Two distinct cDNAs for human IMP dehydrogenase

The conformation of NADH bound to inosine 5'-monophosphate dehydrogenase determined by transferred nuclear Overhauser effect spectroscopy

Inosine 5'-monophosphate dehydrogenase (IMPDH) catalyzes the oxidation of inosine 5-monophosphate (IMP) to xanthosine 5'-monophosphate (XMP). The reaction proceeds with concomitant conversion of NAD+ to NADH and is the rate-limiting step in the de novo biosynthesis of guanosine nucleotides. IMPDH is a target for numerous chemotherapeutic agents. The conformations of enzyme-bound substrates, enzyme-bound products and enzyme-bound ligands in general, are of interest for the understanding of the catalytic mechanism of the enzyme and the design of new inhibitors. Although several of the chemotherapeutic inhibitors of IMPDH are NAD+ or NADH analogues, no structural data for IMPDH-bound NAD+ (or NADH) are available. In the present work, we have used transferred nuclear Overhauser effect spectroscopy (TRNOESY) to determine the conformation of NADH bound to the active site of human type II IMPDH (IMPDH-h2). The inter-proton distances determined from TRNOESY data indicate that NADH binds to the enzyme active site in an overall extended conformation. The adenosine moiety and the nicotinamide riboside moiety are both in the anti conformation about the glycosidic bond, and both ribose rings are in approximately C4'-exo conformations. The nicotinamide amide group was found to be in a cis conformation. The anti conformation of the nicotinamide riboside moiety is in accord with the preferred conformations of several potent and selective dinucleotide inhibitors and is consistent with that implied by the stereospecificity of hydride transfer in the enzymatic reaction. The implications of this conformation for the catalytic mechanism of IMPDH-h2 are discussed.

Isosteric analogues of nicotinamide adenine dinucleotide derived from furanfurin, thiophenfurin, and selenophenfurin as mammalian inosine monophosphate dehydrogenase (type I and II) inhibitors

Dinucleotides TFAD (6), FFAD (7), and SFAD (8), isosteric NAD analogues derived, respectively, from C-nucleosides 5-beta-d-ribofuranosylthiophene-3-carboxamide (thiophenfurin, 1), 5-beta-d-ribofuranosylfuran-3-carboxamide (furanfurin, 2), and 5-beta-d-ribofuranosylselenophene-3-carboxamide (selenophenfurin, 5), were synthesized as human inosine monophosphate dehydrogenase (IMPDH) type I and II inhibitors. The synthesis was carried out by imidazole-catalyzed coupling of the 5'-monophosphate of 1, 2, and 5 with AMP. These dinucleotides, which are also analogues of thiazole-4-carboxamide adenine dinucleotide (TAD) and selenazole-4-carboxamide adenine dinucleotide (SAD), the active metabolites of the oncolytic C-nucleosides 2-beta-D-ribofuranosylthiazole-4-carboxamide (tiazofurin) and 2-beta-D-ribofuranosylselenazole-4-carboxamide (selenazofurin), were evaluated for their inhibitory potency against recombinant human IMPDH type I and II. The order of inhibitory potency found was SAD > SFAD = TFAD = TAD >> FFAD for both enzyme isoforms. No significant difference was found in inhibition of IMPDH type I and II.

Synthesis of a methylenebis(phosphonate) analogue of mycophenolic adenine dinucleotide: a glucuronidation-resistant MAD analogue of NAD

Mycophenolic alcohol (MPAlc), obtained by reduction of the carboxylic group of mycophenolic acid (MPA), was coupled with 2',3'-O-isopropylideneadenosine 5'-methylenebis(phosphonate) (4) in the presence of diisopropylcarbodiimide (DIC) to give P1-(2',3'-O-isopropylideneadenosin-5'-yl)-P2-(mycophenolic alcohol-6'-yl)methylenebis(phosphonate) (8) in 32% yield. Deisopropy-lidenation of 8 with CF3COOH/H2O afforded the methylenebis(phosphonate) analogue 3 of mycophenolic adenine dinucleotide (MAD). Compound 3, beta-methylene-MAD, was found to be a potent inhibitor of inosine monophosphate dehydrogenase (IMPDH) type II (Ki = 0.3 microM) as well as an inhibitor of growth of K562 cells (IC50 = 1.5 microM). In contrast to MPA and mycophenolic alcohol, beta-methylene-MAD was not converted into the glucuronide when incubated with uridine 5'-diphosphoglucuronyltransferase.

IMP-dehydrogenase inhibition in human lymphocytes and lymphoblasts by mycophenolic acid and mycophenolic acid glucuronide

Inosine 5′-monophosphate dehydrogenase (IMP-DH) activities were measured in human lymphocytes (exhibiting type I IMP-DH activity) and human lymphoblasts (exhibiting type II IMP-DH activity) in the presence of various amounts of mycophenolic acid (MPA) (0–20 μmol/L) and MPA glucuronide (MPAG) (0–200 μmol/L). Moreover, the influences of human serum albumin (HSA) and human plasma on the MPA- and MPAG-mediated effects were investigated. In the presence of water, 2.5 μmol/L MPA decreased the IMP-DH activity measured in lymphocytes by 60%, whereas in lymphoblasts a 80% inhibition was detectable. In the presence of ≥10 μmol/L MPA, lymphocytic as well as lymphoblastic IMP-DH activities were reduced in a similar manner. The concentration of MPAG required for 50% inhibition was for both cell types >25 μmol/L and <50 μmol/L, respectively. MPAG (200 μmol/L) reduced lymphocytic as well as lymphoblastic IMP-DH activity by ∼80%. With 100 g/L HSA or human plasma as diluent, the inhibitory effects of MPA and MPAG were significantly (P <0.05) diminished, whereas HSA concentrations ≤25 g/L only slightly influenced the inhibition of IMP-DH activity by MPA and MPAG. In summary, it can be clearly demonstrated that not only MPA but also MPAG contributes to the inhibition of both IMP-DH isoenzymes, which might be relevant for the immunosuppressive properties of mycophenolate mofetil in transplant patients.

Tritrichomonas foetus: a strategy for structure-based inhibitor design of a protozoan inosine-5'-monophosphate dehydrogenase

Inosine-5'-monophosphate dehydrogenase (IMPDH) is an attractive drug target for the control of parasitic infections. The enzyme catalyzes the NAD-dependent oxidation of inosine monophosphate (IMP) to xanthosine monophosphate (XMP), the committed step in guanosine monophosphate (GMP) biosynthesis. We have determined the crystal structures of IMPDH from the protozoan parasite Tritrichomonas foetus in the apo form at 2.3 A resolution and the enzyme-XMP complex at 2.6 A resolution. The enzyme forms a cyclic (C4) homotetramer. The core domain of each monomer forms an eight-stranded parallel beta/alpha barrel with the enzyme active site at the C-termini of the barrel beta strands which lies near the center of the fourfold axis of the tetramer. While the electron-density for XMP in the complex structure is well-defined, the NAD cofactor and a nearby loop containing the catalytic cysteine (Cys-319) are disordered. This disorder at the active site suggests that a high degree of flexibility may be inherent to the catalytic function of IMPDH, making this area a difficult target for structure-based inhibitor design. Unlike IMPDHs from other species, the T. foetus enzyme coordinates the substrate phosphate with a single arginine guanidinium in the active site. Furthermore, a deep groove extends 8 A from the substrate phosphate away from the sugar. This structural uniqueness forms the basis of our efforts to design compounds that specifically inhibit the parasite enzyme.

Regulation of the signal transduction program by drugs

The purpose of this paper was to clarify critical aspects of the behavior of signal transduction activity in normal and cancer cells. 1. Signal transduction activity in the conversion of phosphatidylinositol through PI and PIP kinases and PLC to IP3 is regulated at multiple sites. In liver, hepatomas and human carcinomas PIP kinase is the rate limiting enzyme and PLC activity is present in great excess. 2. The steady-state signal transduction activity as measured by the three enzyme activities and IP3 concentration was markedly up-regulated in rat hepatomas of different growth rates. The steady-state specific activities of the three signal transduction enzymes were elevated in ovarian carcinomas as compared to normal ovary. Increased enzyme activities were also observed in human breast carcinoma cells as compared to normal human breast parenchymal cells. In breast, ovarian and rat hepatoma cells as they go through lag, log and plateau phases, IP3 concentration in the early lag phase increased 4.5- to 20-fold and PI and PIP kinase activities peaked in mid-log phase. These events returned to baseline levels in the plateau phase. PLC activity did not change. 3. The bone marrow PI and PIP kinase activities in 3-day starvation were decreased to 13% and IP3 concentration was reduced to 24%; at 1-day refeeding they returned to normal. PLC activity changed little. These alterations are in line with the rapid t1/2 degradation rates (12 min) of PI and PIP kinases observed in studies with cycloheximide. By contrast, PLC has a long half-life. 4. The molecular action of tiazofurin entails inhibition of IMP DH activity, decrease in GTP and IP3 concentrations, reduction of ras and myc oncogene expression, and signal transduction enzyme activities. These events are followed by induced differentiation and apoptosis. There are also decreases in enzyme activities which have rapid turnover, including TdR kinase, dTMP synthase, and GPRT. In vitro studies indicated that these events are abrogated by addition of guanine which restores GTP concentrations. Therefore, most or all these events were brought about by the reduced GTP concentration in the tiazofurin target cells. 5. Quercetin and genistein are able to inhibit PI and PIP kinase activities and reduce IP3 concentration in vivo and in tissue culture systems. These flavonoids are also inhibitors of cell proliferation and clonogenic ability in rat hepatoma 3924A and in human OVCAR-5 and MDA-MB-435 cells. Quercetin down-regulated the expression of c-myc and Ki-ras oncogenes and led to induced differentiation and apoptosis in K562 cells. Genistein reduced IP3 concentration in vivo and in the tissue culture system. Genistein is antiproliferative and has cytototoxicity in human carcinoma cells. All three drugs, tiazofurin, quercetin and genistein, act, in part at least, through depression of cellular IP3 concentration although the mechanisms may not be identical. 6. Quercetin and genistein, which attack different targets and different phases of the cell cycle, proved to be synergistic in OVCAR-5 cells. The impact of tiazofurin, genistein and quercetin is of interest because the drugs crucially inhibit the display of the neoplastic program of cells and lead to induced differentiation and apoptosis.

Regulation of inosine-5'-monophosphate dehydrogenase type II gene expression in human T cells. Role for a novel 5' palindromic octamer sequence

Expression of the gene encoding human inosine- 5'-monophosphate dehydrogenase (IMPDH) type II, an enzyme catalyzing the rate-limiting step in the generation of guanine nucleotides, is increased more than 10-fold in activated peripheral blood T lymphocytes and is required for T cell activation. We have examined the 5'-regulatory sequences that are important for the transcriptional regulation of this gene in T cells. DNase I mapping of genomic DNA identified a hypersensitive element near the transcription initiation site. Fine mapping by in vivo footprinting demonstrated five transcription factor binding sites that are occupied in both resting and activated peripheral blood T lymphocytes; these are tandem CRE motifs, a Sp1 site, an overlapping Egr-1/Sp1 site, and a novel palindromic octamer sequence (POS). The tandem CRE and POS sites are of major functional importance as judged by mutational and electrophoretic mobility shift analyses. These data provide evidence that expression of the human IMPDH type II gene is predominantly regulated by the nuclear factors ATF-2 and an as yet unidentified POS-binding protein. Additional major protein-DNA interactions do not occur within the promoter region after T lymphocyte activation, indicating a requirement for additional protein-protein interactions and/or post-translational modifications of pre-bound transcription factors to account for the observed increase in IMPDH type II gene expression.

Crystal structure of Tritrichomonas foetus inosine-5'-monophosphate dehydrogenase and the enzyme-product complex

Inosine-5'-monophosphate dehydrogenase (IMPDH) is an attractive drug target for the control of parasitic infections. The enzyme catalyzes the oxidation of inosine monophosphate (IMP) to xanthosine monophosphate (XMP), the committed step in de novo guanosine monophosphate (GMP) biosynthesis. We have determined the crystal structures of IMPDH from the protozoan parasite Tritrichomonas foetus in the apo form at 2.3 A resolution and the enzyme-XMP complex at 2.6 A resolution. Each monomer of this tetrameric enzyme is comprised of two domains, the largest of which includes an eight-stranded parallel beta/alpha-barrel that contains the enzyme active site at the C termini of the barrel beta-strands. A second domain, comprised of residues 102-220, is disordered in the crystal. IMPDH is expected to be active as a tetramer, since the active site cavity is formed by strands from adjacent subunits. An intrasubunit disulfide bond, seen in the crystal structure, may stabilize the protein in a less active form, as high concentrations of reducing agent have been shown to increase enzyme activity. Disorder at the active site suggests that a high degree of flexibility may be inherent in the catalytic function of IMPDH. Unlike IMPDH from other species, the T. foetus enzyme has a single arginine that is largely responsible for coordinating the substrate phosphate in the active site. This structural uniqueness may facilitate structure-based identification and design of compounds that specifically inhibit the parasite enzyme.

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*.

Synthesis, structure, and antiproliferative activity of selenophenfurin, an inosine 5'-monophosphate dehydrogenase inhibitor analogue of selenazofurin

The synthesis and biological activity of selenophenfurin (5-beta-D-ribofuranosylselenophene-3-carboxamide, 1), the selenophene analogue of selenazofurin, are described. Glycosylation of ethyl selenophene-3-carboxylate (6) under stannic chloride-catalyzed conditions gave 2- and 5-glycosylated regioisomers, as a mixture of alpha- and beta-anomers, and the beta-2,5-diglycosylated derivative. Deprotected ethyl 5-beta-D-ribofuranosylselenophene-3-carboxylate (12 beta) was converted into selenophenfurin by ammonolysis. The structure of 12 beta was determined by 1H- and 13C-NMR, crystallographic, and computational studies. Selenophenfurin proved to be antiproliferative against a number of leukemia, lymphoma, and solid tumor cell lines at concentrations similar to those of selenazofurin but was more potent than the thiophene and thiazole analogues thiophenfurin and tiazofurin. Incubation of K562 cells with selenophenfurin resulted in inhibition of IMP dehydrogenase (IMPDH) (76%) and an increase in IMP pools (14.5-fold) with a concurrent decrease in GTP levels (58%). The results obtained confirm the hypothesis that the presence of heteroatoms such as S or Se in the heterocycle in position 2 with respect to the glycosidic bond is essential for both cytotoxicity and IMP dehydrogenase inhibitory activity in this type of C-nucleosides.

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.

Regulation of the human inosine monophosphate dehydrogenase type I gene. Utilization of alternative promoters

Catalysis of guanine nucleotide formation from IMP in the de novo purine synthetic pathway is carried out by two isoforms of the enzyme inosine monophosphate dehydrogenase (IMPDH) that are catalytically indistinguishable but are encoded by separate genes. In order to assess the potential for cell type-specific expression of IMPDH activity, we have characterized the IMPDH type I gene and identified three major RNA transcripts that are differentially expressed from three different promoters. A 4.0-kilobase pair (kb) mRNA containing 1.3 kb of 5'-untranslated region is expressed in activated peripheral blood lymphocytes and to a far lesser extent in cultured tumor cell lines. The P1 promoter that regulates the transcription of this mRNA has a high degree of sequence identity to an Alu repetitive sequence. A transcript of 2.7 kb is found in a subset of the tumor cell lines examined, whereas a 2.5-kb mRNA species is universally expressed and is the prevalent mRNA in most cell lines and tissues. The relative strengths of the three promoter regions and the effects of variable extents of 5'-flanking sequence on the P3 promoter differ in Jurkat T, as compared with Raji B lymphoid cell lines, demonstrating a complex cell type-specific transcriptional regulation of IMPDH type I gene expression.

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.

Rationally designed inhibitors of inosine monophosphate dehydrogenase

Functionalized 2-alkyl derivatives of inosinic acid have been synthesized to serve as reversible as well as irreversible inhibitors of the human type II enzyme inosine monophosphate dehydrogenase. These compounds were designed to react with Cys-331 of the enzyme to form covalent bonds so as to interfere with the normal enzyme mechanism which involves attack of Cys-331 at C-2 of the substrate. Mass spectrometric analysis of the reaction products after enzymatic degradation confirmed the appropriateness of the inhibitor design.

Structure-activity relationships for inhibition of inosine monophosphate dehydrogenase by nuclear variants of mycophenolic acid

Structure-activity relationships in the region of the phthalide ring of the inosine monophosphate dehydrogenase inhibitor mycophenolic acid have been explored. Replacement of the lactone ring with other cyclic moieties resulted in loss of potency, especially for larger groups. Replacement of the ring by acyclic substituents also indicated a strong sensitivity to steric bulk. A phenolic hydroxyl group, with an adjacent hydrogen bond acceptor, was found to be essential for high potency. The aromatic methyl group was essential for activity; the methoxyl group could be replaced by ethyl to give a compound with 2-4 times the potency of mycophenolic acid in vitro and in vivo.

Conformational changes and stabilization of inosine 5'-monophosphate dehydrogenase associated with ligand binding and inhibition by mycophenolic acid

The effects of substrate, product, and inhibitor (mycophenolic acid) binding on the conformation and stability of hamster type II inosine 5'-monophosphate dehydrogenase (IMPDH) have been examined. The protein in various states of ligand occupancy was compared by analyzing susceptibility to in vitro proteolysis, the degree of binding of a hydrophobic fluorescent dye, secondary structure content as determined by far-UV circular dichroism spectra, and urea-induced denaturation curves. These analysis methods revealed consistent evidence that IMPDH undergoes a local reorganization when IMP or XMP bind. NAD+ produced no such effect. In fact, no evidence was found for NAD+ binding independently of IMP. It is proposed that IMPDH adopts an open conformation around its nucleotide binding sites in the absence of substrates and that binding of IMP stabilizes a closed conformation that has a higher affinity for NAD+. The data also suggest the enzyme remains in the closed configuration throughout the catalytic steps and then reverts to the open conformation with XMP release, thereby consummating the enzyme cycle. Mycophenolic acid inhibition appeared to impart even greater stability. We propose that localized conformational changes occur during the normal and mycophenolic acid-inhibited reaction sequences of IMPDH.

Structure and mechanism of inosine monophosphate dehydrogenase in complex with the immunosuppressant mycophenolic acid

The structure of inosine-5'-monophosphate dehydrogenase (IMPDH) in complex with IMP and mycophenolic acid (MPA) has been determined by X-ray diffraction. IMPDH plays a central role in B and T lymphocyte replication. MPA is a potent IMPDH inhibitor and the active metabolite of an immunosuppressive drug recently approved for the treatment of allograft rejection. IMPDH comprises two domains: a core domain, which is an alpha/beta barrel and contains the active site, and a flanking domain. The complex, in combination with mutagenesis and kinetic data, provides a structural basis for understanding the mechanism of IMPDH activity and indicates that MPA inhibits IMPDH by acting as a replacement for the nicotinamide portion of the nicotinamide adenine dinucleotide cofactor and a catalytic water molecule.

Chemical synthesis of benzamide adenine dinucleotide: inhibition of inosine monophosphate dehydrogenase (types I and II)

Treatment of 3-(2,3-O-isopropylidene-beta-D-ribofuranosyl)benzamide (6) with POCl3 in (EtO)3-PO afforded only little phosphorylation product (8, 5%), but the major product was 5'-chlorobenzamide riboside (7, 85%). Reaction of 6 with 2-cyanoethyl N,N-diisopropylchlorophosphoramidite followed by 2-cyanoethanol/tetrazole treatment and oxidation with tert-butyl peroxide gave a 1:1 mixture of the desired 5'-O-bis(2-cyanoethyl) phosphate 9 and the chloro derivative 7. This mixture was treated with methanolic ammonia and partitioned between CHCl3 and water. The 2',3'-O-isopropylidenebenzamide mononucleotide (8) was obtained in 21.2% overall yield from the aqueous layer. Compound 8 was then converted into the corresponding imidazolide 11b which, upon coupling with 2',3'-O-acetonide of AMP, afforded the acetonide of benzamide adenine dinucleotide (15) in 94% yield together with small amounts of symmetrical pyrophosphates P1,P2-bis(2',3'-O-isopropylideneadenosin-5'-yl)pyrophosphate (13, 3%) and P1,P2-bis(2',3'-O-isopropylidene-3-(carbamoylphenyl)-5'-ribosyl)py rophosphate (14, 2%). Deprotection of 15 with Dowex 50/H+ in water afforded the desired benzamide adenine dinucleotide (BAD) in 93% yield. BAD inhibits inosine monophosphate dehydrogenase type I (IC50 = 0.78 microM) and type II (IC50 = 0.88 microM) with same degree of potency.

Inhibition of IMPDH by mycophenolic acid: dissection of forward and reverse pathways using capillary electrophoresis

The objective of this work was to contribute to the understanding of mechanisms for IMPDH inhibition. We over-expressed hamster type II IMPDH in Escherichia coli, purified the protein to apparent homogeneity, and used capillary electrophoresis to quantify enzyme turnover events accompanying inhibition by mycophenolic acid (MPA). We dissected two convergent pathways leading to MPA-inhibition; a rapid "forward" pathway beginning with substrates and linked to enzyme catalysis, and a slower "reverse" pathway apparently not involving catalysis. MPA-inhibition occurred rapidly in the forward direction by interrupting the enzyme turnover cycle, after IMP and NAD+ binding, after hydride transfer, and after NADH release. Slow inhibition, without substrate turnover, was achieved by incubating free enzyme with excess XMP and MPA. We propose that mycophenolic acid inhibits IMPDH by trapping a transient covalent product of the hydride transfer reaction (IMPDH approximately XMP*) before a final hydrolysis step that precedes XMP and enzyme release in the forward reaction pathway. Understanding the ligand occupancy of the protein has also proven important for producing homogeneous, chemically defined complexes for structural studies. IMPDH samples inhibited by MPA in the forward and reverse pathways yielded similar, high-quality crystals that are currently undergoing X-ray diffraction analyses.

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.

Synthesis and modeling studies with monocyclic analogues of mycophenolic acid

Two stepwise procedures, developed for the introduction of the (E)-4-methyl-4-hexenoic acid side chain of mycophenolic acid, were used in the synthesis of monocyclic mycophenolic acid analogues 2a-i. The derivatives with a methyl group or hydrogen at C-4 and lacking the lactone moiety were much less cytotoxic than mycophenolic acid. The monocyclic analogues with a C-4 chloro group did show some activity, albeit much less than mycophenolic acid. The observed differences in potency are rationalized by semiempirical calculations of intramolecular H-bonds.

Inosine monophosphate dehydrogenase expression: transcriptional regulation of the type I and type II genes

Inosine 5'-monophosphate dehydrogenase (IMPDH) is an essential rate-limiting enzyme in the de novo guanine nucleotide synthetic pathway that catalyzes the conversion of IMP to XMP. Enzyme activity is accounted for by the expression of two distinct but closely related genes termed IMPDH I and II. Increased IMPDH activity has been linked to both cellular proliferation and neoplastic transformation and generally ascribed to an increase in the expression of the type II gene. We have characterized the type I and type II genes and identified elements important in the transcriptional regulation of both genes. The type II IMPDH gene contains a 466 bp 5' flanking region spanning the translation start site that contains several transcription factor binding sites and mediates increased transcription of a CAT reporter gene in peripheral blood T lymphocytes when these cells are induced to proliferate. The single functional IMPDH type I gene contains exon-intron boundaries and exon structures that are nearly identical to those in the type II gene. In contrast to the type II gene, however, it contains two putative promoter sites, each with the potential for transcriptional regulation. We conclude that these two genes most probably arose from an early gene duplication event and that their highly conserved structures and differential regulation at the transcriptional level argue strongly for a significant role for each gene in cellular metabolism, growth, and differentiation.

Recombinant human inosine monophosphate dehydrogenase type I and type II proteins. Purification and characterization of inhibitor binding

Inosine monophosphate dehydrogenase (IMPDH) activity results from the expression of two separate genes, and the resulting proteins (type I and type II) are 84% identical at the amino acid level. Although the type II mRNA is expressed at higher levels in proliferating cells, both mRNAs, and by extrapolation both proteins, are present in normal and malignant cells. Since IMPDH is an important target for the development of drugs with both chemotherapeutic and immunosuppressive activity, we have compared the kinetic and physical properties of the two human enzymes expressed in and purified from Escherichia coli. Type I and II IMPDH had kcat values of 1.8 and 1.4 sec-1, respectively, with Km values for IMP of 14 and 9 microM and Km values for NAD of 42 and 32 microM. The two enzymes were inhibited competitively by the immunosuppressive agent mizoribine 5'-monophosphate (MMP) with Ki values of 8 and 4 nM and inhibited uncompetitively by mycophenolic acid with Ki values of 11 and 6 nM. The association of MMP to either isozyme, as monitored by fluorescence quenching, was relatively slow with kon values of 3-8 x 10(4) M-1 sec-1 and koff values of 3 x 10(-4) sec-1 (half-lives of 36-43 min). Thus, MMP is a potent, tight-binding competitive inhibitor that does not discriminate between the two IMPDH isozymes.

Characterization of the human inosine-5'-monophosphate dehydrogenase type II gene

Inosine-5'-monophosphate dehydrogenase (IMPDH) activity and mRNA levels are induced up to 15-fold upon mitogenic or antigenic stimulation of human peripheral blood T lymphocytes. This increase in IMPDH activity is required for cellular proliferation and has been associated with malignant transformation. We have cloned the human IMPDH type II gene and show that it contains 14 exons and is approximately 5.8 kilobases in length. Exons vary in size from 49 to 207 base pairs and introns from 73 to 1065 base pairs. The transcription start site was mapped to a position 50 nucleotides upstream of the translation initiation site. The 5'-flanking region consisting of 463 base pairs upstream of the translation initiation site confers induced transcription and differential regulation upon a chloramphenicol acetyltransferase reporter gene when transfected into Jurkat T cells and human peripheral blood T lymphocytes, respectively. DNase I footprinting analysis using Jurkat T cell nuclear extract identified four protected regions in the promoter which coincide with consensus transcription factor binding sites for the nuclear factors AP2, ATF, CREB, Egr-1, Nm23, and Sp1. These findings suggest that several of these nuclear factors may play a critical role in the regulation of IMPDH type II gene expression during T lymphocyte activation.

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.

Probing the active site of human IMP dehydrogenase using halogenated purine riboside 5'-monophosphates and covalent modification reagents

Active-site amino acid residues of human type II inosine 5'-monophosphate dehydrogenase (IMPDH) were investigated using the covalent modification reagents 6-chloroinosine 5'-monophosphate (6-Cl-IMP) and iodoacetamide. IMPDH was incubated with these reagents in the presence and absence of IMP, NAD, and NADH, and the activity of the enzyme for IMP dehydrogenation or 2-Cl-IMP dehalogenation was followed. IMPDH activity was rapidly lost when the enzyme was incubated with the IMP analog, 6-Cl-IMP, or with iodoacetamide. The enzyme was protected against inactivation in the presence of the substrate IMP. It was not protected against inactivation by NAD alone. Saturating concentrations of IMP and NADH reduced the inactivation rate by about the same amount as with IMP alone. IMPDH samples labeled with 6-Cl-IMP and an unlabeled control were alkylated with iodoacetamide, digested with trypsin, and analyzed by HPLC-mass spectrometry (HPLC-MS). All eight cysteines of human type II IMPDH were found to exist as free sulfhydryls on the active, unlabeled form of the enzyme. At an enzyme/inactivator ratio of 1:4, only one cysteine residue, Cys-331, was found to be covalently modified by 6-Cl-IMP. From the results of the substrate protection experiments and HPLC-MS data, it is concluded that 6-Cl-IMP binds in the IMP binding site of IMPDH and reacts covalently with Cys-331 to form a purine riboside 5'-monophosphate-enzyme adduct.

Tissue-differential expression of two distinct genes for human IMP dehydrogenase (E.C.1.1.1.205)

Human IMP dehydrogenase (E.C. 1.1.1.205) is recently regarded as a potent targeting enzyme for immunosuppressive drugs. Tissue differential expressions of human type I and type II IMP dehydrogenase were investigated in sixteen human adult organs (heart, brain, placenta, lung, liver, skeletal muscle, kidney, pancreas, spleen, thymus, prostate, testis, ovary, small intestine, colon, peripheral blood leukocytes) and five human fetal organs (heart, brain, lung, liver, kidney) using Northern blot analysis. In all tissues examined in this study, the sizes of mRNAs of each isoform were identical, respectively. The 2.3 kb type II mRNA was shown predominantly, and the 3.5 kb type I mRNA level was lower than type II in most human tissues examined. In contrast, type I IMPDH gene expressed higher than type II in peripheral blood leukocytes, uniquely. We also demonstrated that both type I and type II IMPDH genes are widely distributed among various species by Southern blot analysis. Interestingly, type I IMPDH gene may have multiple gene families in primates.

Biochemical and molecular properties of lithium-sensitive myo-inositol monophosphatase

Myo-inositol monophosphatase is a pivotal enzyme of the inositol second messenger system which is specifically inhibited by therapeutic levels of lithium salts, implicating inhibition of this enzyme as a potential site of its action in bipolar disease. This enzyme has a native molecular weight of 59,000, and has traditionally been found in the cytosolic fraction, although a membrane-bound form has also been identified. Possessing two identical subunits, this enzyme hydrolyzes those monophosphates which are equatorially located within the inositol ring, and several nucleoside monophosphates phosphorylated at the 2-position. Each subunit of the native enzyme contains an active site with unusually large caverns as revealed by crystallographic studies, which may explain the accommodation of these structurally unrelated substrates. We have suggested that the uncompetitive inhibition of this phosphatase by lithium ions may prevent the formation of an enzyme-bound non-isomeric (meso) intermediate, Mg(2+)-inositol 1,3 or 4,6 cyclic monophosphate when this enzyme hydrolyzes its respective isomeric substrates.

Identification and regional localization of a human IMP dehydrogenase-like locus (IMPDHL1) at 16p13.13

Sequence-tagged sites (STSs) are versatile chromosomal markers for a variety of genome mapping efforts. In this report, we describe a randomly generated STS (323F4) from human chromosome 16 genomic DNA that has 90.0% sequence identity to the type I human inosine-5'-monophosphate dehydrogenase (IMPDH1) gene and 72% identity to the type II human inosine-5'-monophosphate dehydrogenase (IMPDH2) gene. Additional sequencing by primer walking has provided a total of 1380 bp of the human chromosome 16 sequence. The IMPDH-like sequence 323F4 was regionally localized by PCR analysis of a panel of somatic cell hybrids containing different portions of human chromosome 16 to 16p13.3-13.12, between the breakpoints found in hybrids CY196/CY197 and CY198. This regional mapping assignment was further refined to subband 16p13.13 by high-resolution fluorescence in situ hybridization using cosmid 323F4 as a probe. We conclude that a third, previously undescribed IMPDH locus, termed IMPDHL1, exists at human chromosome 16p13.13.

Inosine monophosphate dehydrogenase from porcine (Sus scrofa domestica) thymus: purification and properties

1. IMP dehydrogenase (EC 1.1.1.205) from porcine thymus glands has been purified to homogeneity. 2. The enzyme has a subunit MW of 57 kDa and an amino acid composition similar to those obtained from other normal and cancerous mammalian cells. 3. The apparent Km values at pH 8.0 for IMP and NAD+ are 7 and 16 microM, respectively. 4. GMP, XMP and AMP are competitive inhibitors towards IMP and Ki values of 50, 85 and 282 microM, respectively. 5. The effectiveness of nucleotides to protect inactivation by CI-IMP is IMP > GMP > XMP > AMP.

Reciprocal alterations of GMP reductase and IMP dehydrogenase activities during differentiation in HL-60 leukemia cells

The study was undertaken to elucidate the regulatory roles of GMP reductase (GMPR) and IMP dehydrogenase (IMPDH) on purine interconversion during differentiation. Treatment of HL-60 cells with retinoic acid (1 microM) induced granulocytic differentiation which was accompanied with a 2.4-fold increase in GMPR and 55% decrease in IMPDH activities. Maturation induced by 12-O-tetradecanoylphorbol 13-acetate or dimethylsulfoxide was also associated with similar reciprocal alterations. Incubation with guanosine (200 microM), which expands the guanine nucleotide pool, elevated GMPR (1.9-fold) and decreased IMPDH (73%) activities. The synchronous and opposing alterations in GMPR and IMPDH activities should amplify the metabolic response due to differentiation or guanylate pool expansion.

Mycophenolic acid inhibits the degranulation of rat peritoneal mast cells

The effect of mycophenolic acid (MPA), a potent inhibitor of inosine monophosphate dehydrogenase, on the degranulation of rat peritoneal mast cells (RMC) was studied. RMC were pretreated for 48 hr with 0.1-10 microM MPA before the cells were sensitized with IgE and triggered with specific Ag. The net amount of [3H]5-HT released from granules was decreased by 44 and 32% with 1 and 10 microM MPA treatment, respectively. MPA inhibition of degranulation was completely reversed by the addition of 30 microM guanosine to the incubation medium. There was no difference in the apparent number or affinity of IgE binding sites between control and MPA-treated RMC. MPA pretreatment also had no effect on the IgE receptor-mediated production of PGD2 in RMC. These results suggest that depletion of intracellular GTP pools by MPA can disrupt the signaling between the IgE receptor and the secretory granules and that, under these same conditions, the release and metabolism of arachidonic acid are unaffected.

De novo purine nucleotide biosynthesis: cloning, sequencing and expression of a chicken PurH cDNA encoding 5-aminoimidazole-4-carboxamide-ribonucleotide transformylase-IMP cyclohydrolase

The purH cDNA, encoding 5-aminoimidazole-4-carboxamide-ribonucleotide (AICAR) transformylase-inosine monophosphate cyclohydrolase (ATIC), was cloned by functional complementation of an Escherichia coli purH mutant using a chicken liver cDNA expression library. This represents the first report of the cloning of any eukaryotic ATIC-encoding cDNA (PurH). The avian ATIC mRNA is 2.3 kb long and encodes a protein with an Mr of 64,422. The deduced amino acid sequence is 36% identical to the bacterial purH-encoded enzymes from Bacillus subtilis and E. coli. The avian cDNA was expressed as a glutathione S-transferase (GST) fusion protein that was purified in a single step by affinity chromatography. A novel vector was employed which permits rapid and highly efficient cleavage of the GST fusion protein yielding 10 mg of purified PurH product per liter of bacterial culture. Km values were determined with the purified fusion protein utilizing AICAR and (6-R)N10-formyl-tetrahydrofolate as substrates. These values compare favorably with the isolated avian enzyme, supporting the idea that kinetic, as well as other physical properties of the recombinant fusion protein are similar to the native avian enzyme. Large quantities of purified enzyme and the ability to generate site-directed mutations should make mechanistic studies possible. The recombinant enzyme also affords a simple and reliable approach to identifying new antifolates.