Real-Time Monitoring of Human Guanine Deaminase Activity by an Emissive Guanine Analog.

Guanine deaminase (GDA) deaminates guanine to xanthine. Despite its significance, the study of human GDA remains limited compared to other metabolic deaminases. As a result, its substrate and inhibitor repertoire are limited, and effective real-time activity, inhibitory, and discovery assays are missing. Herein, we explore two emissive heterocyclic cores, based on thieno[3,4-d]pyrimidine (thN) and isothiazole[4,3-d]pyrimidine (tzN), as surrogate GDA substrates. We demonstrate that, unlike the thieno analog, thGN, the isothiazolo guanine surrogate, tzGN, does undergo effective enzymatic deamination by GDA and yields the spectroscopically distinct xanthine analog, tzXN. Further, we showcase the potential of this fluorescent nucleobase surrogate to provide a visible spectral window for a real-time study of GDA and its inhibition.

Cypin: A novel target for traumatic brain injury.

Cytosolic PSD-95 interactor (cypin), the primary guanine deaminase in the brain, plays key roles in shaping neuronal circuits and regulating neuronal survival. Despite this pervasive role in neuronal function, the ability for cypin activity to affect recovery from acute brain injury is unknown. A key barrier in identifying the role of cypin in neurological recovery is the absence of pharmacological tools to manipulate cypin activity in vivo. Here, we use a small molecule screen to identify two activators and one inhibitor of cypin's guanine deaminase activity. The primary screen identified compounds that change the initial rate of guanine deamination using a colorimetric assay, and secondary screens included the ability of the compounds to protect neurons from NMDA-induced injury and NMDA-induced decreases in frequency and amplitude of miniature excitatory postsynaptic currents. Hippocampal neurons pretreated with activators preserved electrophysiological function and survival after NMDA-induced injury in vitro, while pretreatment with the inhibitor did not. The effects of the activators were abolished when cypin was knocked down. Administering either cypin activator directly into the brain one hour after traumatic brain injury significantly reduced fear conditioning deficits 5 days after injury, while delivering the cypin inhibitor did not improve outcome after TBI. Together, these data demonstrate that cypin activation is a novel approach for improving outcome after TBI and may provide a new pathway for reducing the deficits associated with TBI in patients.

Analogs of iso-azepinomycin as potential transition-state analog inhibitors of guanase: synthesis, biochemical screening, and structure-activity correlations of various selectively substituted imidazo[4,5-e][1,4]diazepines.

Guanase is an important enzyme of the purine salvage pathway of nucleic acid metabolism and its inhibition has beneficial implications in viral, bacterial, and cancer therapy. The work described herein is based on a hypothesis that azepinomycin, a heterocyclic natural product and a purported transition state analog inhibitor of guanase, does not represent the true transition state of the enzyme-catalyzed reaction as closely as does iso-azepinomycin, wherein the 6-hydroxy group of azepinomycin has been translocated to the 5-position. Based on this hypothesis, and assuming that iso-azepinomycin would bind to guanase at the same active site as azepinomycin, several analogs of iso-azepinomycin were designed and successfully synthesized in order to gain a preliminary understanding of the hydrophobic and hydrophilic sites surrounding the guanase binding site of the ligand. Specifically, the analogs were designed to explore the hydrophobic pockets, if any, in the vicinity of N1, N3, and N4 nitrogen atoms as well as O(5) oxygen atom of iso-azepinomycin. Biochemical inhibition studies of these analogs were performed using a mammalian guanase. Our results indicate that (1) increasing the hydrophobicity near O(5) results in a negative effect, (2) translocating the hydrophobicity from N3 to N1 also results in decreased inhibition, (3) increasing the hydrophobicity near N3 or N4 produces significant enhancement of inhibition, (4) increasing the hydrophobicity at either N3 or N4 with a simultaneous increase in hydrophobicity at O(5) considerably diminishes any gain in inhibition made by solely enhancing hydrophobicity at N3 or N4, and (5) finally, increasing the hydrophilic character near N3 has also a deleterious effect on inhibition. The most potent compound in the series has a Ki value of 8.0±1.5µM against rabbit liver guanase.

Investigations into specificity of azepinomycin for inhibition of guanase: discrimination between the natural heterocyclic inhibitor and its synthetic nucleoside analogues.

In our long and broad program to explore structure-activity relationships of the natural product azepinomycin and its analogues for inhibition of guanase, an important enzyme of purine salvage pathway of nucleic acid metabolism, it became necessary to investigate if the nucleoside analogues of the heterocycle azepinomycin, which are likely to be formed in vivo, would be more or less potent than the parent heterocycle. To this end, we have resynthesized both azepinomycin (1) and its two diastereomeric nucleoside analogues (2 and 3), employing a modified, more efficient procedure, and have biochemically screened all three compounds against a mammalian guanase. Our results indicate that the natural product is at least 200 times more potent toward inhibition of guanase as compared with its nucleoside analogues, with the observed K(i) of azepinomycin (1) against the rabbit liver guanase=2.5 (±0.6)×10(-6) M, while K(i) of Compound 2=1.19 (±0.02)×10(-4) M and that of Compound 3=1.29 (±0.03)×10(-4) M. It is also to be noted that while IC(50) value of azepinomycin against guanase in cell culture has long been reported, no inhibition studies nor K(i) against a pure mammalian enzyme have ever been documented. In addition, we have, for the first time, determined the absolute stereochemistry of the 6-OH group of 2 and 3 using conformational analysis coupled with 2-D (1)H NMR NOESY.

A novel transition state analog inhibitor of guanase based on azepinomycin ring structure: Synthesis and biochemical assessment of enzyme inhibition.

Synthesis and biochemical inhibition studies of a novel transition state analog inhibitor of guanase bearing the ring structure of azepinomycin have been reported. The compound was synthesized in five-steps from a known compound and biochemically screened against the rabbit liver guanase. The compound exhibited competitive inhibition profile with a K(i) of 16.7±0.5µM.

Identification of small molecule compounds with higher binding affinity to guanine deaminase (cypin) than guanine.

Guanine deaminase (GDA; cypin) is an important metalloenzyme that processes the first step in purine catabolism, converting guanine to xanthine by hydrolytic deamination. In higher eukaryotes, GDA also plays an important role in the development of neuronal morphology by regulating dendritic arborization. In addition to its role in the maturing brain, GDA is thought to be involved in proper liver function since increased levels of GDA activity have been correlated with liver disease and transplant rejection. Although mammalian GDA is an attractive and potential drug target for treatment of both liver diseases and cognitive disorders, prospective novel inhibitors and/or activators of this enzyme have not been actively pursued. In this study, we employed the combination of protein structure analysis and experimental kinetic studies to seek novel potential ligands for human guanine deaminase. Using virtual screening and biochemical analysis, we identified common small molecule compounds that demonstrate a higher binding affinity to GDA than does guanine. In vitro analysis demonstrates that these compounds inhibit guanine deamination, and more surprisingly, affect GDA (cypin)-mediated microtubule assembly. The results in this study provide evidence that an in silico drug discovery strategy coupled with in vitro validation assays can be successfully implemented to discover compounds that may possess therapeutic value for the treatment of diseases and disorders where GDA activity is abnormal.

RhoA regulates dendrite branching in hippocampal neurons by decreasing cypin protein levels.

The way a dendrite is patterned determines how a neuron will receive information. The Rho GTPases have been reported to play increasingly well defined roles in determining dendritic branch and spine development and morphology. Much is known about how these small GTPases regulate the actin cytoskeleton; however, very little is known about how they alter the microtubule cytoskeleton. Our laboratory previously cloned and characterized cypin, a guanine deaminase that increases dendrite number by binding to tubulin heterodimers and promoting microtubule assembly. In the present study, we show that cypin and RhoA are part of a common pathway that regulates dendrite number. Inhibition of Rho kinase activity does not affect cypin-mediated dendrite branching. Furthermore, cypin does not affect the activity of RhoA, as measured by GTP binding to RhoA. In fact, activated RhoA acts to inhibit cypin protein expression and, by doing so, decreases dendrite number. In addition, this decrease in cypin protein occurs via a translation-dependent mechanism. Together, our data suggest that cypin acts downstream of the small GTPase RhoA to regulate dendrite branching in hippocampal neurons, providing a novel mechanism for RhoA action on microtubule dynamics.

Design of inhibitors against guanase: synthesis and biochemical evaluation of analogues of azepinomycin.

As part of a program to design rational, mechanism-based inhibitors of guanase, we report here the synthesis and biochemical screening of two analogues of azepinomycin (1 and 2), a naturally occurring inhibitor of guanase, known to mimic the transition-state of the enzyme-catalyzed reaction. Our biochemical results show that compounds 1 and 2 are competitive inhibitors with K(i) of 2.01+/-0.16 x 10(-5) and 5.36+/-0.14 x 10(-5) M, respectively.

Pyrazolo[4,3-e][1,2,4]triazines: purine analogues with electronic absorption in the visible region.

Synthesis of several pryrazolo[4,3-e][1,2,4]-triazines is described. The absorption spectrum of some 5-substituted derivatives was found to extend to the visible region. These compounds were found to inhibit some enzymes of purine metabolism, like xanthine oxidase or bacterial purine-nucleoside phosphorylase with Ki values in the 10(-3) -10(-5) M range.

A unique ring-expanded acyclic nucleoside analogue that inhibits both adenosine deaminase (ADA) and guanine deaminase (GDA; guanase): synthesis and enzyme inhibition studies of 4,6-diamino-8H-1-hydroxyethoxymethyl-8-iminoimidazo[4,5-e][1,3]diazepine.

The synthesis and enzyme inhibition studies of a novel ring-expanded acyclic nucleoside analogue are reported. Compound has been found to be a competitive inhibitor of both adenosine deaminase (ADA) and guanine deaminase (GDA; guanase) with K(i)'s equal to 1.52+/-0.34 x 10(-4) M and 2.97+/-0.25 x 10(-5) M, respectively. Inhibition of two enzymes of purine metabolism may bear beneficial implications in antiviral therapy.

Investigations into biochemical mode of inhibition of guanase by azepinomycin: synthesis and biochemical screening of several analogues of azepinomycin.

In an effort to biochemical mode of guanase inhibition as well as the structure-activity relationships of azepinomycin, five analogues (I-V) of azepinomycin were synthesized and screened against guanase from rabbit liver. Our results suggest that while the 6-hydroxy group of azepinomycin is crucial for activity, its putative transition state mode of inhibition of guanase is questionable. The additional H-bonding sites at position 5, and hydrophobic groups in and around position 3 of azepinomycin appear to be tolerated, and may in fact enhance the potency of inhibition.

Analogues of azepinomycin as inhibitors of guanase.

Synthesis and biochemical screening against guanase of analogues of the naturally occurring guanase inhibitor azepinomycin (2) are reported. Compound 6-amino-5,6,7,8,-tetrahydro-4H-imidazo[4,5-e][1,4]diazepine-5,8-dione (3) was synthesized in six steps commencing with 1-benzyl-5-nitroimidazole-4-carboxylic acid (5). Compound 3 and its synthetic precursor 3-benzyl-6-(N-benzyloxycarbonyl)amino-5,6,7,8-tetrahydro-4H-imidazo[4,5- e] [1,4]diazepine-5,8-dione (12) were screened against rabbit liver guanase. Both were found to be moderate inhibitors of the enzyme with K's in the range of 10(-4) M.

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.

Synthesis and guanase inhibition studies of a novel ring-expanded purine analogue containing a 5:7-fused, planar, aromatic heterocyclic ring system.

The synthesis of a novel planar, potentially aromatic, ring-expanded xanthine analogue (1), containing the 5:7-fused imidazo[4,5-e][1,4]diazepine ring system, along with guanase inhibition studies are reported. The compound was synthesized in six steps, starting from 1-benzyl-5-nitroimidazole-4-carboxylic acid (2), and was biochemically screened against rabbit liver guanase. Compound 1 is a moderate competitive inhibitor of the enzyme with a Ki of 2.27 +/- 0.66 x 10(-4) M.

Isolation and characterization of human liver guanine deaminase.

Guanine deaminase (EC 3.5.4.3, guanine aminohydrolase [GAH]) was purified 3248-fold from human liver to homogeneity with a specific activity of 21.5. A combination of ammonium sulfate fractionation, and DEAE-cellulose, hydroxylapatite, and affinity chromatography with guanine triphosphate ligand were used to purify the enzyme. The enzyme was a dimer protein of a molecular weight of 120,000 with each subunit of 59,000 as determined by gel filtration and sodium dodecyl sulfate-gel electrophoresis. Isoelectric focusing gave a pI of 4.76. It was found to be an acidic protein, as evidenced by the amino acid analysis, enriched with glutamate, aspartate, alanine and glycine. It showed a sharp pH optimum of 8.0. The apparent Km for guanine was determined to be 1.53 X 10(-5) M at pH 6.0 and 2 X 10(-4) M for 8-azaguanine as a substrate at pH 6.0. The enzyme was found to be sensitive to p-hydroxymercuribenzoate inhibition with a Ki of 1.53 X 10(-5) M and a Ki of 5 X 10(-5) M with 5-aminoimidazole-4-carboxamide as an inhibitor. The inhibition with iodoacetic acid showed only a 7% loss in the activity at 1 X 10(-4) M and a 24% loss at 1 X 10(-3) M after 30 min of incubation, whereas p-hydroxymercuribenzoate incubation for 30 min resulted in a 91% loss of activity at a concentration of 1 X 10(-4) M. Guanine was the substrate for all of the inhibition studies. The enzyme was observed to be stable up to 40 degrees C, with a loss of almost all activity at 65 degrees C with 30 min incubation. Two pKa values were obtained at 5.85 and 8.0. Analysis of the N-terminal amino acid proved to be valine while the C-terminal residue was identified as alanine.

Consequences of the salvage of purine compounds on the proliferation of rat T-lymphocytes with normal or inhibited purine de novo synthesis.

We studied the ability of purine compounds to restore the proliferation of concanavalin-A-stimulated rat T-lymphocytes under conditions of purine de novo synthesis inhibition and, on the other hand, the inhibition by purine nucleosides of the response of these cells to a mitogenic stimulation under conditions of normal purine de novo synthesis. The use of 50 microM azaserine, a potent inhibitor of purine de novo synthesis, allowed us to define the physiologically active salvage pathways of purine bases, ribo- and deoxyribonucleosides in concanavalin-A-stimulated rat T-lymphocytes. Except for guanylic compounds, all purines completely restored cell proliferation at a concentration of 50 microM. Guanine, guanosine and 2'-deoxyguanosine at concentrations up to 500 microM did not allow us to restore more than 50% of the cell proliferation. In conditions of normal purine de novo synthesis, the addition of 1000 microM adenine, adenosine, 2'-deoxyadenosine or 100 microM 2'-deoxyguanosine inhibited rat T-lymphocyte proliferation. The differences between the degree of inhibition of cell proliferation could be explained only in part by the differences between the capacities of salvage of these compounds. Furthermore, the fact that 2'-deoxyguanosine toxicity was dependent and 2'-deoxyadenosine toxicity independent on the activation state of the cells provided more evidence that the biochemical mechanisms of inhibition of cell proliferation should be different for these two nucleosides.

New guanine deaminase inhibitors.

2-Aryl/alkyl-3-(5'-carboxamido-imidazol-4'-yl)-quinazolin-4-(3H)-o nes have been reported as Guanine deaminase inhibitors. These compounds were prepared from 4-amino-5-imidazol-carboxamide and suitable 2-aryl/alkyl-benzoxazin-4-ones. Compounds were characterized by their m.ps., elemental analyses and I. R. spectra. These compounds have shown significant inhibition, aryl substituent was more effective than alkyl group. This is the first report when quinazolones have been reported as Guanine deaminase inhibitors. Inhibitors of this enzyme can enhance the bioavailability of purine antagonists and in turn are helpful for the chemotherapy of cancer.

Effect of inhibition of purine enzymes on benzodiazepine binding in the human brain.

Since inosine is an inhibitory ligand for benzodiazepine binding, and since several of the purine enzymes have a specific localization, it was hypothesized that the unique distribution of benzodiazepine receptors may be dependent on the regional concentrations and specific actions of these enzymes in increasing or decreasing the amount of inosine. To test the above theory, the binding of 3H-flunitrazepam to receptors was studied on homogenates of various regions of autopsied human brain before and after treatment with irreversible potent inhibitors of the purine enzymes guanine deaminase and adenosine deaminase. As predicted, inhibition of guanase, which metabolizes guanine and hypoxanthine to xanthine, caused a marked inhibition of binding in the cerebral cortex and midbrain, where there is an abundance of enzyme, and only slight change in binding in the medulla, cerebellum or pons, where there is little enzyme. When adenosine deaminase, which converts adenosine to inosine, was inhibited, there was increased binding, with as much as a 4-fold increase in the frontal lobe, and very little effect in the cerebellum, medulla or temporal lobe.

Guanine deaminase from rat brain. Purification, characteristics, and contribution to ammoniagenesis in the brain.

1. Guanine deaminase [EC 3.5.4.3] was purified to a homogeneous state from rat brain by a procedure involving ammonium sulfate fractionation, DE-52 column chromatography, hydroxylapatite column chromatography, gel filtration on ACA-34 and isoelectric focusing. Homogeneity was shown by polyacrylamide gel electrophoresis in the presence and absence of SDS. 2. The molecular weight of the enzyme was determined by gel filtration (105,000), and that of its subunit by SDS-polyacrylamide gel electrophoresis (52,000). From these findings, we concluded that the native enzyme consisted of two identical subunits. 3. The Km values for guanine and 8-azaguanine were calculated to be 0.17 mM and 0.67 mM, respectively. This enzyme was markedly inhibited by 5-amino 4-imidazolecarboxamide (AICA), a precursor of purine nucleotide synthesis, with a K1 value of 82 micrometer. 4. Guanine deaminase was purified from rat liver by the procedure used for purification of the brain enzyme and anti-liver enzyme serum was raised in rabbits. This antiserum cross-reacted with the brain enzyme without spur formation in the Ouchterlony double diffusion test. 5. The gamma-globulin fraction of the anti-guanine deaminase serum and AICA inhibited more than 50% of the ammoniagenesis in the brain system in which the purine nucleotide cycle operates. It was also shown that guanine nucleotides were degraded via guanosine and guanine liberating ammonia in the same brain system when substrates for the purine nucleotide cycle were omitted. On the basis of these findings it is suggested that guanine deaminase contributes to ammoniagenesis in the brain.