine and guanine nucleotides during development from the encysted gastrula to the free swimming larva. Further development of the larvae depends on a dietary source of purine rings. We have investigated the growth of Artemia in axenic cultures supplemented with 0.6 mg ml-1 of adenosine, guanosine, inosine or xanthosine. The total protein and soluble nucleotide content of Artemia grown in the presence of adenosine, guanosine or inosine was very similar, around (2 A260 units and 500 mg protein) and (4 A260 units and 1000 mg protein) after 4 and 6 days of postlarval development, respectively. The nucleotide pattern of those extracts subjected to HPLC were almost identical, the major peaks corresponding to ATP, ADP and AMP. Other nucleotides, not well characterized, were also present in those extracts. Mycophenolic acid (10 micrograms ml-1) inhibited the growth of Artemia (as measured by their protein and soluble nucleotide content) in the presence of adenosine and inosine as the purine source, and had no appreciable effect in the presence of guanosine. A quantitative analysis of the chromatographic peaks obtained from Artemia grown in the presence of any of the three nucleosides +/- mycophenolic acid showed that the effect of the antibiotic on each one of the chromatographic peaks was very similar, suggesting that Artemia, and probably other organisms as well, tend to maintain a balance between all nucleotides and to adjust the overall level to the limiting step(s) in their rates of synthesis/interconversion. Xanthosine was not able to support the development of Artemia.

A biochemical basis for synergism of 6-mercaptopurine and mycophenolic acid in Molt F4, a human malignant T-lymphoblastic cell line

6-Mercaptopurine (6MP) cytotoxicity was studied in Molt F4 cells, a T-cell acute lymphoblastic leukemia (ALL) cell line. The effects on cytotoxicity were concentration-dependent. Measurements of intracellular thionucleotide intermediates of 6MP demonstrated a rapid rise of thio-IMP (tIMP) levels, and subsequently a rapid decrease. Thio-GMP (tGMP) and methyl-thio-IMP (Me-tIMP) appeared later in time, and persisted longer. Mycophenolic acid (MPA), a specific inhibitor of IMP dehydrogenase (IMPDH), was used to inhibit the conversion of tIMP into tGMP, thereby decreasing the incorporation of 6MP into DNA. A synergistic effect on cell viability and cell growth was observed when cells were treated with a combination of 2 microM 6MP and 0.5 microM MPA. Also, intracellular Me-tIMP increased 5 times with the combination. Based on the increase of Me-tIMP concentration and the observed synergism between 6MP and MPA, we conclude that methylation of tIMP into Me-tIMP is an important alternative route for 6MP cytotoxicity.

6-Azauracil inhibition of GTP biosynthesis in Saccharomyces cerevisiae

The addition of 6-azauracil to the growth medium causes a strong reduction of the GTP level in the nucleotide pool of Saccharomyces cerevisiae. In-vitro experiments show a strong inhibition of IMP dehydrogenase activity by 6-azaUMP explaining the preceding effect. PPR2 mutants, previously characterized by an increased sensitivity to 6-azauracil compared to the wild-type, are specifically susceptible to the lowering of the GTP pool, and are able to grow in presence of 6-azauracil when guanine is added to the medium.

Isolation and sequence of a cDNA encoding mouse IMP dehydrogenase

Inosinic acid (IMP) dehydrogenase (IMPD) catalyzes the conversion of IMP to XMP as the first committed step in GMP biosynthesis de novo. We have isolated a cDNA containing the complete coding region of mouse IMPD by its ability to complement a bacterial mutant lacking IMPD activity. Two independent cDNA clones were isolated by complementation, of which the longest was 1.7 kb in length. Northern analyses, using the IMPD cDNA as a probe, indicated that mature IMPD mRNA was a single species approx. 2.0 kb in size. Mouse IMPD is almost identical to Chinese hamster and human IMPDs and is highly conserved between Escherichia coli and mouse, with a direct amino acid (aa) identity of 39%, which increases to 60% if conserved aa are considered. The leader region of our longest cDNA clone is G + C-rich and contains two tandem copies of a G + C-rich direct repeat.

Anti-proliferative activity of L-651,582 correlates with calcium-mediated regulation of nucleotide metabolism at phosphoribosyl pyrophosphate synthetase

L-651,582, 5-amino-[4-(4-chlorobenzoyl)-3,5-dichlorobenzyl]-1, 2,3-triazole-4-carboxamide, is an antiproliferative and antiparasitic agent which inhibits nucleotide metabolism in mammalian cells. The drug equivalently inhibited 3H-hypoxanthine, 14C-adenine, and 14C-formate incorporation into nucleotide pools in Madin-Darby bovine kidney (MDBK) cells, suggesting depletion of the supply of phosphoribosyl pyrophosphate, (PRPP), required for each of these independent pathways. Inhibition of nucleotide metabolism correlated with inhibition of proliferation for three cell types with differing sensitivities toward the drug. L-651,582 inhibited incorporation of 3H-hypoxanthine into nucleotide pools with either glucose, uridine, or ribose as carbon source suggesting a block at PRPP synthetase, rather than a block in a pathway supplying ribose-5-phosphate. PRPP synthetase was not inhibited directly by the compound, indicating regulation of the enzyme in intact cells. Drug treatment did not kill cells but reduced the fraction of cells in S and G2/M while increasing the population in G1. Inhibition of uptake of 45Ca was demonstrated at concentrations identical to those required for inhibition of nucleotide metabolism or proliferation. Inhibition of cellular PRPP biosynthesis rates were also observed using EGTA to lower calcium levels. These data suggest a previously unrecognized link between calcium entry, the regulation of nucleotide biosynthesis at PRPP synthetase, and the rate of proliferation of mammalian cells.

Mycophenolic acid and thiazole adenine dinucleotide inhibition of Tritrichomonas foetus inosine 5'-monophosphate dehydrogenase: implications on enzyme mechanism

Inosine 5'-monophosphate dehydrogenase (IMPDH) catalyzes the oxidation of inosine 5'-monophosphate (IMP) to xanthosine 5'-monophosphate (XMP) with the conversion of NAD to NADH. An ordered sequential mechanism where IMP is the first substrate bound and XMP is the last product released was proposed for Tritrichomonas foetus IMPDH on the basis of product inhibition studies. Thiazole adenine dinucleotide (TAD) is an uncompetitive inhibitor versus IMP and a noncompetitive inhibitor versus NAD, which suggests that TAD binds to both E-IMP and E-XMP. Mycophenolic acid is also an uncompetitive inhibitor versus IMP and noncompetitive versus NAD. Multiple-inhibitor experiments show that TAD and mycophenolic acid are mutually exclusive with each other and with NADH. Therefore, mycophenolic acid most probably binds to the dinucleotide site of T. foetus IMPDH. The mycophenolic acid binding site was further localized to the nicotinamide subsite within the dinucleotide site: mycophenolic acid was mutually exclusive with tiazofurin, but could form ternary enzyme complexes with ADP or adenosine diphosphate ribose. NAD inhibits the IMPDH reaction at concentrations greater than 3 mM. NAD substrate inhibition is uncompetitive versus IMP, which suggests that NAD inhibits by binding to E-XMP. TAD is mutually exclusive with both NAD and NADH in multiple-inhibitor experiments, which suggests that there is one dinucleotide binding site. The ordered mechanism predicts that multiple-inhibitor experiments with XMP and TAD, mycophenolic acid, or NAD should have an interaction constant (alpha) between 0 and 1. However, alpha was greater than 1 in all cases.(ABSTRACT TRUNCATED AT 250 WORDS)