Clinical significance of serum guanase activity in various liver diseases

Homopurine Alkaloids: A Brief Overview

The isolation, structure elucidation, synthesis, biological properties, and biosynthesis of the homopurine alkaloids are reviewed, with an emphasis on the “victim-guardian” relationships between co-occurring alkaloids.

Mechanisms for the deamination reaction of 8-oxoguanine catalyzed by 8-oxoguanine deaminase: A combined QM/MM molecular dynamics study

The deamination reaction of 8-oxoguanine (8-oxoG) catalyzed by 8-oxoguanine deaminase (8-oxoGD) plays a critically important role in the DNA repair activity for oxidative damage. In order to elucidate the complete enzymatic catalysis mechanism at the stages of 8-oxoguanine binding, departure of 2-hydroxy-1H-purine-6,8(7H,9H)-dione from the active site, and formation of 8-oxoxanthine, extensive combined QM(PM3)/MM molecular dynamics simulations have been performed. Computations show that the rate-limiting step corresponds to the nucleophilic attack from zinc-coordinate hydroxide group to free 8-oxoguanine. Through conformational analyses, we demonstrate that Trp115, Trp123 and Leu119 connect to O8@8-oxoguanine with hydrogen bonds, and we suggest that mutations of tryptophan (115 and 123) to histidine or phenylalanine and mutation of leucine (119) to alanine could potentially lead to a mutant with enhanced activity. On this ground, a proton transfer mechanism for the formation of 8-oxoxanthine was further discussed. Both Glu218 and water molecule could be used as proton shuttles, and water molecule plays a major role in proton transfer in substrate. On the other hand, comparative simulations on the deamination of guanine and isocytosine reveal that, for the helping of hydrogen bonds between O8@8-oxoguanine and enzyme, O8@8-oxoguanine is the fastest to be deaminated among the three substrates which are also supported by the experimental kinetic constants.

WHICH IS THE PROTON-SHUTTLE IN ISOXANTHOPTERIN DEAMINASE? QM/MM MD UNDERSTANDING

The deamination process of isoxanthopterin catalyzed by isoxanthopterin deaminase was determined using the combined QM(PM3)/MM molecular dynamics simulations. In this paper, the updated PM3 parameters were employed for zinc ions and the initial model was built up based on the crystal structure. Proton transfer and following steps have been investigated in two paths: Asp336 and His285 serve as the proton shuttle, respectively. Our simulations showed that His285 is more effective than Aap336 in proton transfer for deamination of isoxanthopterin. As hydrogen bonds between the substrate and surrounding residues play a key role in nucleophilic attack, we suggested mutating Thr195 to glutamic acid, which could enhance the hydrogen bonds and help isoxanthopterin get close to the active site. The simulations which change the substrate to pterin 6-carboxylate also performed for comparison. Our results provide reference for understanding of the mechanism of deaminase and for enhancing the deamination rate of isoxanthopterin deaminase.

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.

β3-adrenoceptor-mediated increased circulating transaminase levels in mice treated with its agonist BRL 37344

Treatment with the selective β(3)-adrenoceptor agonist BRL 37344 increased circulating levels of alanine transaminase (ALT) and aspartate transaminase (AST) in mice without causing hepatocellular injury. To clarify whether this was a β(3)-adrenoceptor-mediated effect, the inhibitory effect of the selective β(3)-adrenoceptor antagonist SR 59230A on the increase in circulating transaminase levels induced by BRL 37344 was examined. A single intraperitoneal dose of BRL 37344 alone initially increased insulin and non-esterified fatty acid (NEFA) dose-proportionally at 0.5 hr post-dose, findings considered attributable to β(3)-adrenoceptor-stimulating effects. Levels of the gluconeogenic precursors pyruvate (PA) and lactate (LA) were increased corresponding to the change in insulin. Thereafter, glucose (GLU) level was decreased at 4 and 8 hr post-dose, suggesting disruption of glucose homeostasis. In association with these changes in glucose metabolism, transaminase levels were increased maximally at 4 hr post-dose. The transaminase changes were not accompanied by increases in circulating levels of other hepatocellular enzymes, including guanine deaminase (GUA), glutamate dehydrogenase (GLDH), and lactate dehydrogenase (LDH), or any morphological hepatocellular injury. Intraperitoneal pre-treatment with SR 59230A partly inhibited the effects of BRL 37344 alone, indicating that the increase in levels of circulating ALT by BRL 37344 was attributable to a β(3)-adrenoceptor-stimulating effect. In conclusion, the β(3)-adrenoceptor agonist BRL 37344 was shown to increase circulating transaminase levels in mice accompanied with dynamic changes in glucose metabolism. These findings suggest the possibility that circulating transaminase levels are increased as pharmacological effects of drugs disrupting glucose metabolism, and that hepatotoxic markers should be selected considering these effects to distinguish between acceptable pharmacology and toxicity.

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.

Chapter 2: Ring-Expanded (‘Fat‘) Purines and their Nucleoside/Nucleotide Analogues as Broad-Spectrum Therapeutics

This chapter describes a family of ring-expanded purines, informally referred to as “fat” or f-purines, as well as their nucleoside/nucleotide analogues (RENs/RENTs) that have broad applications in chemistry, biology, and medicine. Although purine itself has never been found in nature, substituted purines, such as adenine and guanine, or their respective nucleoside derivatives, adenosine and guanosine, are the most ubiquitous class of nitrogen heterocycles and play crucial roles in wide variety of functions of living beings As nucleotides (AMP,GMP), they are the building blocks of nucleic acids (RNA/DNA). They serve as energy cofactors (ATP, GTP), as part of coenzymes (NAD/FAD) in oxidation-reduction reactions, as important second messengers in many intracellular signal transduction processes (cAMP/cGMP), or as direct neurotransmitters by binding to purinergic receptors (adenosine receptors). Therefore, it is not surprising that the analogues of purines have found utility both as chemotherapeutics (antiviral, antibiotic, and anticancer agents) and pharmacodynamic entities (the regulation of myocardial oxygen consumption and cardiac blood flow). While they can act as substrates or the inhibitors of the enzymes of purine metabolism to render their chemotherapeutic action, their ability to act as agonists or antagonists of A1/A2A receptors is the basis for the modulation of pharmacodynamic property.

Catalytic mechanism of guanine deaminase: an ONIOM and molecular dynamics study

The catalytic mechanism of Bacillus subtilis guanine deaminase (bGD), a Zn metalloenzyme, has been investigated by a combination of quantum mechanical calculations using the multilayered ONIOM method and molecular dynamics simulations. In contrast to a previously proposed catalytic mechanism, which requires the bound guanine to assume a rare tautomeric state, the ONIOM calculations showed that the active-site residues of the enzyme do not affect the tautomeric state of guanine, and consequently the bound guanine is a tautomer that is the most abundant in aqueous solution. Two residues, Glutamate 55 and Aspartate 114, were found to play important roles in proton shuttling in the reaction. The proposed reaction path is initiated by proton transfer from a Zn-bound water to protonate Asp114. This process may be quite complex and rather dynamic in nature, as revealed by the molecular dynamics (MD) simulations, whereby another water may bridge the Zn-bound water and Asp114, which then is eliminated by positioning of guanine in the active site. The binding of guanine stabilizes protonated Asp114 by hydrogen bond formation. Asp114 can then transfer its proton to the N3 of the bound guanine, facilitating the nucleophilic attack on C2 of the guanine by the Zn-bound hydroxide to form a tetrahedral intermediate. This occurs with a rather low barrier. Glu55 then transfers a proton from the Zn-hydroxide to the amino group of the reaction intermediate and, at this point, the C2-N2 bond has lengthened by 0.2 A compared to guanine, making C2-N2 bond cleavage more facile. The C2-N2 bond breaks forming ammonia, with an energy barrier of approximately 8.8 kcal/mol. Ammonia leaves the active site, and xanthine is freed by the cleavage of the Zn-O2 bond, with a barrier approximately 8.4 kcal/mol. Along this reaction path, the highest barrier comes from C2-N2 bond cleavage, while the barrier from the cleavage of the Zn-O2 bond is slightly smaller. The Zn-O2 bond can be broken without the assistance of water during the release of xanthine.

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.

Proteomic analysis of differentially expressed proteins in hepatocellular carcinoma developed in patients with chronic viral hepatitis C

Hepatocellular carcinoma (HCC) is a major complication of chronic viral hepatitis C. Therapy for HCC is still disappointing. It is thus of great importance to identify novel HCC markers for early detection of the disease, and tumor-specific proteins as potential therapeutic targets. We have used a proteomic approach to identify new proteins involved in HCC development. Four cases of HCC developing from chronic viral hepatitis C were analyzed by two-dimensional electrophoresis (2-DE), and results were compared to those of paired adjacent non-tumorous liver tissues. For MS fingerprinting, protein spots with differential intensity between HCC and non-tumorous liver were directly cut out of gels and processed for MALDI-MS and nano-LC-ESI-MS/MS analysis. Approximately 850 spots were visualized in each gel. The comparative analysis of paired samples indicated that 345 protein spots showed significant differences in expression level between non-tumor and tumor tissue. Among the 345 protein spots analyzed, 238 spots corresponding to 155 different proteins were identified; 49 proteins were up-regulated, whereas 106 proteins were down-regulated. Among these 155 proteins, 91 proteins were regulated in at least three cases. Although 52 out of these 91 proteins have been already described by previous proteomic or transcriptomic studies, or are already known to be involved in hepatocarcinogenesis, this experiment revealed 39 new proteins differentially expressed in HCC developing from viral hepatitis C. Variations in protein accumulation were confirmed for two selected proteins (apolipoprotein E, chloride intracellular channel 1) by Western blotting in ten additional cases of HCC developing in patients with viral hepatitis C.

Estimation of guanine deaminase using guanosine as a "prosubstrate"

Plasma guanine deaminase (guanase; GD) is well established as an indicator of hepatocellular disease, recently being applied in the detection of hepatitis C in donor blood and in the diagnosis of hepatoma. No totally efficient, simple method for the estimation of plasma GD activity is routine since both guanine and 8-azaguanine, the substrates of the enzyme, are scarcely soluble in water. This difficulty in preparing stable substrates of sufficient concentration has resulted in methods that are both troublesome and inaccurate. Here we describe the development of new colorimetric and high-performance liquid chromatography (HPLC) methods utilizing guanosine as a "prosubstrate." After an initial breakdown of the guanosine to guanine using purine nucleoside phosphorylase, the ammonia formed as a result of the breakdown of the guanine by GD was estimated colorimetrically by the Berthelot reaction. As an alternative or a complementary assay, the xanthine also formed was measured using an isocratic HPLC method. These methods are suitable for routine assays for measuring plasma GD over a wide range of activities.

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.

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.

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.