Relationship between intracellular concentration of S-adenosylhomocysteine and inhibition of vaccinia virus replication and inhibition of murine L-929 cell growth

Serendipitous One-Step Synthesis of Cyclopentene Derivatives from 5'-Deoxy-5'-heteroarylsulfonylnucleosides as Nucleoside-Derived Julia-Kocienski Reagents

A serendipitous one-step transformation of 5'-deoxy-5'-heteroarylsulfonylnucleosides into cyclopentene derivatives is reported. This unique transformation likely proceeds via a domino reaction initiated by α-deprotonation of the heteroaryl sulfone and subsequent elimination reaction to generate a nucleobase and an α,β-unsaturated sulfone that contains a formyl group. The Michael addition of the nucleobase to the α,β-unsaturated sulfone and the subsequent intramolecular Julia-Kocienski reaction eventually generate the cyclopentene ring. Heteroarylthio and acylthio groups can be incorporated into the cyclopentene core in place of the nucleobase by conducting this reaction in the presence of a heteroarylthiol and a thiocarboxylic acid, respectively. cis,cis-Trisubstituted cyclopentene derivatives are obtained as a single stereoisomer from ribonucleoside-derived Julia-Kocienski sulfones.

Synergistic Antiviral Activity of Inhibitors of S-Adenosylhomocysteine Hydrolase and Ribavirin

(1′R,2′S,3′R)-9-(2′,3′-Dihydroxycycloperrt-4′-en-1′-yl)-adenine (DHCeA) and -3-deazaadenine (3-deaza-DHCeA), which are potent inhibitors of S-adenosylho-mocysteine (AdoHcy) hydrolase, and ribavirin, which is an inhibitor of IMP-dehydrogenase, were found in this study to have synergistic effects on inhibiting vaccinia virus replication in murine L929 cells without creating a synergistic effect on cellular toxicity. Thus, the antiviral effectiveness of this drug combination was 5–10× higher than the antiviral effectiveness observed with the AdoHcy hydrolase inhibitors alone. Ribavirin does not alter the ability of DHCeA and 3-deaza-DHCeA to elevate the intracellular AdoHcy/S-adeno-sylmethionine (AdoMet) ratio. Increases in this ratio were shown earlier to correlate with the antiviral effects of these carbocyclic nucleosides. Ribavirin was also shown to significantly reduce the cellular level of GTP, which is consistent with its activity as an inhibitor of IMP-dehydrogenase and its proposed mechanism of antiviral action, inhibiting the formation of the ‘capped methylated structure’ at the 5′-end of viral mRNA.

(1′R, 2′S, 3′R)-9-(2′, 3′-Dihydroxycyclopentan-1′-yl)-Adenine and −3-Deaza-Adenine: Analogues of Aristeromycin Which Exhibit Potent Antiviral Activity with Reduced Cytotoxicity

Two synthetic analogues of aristeromycin, which were shown in a separate study to be inhibitors of S-adenosylhomocysteine hydrolase and devoid of substrate activity with adenosine kinase and adenosine deaminase, were found in this study to inhibit vaccinia virus replication in murine L929 cells and to have reduced cytotoxicity compared with that of the parent compound. Aristeromycin was shown to produce cytocidal effects on murine L929 cells, whereas the synthetic analogues produced cytostatic effects on cell growth. The antiviral effects of these synthetic analogues are correlated with their ability to elevate the intracellular ratio of S-adenosylhomocysteine/ S-adenosylmethionine. These results confirm that S-adenosylhomocysteine hydrolase is the molecular target which mediates the antiviral effects of aristeromycin and that transformation of aristeromycin by cellular adenosine kinase mediates its cytocidal properties.

Proteomic analysis by iTRAQ in red claw crayfish, Cherax quadricarinatus, hematopoietic tissue cells post white spot syndrome virus infection

To elucidate proteomic changes of Hpt cells from red claw crayfish, Cherax quadricarinatus, we have carried out isobaric tags for relative and absolute quantitation (iTRAQ) of cellular proteins at both early (1 hpi) and late stage (12 hpi) post white spot syndrome virus (WSSV) infection. Protein database search revealed 594 protein hits by Mascot, in which 17 and 30 proteins were present as differentially expressed proteins at early and late viral infection, respectively. Generally, these differentially expressed proteins include: 1) the metabolic process related proteins in glycolysis and glucogenesis, DNA replication, nucleotide/amino acid/fatty acid metabolism and protein biosynthesis; 2) the signal transduction related proteins like small GTPases, G-protein-alpha stimulatory subunit, proteins bearing PDZ- or 14-3-3-domains that help holding together and organize signaling complexes, casein kinase I and proteins of the MAP-kinase signal transduction pathway; 3) the immune defense related proteins such as α-2 macroglobulin, transglutaminase and trans-activation response RNA-binding protein 1. Taken together, these protein information shed new light on the host cellular response against WSSV infection in a crustacean cell culture.

HaCaT Keratinocytes Response on Antimicrobial Atelocollagen Substrates: Extent of Cytotoxicity, Cell Viability and Proliferation

The effective and widely tested biocides: Benzalkonium chloride, bronopol, chitosan, chlorhexidine and irgasan were added in different concentrations to atelocollagen matrices. In order to assess how these antibacterial agents influence keratinocytes cell growth, cell viability and proliferation were determined by using MTT assay. Acquired data indicated a low toxicity by employing any of these chemical substances. Furthermore, cell viability and proliferation were comparatively similar to the samples where there were no biocides. It means that regardless of the agent, collagen-cell-attachment properties are not drastically affected by the incorporation of those biocides into the substrate. Therefore, these findings suggest that these atelocollagen substrates enhanced by the addition of one or more of these agents may render effectiveness against bacterial stains and biofilm formation, being the samples referred to herein as “antimicrobial substrates” a promising view in the design of novel antimicrobial biomaterials potentially suitable for tissue engineering applications.

Small molecule inhibitors that selectively block dengue virus methyltransferase

Crystal structure analysis of Flavivirus methyltransferases uncovered a flavivirus-conserved cavity located next to the binding site for its cofactor, S-adenosyl-methionine (SAM). Chemical derivatization of S-adenosyl-homocysteine (SAH), the product inhibitor of the methylation reaction, with substituents that extend into the identified cavity, generated inhibitors that showed improved and selective activity against dengue virus methyltransferase (MTase), but not related human enzymes. Crystal structure of dengue virus MTase with a bound SAH derivative revealed that its N6-substituent bound in this cavity and induced conformation changes in residues lining the pocket. These findings demonstrate that one of the major hurdles for the development of methyltransferase-based therapeutics, namely selectivity for disease-related methyltransferases, can be overcome.

The IRBIT domain adds new functions to the AHCY family

During the past few years, the IRBIT domain has emerged as an important add-on of S-adenosyl-L-homocystein hydrolase (AHCY), thereby creating the new family of AHCY-like proteins. In this review, we discuss the currently available data on this new family of proteins. We describe the IRBIT domain as a unique part of these proteins and give an overview of its regulation via (de)phosphorylation and proteolysis. The second part of this review is focused on the potential functions of the AHCY-like proteins. We propose that the IRBIT domain serves as an anchor for targeting AHCY-like proteins towards cytoplasmic targets. This leads to regulation of (i) intracellular Ca2+ via the inositol 1,4,5-trisphosphate receptor (IP3R), (ii) intracellular pH via the Na+/HCO3 - cotransporters (NBCs); whereas inactivation of the IRBIT domain induces (iii) nuclear translocation and regulation of AHCY activity. Dysfunction of AHCY-like proteins will disturb these three important functions, with various biological implications.

Synthesis and antimicrobial activity of some novel nucleoside analogues of adenosine and 1,3-dideazaadenosine

A number of nucleoside analogues have been synthesized and evaluated for their antibacterial and antifungal activities against Staphylococcus aureus, Group D Streptococcus, Pseudomonas aeruginosa, Proteus spp., Salmonella spp., Aspergillus fumigatus, Penicillium marneffei, Candida albicans, Cryptococcus neoformans, and Mucor spp. The compounds 1, 4, and 6 emerged as potent antibacterial agents with MIC values of 0.75, 0.38, and 0.19 microM, respectively, against group D Streptococcus. Further, the results suggest that the molecules 4, 6, and 7 would be potent antifungal agents as they show substantial degree of inhibition toward the growth of pathogenic fungi with MICs of 0.75, 0.38, and 0.38 microM, respectively.

Catalytic mechanism of S-adenosylhomocysteine hydrolase: roles of His 54, Asp130, Glu155, Lys185, and Aspl89

S-adenosylhomocysteine hydrolase (AdoHcyase) catalyzes the hydrolysis of S-adenosylhomocysteine (AdoHcy) to form adenosine and homocysteine. The crystal structure of the K185N mutated enzyme, which has weak catalytic activity (0.1%), has been determined at 2.8 A resolution and supports the previously predicted mechanism [Takata, Y., Yamada, T., Huang, Y., Komoto, J., Gomi, T., Ogawa, H., Fujioka, M., & Takusagawa, F. (2002). Catalytic mechanism of S-adenosylhomocysteine hydrolase. Site-directed mutagenesis of Asp-130, Lys-185, Asp-189, and Asn-190. J. Biol. Chem. 277, 22670-22676]. The mutated enzyme has an intermediate structure between the open and closed conformation, observed in the substrate-free enzyme and in the inhibitor complexes, respectively. H54, H300, and H352 were mutated to asparagine, respectively, to identify the roles of the histidine residues in catalysis. The kinetic data of H54N, H300N, and H354N mutated enzymes suggest that H54 is the amino acid residue that acts as a general acid/base to cleave the C5'-S(D) bond of AdoHcy. The E155Q mutated enzyme retained a large portion of the catalytic activity (31%), while the E155D mutated enzyme lost most of it (0.3%). The NADH accumulation measurements of the mutated enzymes indicated that the C3'-oxidation and the C4'-proton abstraction are a concerted event and the C5'-S(D) bond cleavage is an independent event. The C4'-proton exchange measurements indicate that the enzyme has an open conformation when AdoHcy is converted to 3'-keto-4', 5'-dehydro-Ado in the active site. With the results of this study and those of the previous studies, a detailed catalytic mechanism of AdoHcyase is described. K185 facilitates the C3'-oxidation, D130 abstracts the C4'-proton, D189, and E155 act as a communicator between the concerted C3'-oxidation and C4'-proton abstraction, and H54 plays as a general acid to cleave the C5'-S(D) bond of AdoHcy.

Structure-activity relationship of 5'-substituted fluoro-neplanocin a analogues as potent inhibitors of S-adenosylhomocysteine hydrolase

Four 5'-substituted fluoro-neplanocin A analogues la-d were designed and synthesized, and the inhibitory activity against SAH was in the following order: NH2 > SH > F, N3, indicating a hydrogen bonding donor is essential for inhibitory activity.

Synthesis of 5′-substituted fluoro-neplanocin A analogues: importance of a hydrogen bonding donor at 5′-position for the inhibitory activity of S-adenosylhomocysteine hydrolase

Four 5'-substituted fluoro-neplanocin A analogues 1a-d were designed and synthesized, using cyclopentenone derivative 2 as a key intermediate. The inhibitory activity against SAH was in the following order: NH(2)>SH>F, N(3), indicating a hydrogen bonding donor such as OH or NH(2) was essential for inhibitory activity. All the final compounds showed much less decreased cytotoxicity in two cancer cell lines (Col2 and A549), implying that phosphorylation of the 5'-hydroxyl group of fluoro-neplanocin A is closely related to its high cytotoxicity.

Association of diamine oxidase and S-adenosylhomocysteine hydrolase in Nicotiana tabacum extracts

The oxidative deamination of methylated putrescine by a diamine oxidase activity (DAO) is an important step in the biosynthesis of nicotine in tobacco and tropane alkaloids in several Solanaceous plants. A polyclonal rabbit antiserum was previously developed to a purported purified DAO enzyme from Nicotiana tabacum. The antiserum bound to a single 53 kDa protein and immunoprecipitated 80% of DAO activity from tobacco root extracts. In an effort to obtain DAO cDNAs, this antiserum was used to screen a tobacco cDNA expression library and three distinct immunoreactive cDNA clones were isolated. These cDNAs encoded predicted proteins that were either identical or nearly identical to predicted S-adenosylhomocysteine hydrolase (SAHH) from two Nicotiana species. Thus, the rabbit antiserum was not specific to DAO, even though it immunodepleted the majority of DAO activity from root extracts. Alternative hypotheses to explain the DAO immunodepletion results (such as poisoning of DAO activity or that SAHH is a bifunctional enzyme) were tested and ruled out. Therefore, we hypothesize that SAHH associates with DAO as part of a larger multienzyme complex that may function in planta as a nicotine metabolic channel.

Inhibition of S-adenosylhomocysteine hydrolase by acyclic sugar adenosine analogue D-eritadenine. Crystal structure of S-adenosylhomocysteine hydrolase complexed with D-eritadenine

D-eritadenine (DEA) is a potent inhibitor (IC(50) = 7 nm) of S-adenosyl-l-homocysteine hydrolase (AdoHcyase). Unlike cyclic sugar Ado analogue inhibitors, including mechanism-based inhibitors, DEA is an acyclic sugar Ado analogue, and the C2' and C3' have opposite chirality to those of the cyclic sugar Ado inhibitors. Crystal structures of DEA alone and in complex with AdoHcyase have been determined to elucidate the DEA binding scheme to AdoHcyase. The DEA-complexed structure has been analyzed by comparing it with two structures of AdoHcyase complexed with cyclic sugar Ado analogues. The DEA-complexed structure has a closed conformation, and the DEA is located near the bound NAD(+). However, a UV absorption measurement shows that DEA is not oxidized by the bound NAD(+), indicating that the open-closed conformational change of AdoHcyase is due to the substrate/inhibitor binding, not the oxidation state of the bound NAD. The adenine ring of DEA is recognized by four essential hydrogen bonds as observed in the cyclic sugar Ado complexes. The hydrogen bond network around the acyclic sugar moiety indicates that DEA is more tightly connected to the protein than the cyclic sugar Ado analogues. The C3'-H of DEA is pointed toward C4 of the bound NAD(+) (C3'...C4 = 3.7 A), suggesting some interaction between DEA and NAD(+). By placing DEA into the active site of the open structure, the major forces to stabilize the closed conformation of AdoHcyase are identified as the hydrogen bonds between the backbone of His-352 and the adenine ring, and the C3'-H...C4 interaction. DEA has been believed to be an inactivator of AdoHcyase, but this study indicates that DEA is a reversible inhibitor. On the basis of the complexed structure, selective inhibitors of AdoHcyase have been designed.

Synthesis, mechanism of action, and antiviral activity of a new series of covalent mechanism-based inhibitors of S-adenosyl-L-homocysteine hydrolase

A direct method for the preparation of 5'-S-alkynyl-5'-thioadenosine and 5'-S-allenyl-5'-thioadenosine has been developed. Treatment of a protected 5'-acetylthio-5'-deoxyadenosine with sodium methoxide and propargyl bromide followed by deprotection gave the 5'-S-propargyl-5'-thioadenosine 4. Under controlled base-catalysis with sodium tert-butoxide in tert-butyl alcohol 4 was quantitatively converted into 5'-S-allenyl-5'-thioadenosine 5 or 5'-S-propynyl-5'-thioadenosine 6. Incubation of recombinant human placental AdoHcy hydrolase with 4, 5, or 6 resulted in time- and concentration-dependent inactivation of the enzyme (K(i): 45 +/- 0.5, 16 +/- 1, and 15 +/- 1 microM, respectively). Compound 4 caused complete conversion of the enzyme from its E-NAD(+) to E-NADH form during the inactivation process. This indicates that 4 is a substrate for the 3'-oxidative activity of AdoHcy hydrolase (type I inhibitor). In contrast, the NAD(+)/NADH content of the enzyme was not affected during the inactivation process with 5 and 6, and their mechanism of inactivation was further investigated. Addition of enzyme-sequestered water on the S-allenylthio group of 5 or S-propynylthio group of 6 within the active site should lead to the formation of the corresponding thioester 7. This acylating-intermediate agent could then undergo nucleophilic attack by a protein residue, leading to a type II mechanism-based inactivation. ElectroSpray mass spectra analysis of the inactivated protein by 5 supports this mechanistic proposal. Further studies (MALDI-TOF and ESI/MS(n) experiments) of the trypsin and endo-Lys-C proteolytic cleavage of the fragments of inactivated AdoHcy hydrolase by 5 were carried out for localization of the labeling. The antiviral activity of 4, 5, and 6 against a large variety of viruses was determined. Significant activity (EC(50): 1.9 microM) was noted with 5 against vaccinia virus.

The mechanism of inactivation of S-adenosylhomocysteine hydrolase by fluorinated analogs of 5'-methylthioadenosine

5'-Deoxy-5'-difluoromethylthioadenosine (DFMTA) 1a and 5'-deoxy-5'-trifluoromethyl-thioadenosine (TFMTA) 1b are inhibitors of beef liver S-adenosyl-L-homocysteine hydrolase. DFMTA and TFMTA are time-dependent and irreversible inhibitors of the enzyme. Both 1a and 1b are oxidized by E-NAD+ to produce E-NADH and fluoride anion is formed in the inactivation reaction (2.2 mol of fluoride/mole of enzyme subunit and 3.1 fluoride/mole of enzyme subunit from DFMTA and TFMTA respectively). Using [8-3H]-1a or [8-3H]-1b no trace of labelled adenosine was detected during the inactivation reaction but adenine was formed. The mechanism of inhibition of S-adenosyl-L-homocysteine hydrolase by these two fluorinated nucleosides is discussed.

A new series of S-adenosyl-L-methionine synthetase inhibitors

A new series of epithio and epoxy amino acid analogues of L-methionine or L-methoxinine were examined as potential inhibitors of the enzyme S-adenosylmethionine (AdoMet) synthetase. The kinetic behaviour of these compounds was studied using recombinant rat liver S-adenosyl-L-methionine sythetase (alpha-isoform) fractionated from E. coli, transformed with the plasmid pSSRL-T7N. All the compounds tested were competitive inhibitors with respect to L-methionine and the (2S, 4S)-2-amino-4,5-epoxy pentanoic acid was found to be a very potent inhibitor of the enzyme compared to those already reported for AdoMet synthetase from other mammalian tissues.

A novel mechanism-based inhibitor (6'-bromo-5', 6'-didehydro-6'-deoxy-6'-fluorohomoadenosine) that covalently modifies human placental S-adenosylhomocysteine hydrolase

Most inhibitors of S-adenosylhomocysteine (AdoHcy) hydrolase function as substrates for the "3'-oxidative activity" of the enzyme and convert the enzyme from its active form (NAD+) to its inactive form (NADH) (Liu, S., Wolfe, M. S., and Borchardt, R. T. (1992) Antivir. Res. 19, 247-265). In this study, we describe the effects of a mechanism-based inhibitor, 6'-bromo-5', 6'-didehydro-6'-deoxy-6'-fluorohomoadenosine (BDDFHA), which functions as a substrate for the "6'-hydrolytic activity" of the enzyme with subsequent formation of a covalent linkage with the enzyme. Incubation of human placental AdoHcy hydrolase with BDDFHA results in a maximum inactivation of 83% with the remaining enzyme activity exhibiting one-third of the kcat value of the native enzyme. This partial inactivation is concomitant with the release of both Br- and F- ions and the formation of adenine (Ade). The enzyme can be covalently labeled with [8-3H]BDDFHA, resulting in a stoichiometry of 2 mol of BDDFHA/mol of the tetrameric enzyme. The 3H-labeled enzyme retains its original NAD+/NADH content. Tryptic digestion and subsequent protein sequencing of the [8-3H]BDDFHA-labeled enzyme revealed that Arg196 is the residue that is associated with the radiolabeled inhibitor. The partition ratio of the Ade formation (nonlethal event) to covalent acylation (lethal event) is approximately 1:1. From these experimental results, a possible mechanism by which BDDFHA inactivates AdoHcy hdyrolase is proposed: enzyme-mediated water addition at the C-6' position of BDDFHA followed by elimination of Br- ion results in the formation of homoAdo 6'-carboxyl fluoride (HACF). HACF then partitions in two ways: (a) attack by a proximal nucleophile (Arg196) to form an amide bond after expulsion of F- ion (lethal event) or (b) depurination to form Ade and hexose-derived 6-carboxyl fluoride (HDCF), which is further hydrolyzed to hexose-derived 6-carboxylic acid (HDCA) and F- ion (nonlethal event).

Effect of limited proteolysis on the stability and enzymatic activity of human placental S-adenosylhomocysteine hydrolase

Human placental S-adenosylhomocysteine (AdoHcy) hydrolase was subjected to limited papain digestion. The multiple cleavage sites in the enzyme were identified to be Lys94-Ala95, Tyr100-Ala101, Glu243-Ile244, Met367-Ala368, Gln369-Ile370, and Gly382-Val383. Despite multiple cleavage sites in the backbone of the protein, the digested enzyme was able to maintain its quaternary structure and retain its full catalytic activity. The enzyme activity of the partially digested AdoHcy hydrolase was essentially identical to that of the native enzyme at several pH values. The thermal stabilities of the native and partially digested enzymes were only slightly different at all temperatures tested. The stability of both native and partially digested enzymes were examined in guanidine hydrochloride and equilibrium unfolding transitions were monitored by CD spectroscopy and tryptophan fluorescence spectroscopy. The results of these experiments can be summarized as follows: (1) CD spectroscopic analysis showed that the overall secondary and tertiary structures of the partially digested enzyme are essentially identical with those of the native enzyme; and (2) tryptophan fluorescence spectroscopic analysis indicated that there are small differences in the environments of surface-exposed tryptophan residues between the partially digested enzyme and the native enzyme under unfolding conditions. The differences in the free energy of unfolding, delta(delta Gu) [delta Gu(native)-delta Gu(digested)], is approximately 1.3 kcal/mol. When NAD+ was removed from the partially digested enzyme, the secondary and tertiary structures of the apo form of the digested AdoHcy hydrolase were completely lost and the enzymatic activity could not be recovered by incubation with excess NAD+. These results suggest that AdoHcy hydrolase exists as a very compact enzyme with extensive intramolecular bonding, which contributes significantly to the overall global protein stabilization. Identification of the surface-exposed peptide bonds, which are susceptible to papain digestion, has provided some constraints on the spatial orientations of subunits of the enzyme. This information, in turn, has provided supplemental data for X-ray crystallographic studies currently ongoing in our laboratories.

Inactivation of S-adenosyl-L-homocysteine hydrolase by amide and ester derivatives of adenosine-5'-carboxylic acid

S-Adenosyl-L-homocysteine (AdoHcy) hydrolase has been shown to have (5'/6') hydrolytic activity with vinyl (5') or homovinyl (6') halides derived from adenosine (Ado). This hydrolytic activity is independent of its 3'-oxidative activity. The vinyl (or homovinyl) halides are converted into 5'(or 6')-carboxaldehydes by the hydrolytic activity of the enzyme, and inactivation occurs via the oxidative activity. Amide and ester derivatives of Ado-5'-carboxylic acid were prepared to further probe the hydrolytic capability of AdoHcy hydrolase. The oxidative activity (but not the hydrolytic activity) is involved in the mechanism of inhibition of the enzyme by the ester and amide derivatives of Ado-5'-carboxylic acid, in contrast to the inactivation of this enzyme by adenosine-derived vinyl or homovinyl halide analogues during which both activities are manifested.

Aristeromycin-5'-carboxaldehyde: a potent inhibitor of S-adenosyl-L-homocysteine hydrolase

In an earlier study, Liu et al. (Bioorg. Med. Chem. Lett. 1992, 2, 1741-1744) showed that both the E and Z isomers of 4',5'-didehydro-5'-fluoroaristeromycin were very potent irreversible inhibitors of S-adenosylhomocysteine (AdoHcy) hydrolase. However, it was unclear from a mechanistic standpoint whether these vinyl fluorides were themselves type-I mechanism-based inhibitors causing reduction of enzyme-bound NAD+ or whether they were prodrug for aristeromycin-5'-carboxaldehyde, which was the ultimate type-I inhibitor. To elucidate this mechanism of enzyme inhibition, (4'S)- and (4'R)-aristeromycin-5'-carboxaldehydes (1a,b) were synthesized in this study and shown to be potent type-I mechanism-based inhibitors of AdoHcy hydrolase with k2/Ki values of 4.4 x 10(6) and 8.2 x 10(4)M-1min-1, respectively. However, Using 19F NMR and HPLC, it was shown that (4'S)-4,5'-dedehydro-5'-fluoraristeromycin in the presence of AdoHcy hydrolase did not release fluoride ion or generate aristeromycin-5'-carboxaldehyde (1a,b). These results suggest that the E and Z isomers of 4',5'-didehydro-5'-fluoroaristeromycin are inactivating AdoHcy hydrolase by directly reducing NAD+ to NADH and not using the hydrolytic activity of the enzyme to generate aristeromycin-5'-carboxaldehyde.

Induction of murine erythroleukemia cell differentiation is associated with methylation and differential stability of poly(A)+ RNA transcripts

Murine erythroleukemia (MEL) cells exposed to DMSO were assessed for their ability to methylate poly(A)+ RNA and accumulate RNA transcripts of globin and nonglobin genes (c-myc, beta-actin and MER5). Cells were pulse-labeled with L-[methyl-3H]methionine, cytoplasmic RNA was isolated, selected for poly(A)+ RNA and analyzed by HPLC chromatography for methylated nucleosides. When MEL cells were exposed to inhibitors of RNA methylation (neplanocin A, 3-deazaneplanocin A and cycloleucine) and assessed for their ability to differentiate by DMSO, accumulate RNA transcripts, produce hemoglobin, methylate poly(A)+ and poly(A)- RNA and synthesize S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH), we observed the following: (a) MEL cells treated with DMSO underwent hypermethylation in poly(A)+ RNA that preferentially occurred at the 5'-cap structures (7-methylguanosine and 2'-O-methylcytidine and 2'-O-methyluridine); (b) inducer-treated MEL cells exhibited a decrease in the intracellular level of SAH that led to a lower ratio of SAH/SAM, an event that favors methylation; and (c) treatment of MEL cells with inhibitors of RNA methylation suppressed methylation of poly(A)- and poly(A)+ RNA, reversed the ratio SAH/SAM seen in differentiated MEL cells and prevented differentiation to occur. Moreover, we observed that treatment of MEL cells with selective inhibitors of RNA methylation caused fragmentation of beta major globin and c-myc mRNAs, two RNA transcripts coded by developmentally regulated genes, while had no detectable effect on the structural integrity of poly(A)+ RNA transcripts transcribed by two housekeeping genes (beta-actin and MER5). These data indicate that induction of erythroid cell differentiation of MEL cells is associated with changes in methylation of poly(A)+ RNA and selective differential stability of RNA transcripts, two events that might be related to each other.

(E)-5',6'-didehydro-6'-deoxy-6'-fluorohomoadenosine: a substrate that measures the hydrolytic activity of S-adenosylhomocysteine hydrolase

(E)-5',6'-Didehydro-6'-deoxy-6'-fluorohomoadenosine (EDDFHA), which is a poor substrate for the oxidative activity of S-adenosyl-L-homocysteine (AdoHcy) hydrolase and thus a poor mechanism-based inhibitor was shown to be a good substrate for the hydrolytic activity of this enzyme. Incubation of EDDFHA with AdoHcy hydrolase (NAD+ form) produces a large molar excess of hydrolytic products [e.g., fluoride ion, adenine (Ade) derived from chemical degradation of homoadenosine 6'-carboxaldehyde (HACA), and 6'-deoxy-6'-fluoro-5'-hydroxyhomoadenosine (DFHHA)] accompanied by a slow irreversible inactivation of the enzyme. The enzyme inactivation was shown to be time-dependent, biphasic, and concomitant with the reduction of the enzyme-bound NAD+ (E.NAD+) to E-NADH. The reaction of EDDFHA with AdoHcy hydrolase was shown to proceed by three pathways: pathway a, water attack at the 6'-position of EDDFHA and elimination of fluoride ion results in the formation of HACA, which degrades chemically to form Ade; pathway b, water attack at the 5'-position of EDDFHA results in the formation of DFHHA; and pathway c, oxidation of EDDFHA results in formation of the NADH form of the enzyme (inactive) and 3'keto-EDDFHA, which could react with water at either the C5' or C6' positions. The partition ratios among the three pathways were determined to be k3':k6':k5' = 1:29:79 with one lethal event (enzyme inactivation) occurring every 108 nonlethal turnovers.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular approaches for the treatment of hemorrhagic fever virus infections

Viruses causing hemorrhagic fevers in man belong to the following virus groups: togavirus (Chikungunya), flavivirus (dengue, yellow fever, Kyasanur Forest disease, Omsk hemorrhagic fever), arenavirus (Argentinian hemorrhagic fever, Bolivian hemorrhagic fever, Lassa fever), filovirus (Ebola, Marburg), phlebovirus (Rift Valley fever), nairovirus (Crimian-Congo hemorrhagic fever) and hantavirus (hemorrhagic fever with renal syndrome, nephropathic epidemia). Hemorrhagic fever virus infections can be approached by different therapeutic strategies: (i) vaccination; (ii) administration of high-titered antibodies; and (iii) treatment with antiviral drugs. Depending on the molecular target of their interaction, antiviral agents could be classified as follows: IMP dehydrogenase inhibitors (i.e., ribavirin and its derivatives); OMP decarboxylase inhibitors (i.e., pyrazofurin); CTP synthetase inhibitors (i.e., cyclopentylcytosine and cyclopentenylcytosine); SAH hydrolase inhibitors (i.e., neplanocin A); polyanionic substances (i.e., sulfated polymers); interferon and immunomodulators.

Inhibitory activity of S-adenosylhomocysteine hydrolase inhibitors against human cytomegalovirus replication

Various acyclic and carbocyclic adenosine analogues, which are apparently targeted at the S-adenosylhomocysteine (AdoHcy) hydrolase have been reported to inhibit the replication of a number of pox-, rhabdo-, paramyxo-, arena-, and reoviruses. Here we show that this activity spectrum extends to human cytomegalovirus (HCMV). Of the compounds tested, neplanocin A, 3-deazaneplanocin A, 6'-C-methylneplanocin A and 5'-noraristeromycin were found to be the most potent inhibitors of HCMV replication in vitro. Their 50% inhibitory concentration ranged from 0.05 to 1.35 micrograms/ml. In general, the anti-HCMV activity of the adenosine analogues correlated well with their affinity (Ki) for AdoHcy hydrolase, suggesting that AdoHcy hydrolase may be considered as a target enzyme for anti-HCMV agents. For four compounds (3-deazaneplanocin A, 6'-C-methylneplanocin A (isomers I and II) and 3-deazaadenosine), anti-HCMV potency was greater than could be expected solely from their interaction with AdoHcy hydrolase, suggesting that these compounds may be functioning by an additional mechanism.

Antiviral agents: characteristic activity spectrum depending on the molecular target with which they interact

The target protein (enzyme) with which antiviral agents interact determines their antiviral activity spectrum. Based on their activity spectrum, antiviral compounds could be divided into the following classes: (1) sulfated polysaccharides (i.e., dextran sulfate), which interact with the viral envelope glycoproteins and are inhibitory to a broad variety of enveloped viruses (i.e., retro-, herpes-, rhabdo-, and arenaviruses): (2) SAH hydrolase inhibitors (i.e., neplanocin A derivatives), which are particularly effective against poxvirus, (-)RNA viruses (paramyxovirus, rhabdovirus), and (+/-)RNA virus (reovirus); (3) OMP decarboxylase inhibitors (i.e., pyrazofurin) and CTP synthetase inhibitors (i.e., cyclopentenylcytosine), which are active against a broad range of DNA, (+)RNA, (-)RNA, and (+/-)RNA viruses; (4) IMP dehydrogenase inhibitors (i.e., ribavirin), which are also active against various (+)RNA and (-)RNA viruses and, in particular, ortho- and paramyxoviruses; (5) acyclic guanosine analogs (i.e., ganciclovir) and carbocyclic guanosine analogs (i.e., cyclobut-G), which are particularly active against herpesviruses (i.e., HSV-1, HSV-2, VZV, CMV); (6) thymidine analogs (i.e., BVDU, BVaraU), which are specifically active against HSV-1 and VZV because of their preferential phosphorylation by the virus-encoded thymidine kinase; (7) acyclic nucleoside phosphonates (i.e., HPMPA, HPMPC, PMEA, FPMPA), which, depending on the structure of the acyclic side chain, span an activity spectrum from DNA viruses (papova-, adeno-, herpes-, hepadna-, and poxvirus) to retroviruses (HIV); (8) dideoxynucleoside analogs (i.e., AZT, DDC), which act as chain terminators in the reverse transcriptase reaction and thus block the replication of retroviruses as well as hepadnaviruses; and (9) the TIBO, HEPT, and other TIBO-like compounds, which interact specifically with the reverse transcriptase of HIV-1 and thus block the replication of HIV-1, but not of HIV-2 or any other retrovirus.

Antiretroviral activity of mechanism-based irreversible inhibitors of S-adenosylhomocysteine hydrolase

S-Adenosylhomocysteine hydrolase (AdoHcy-nase) is a key enzyme in transmethylation reactions. The objective of the present study was to examine the potential antiretroviral activities of novel mechanism-based irreversible AdoHcy-nase inhibitors. (Z)-4',5'-didehydro-5'-deoxy-5'-fluoroadenosine (ZDDFA), (E)-4',5'-didehydro-5'-deoxy-5'-fluoroadenosine (EDDFA), (Z)-4',5'-didehydro-5'-deoxy-5'-chloroadenosine (ZDDCA) and 5'-deoxy-5'-acetylenic adenosine (DAA) inhibited AdoHcy-nase activity with Ki values of 0.55, 1.04, greater than 10.0 and 3.30 microM, respectively. These four compounds were tested for antiviral activity in vitro against Moloney leukemia virus (MoLV) in the XC-plaque assay. MoLV replication in murine fibroblasts (SC-1) was inhibited by ZDDFA, EDDFA and DAA with IC50 values of 0.05, 0.25 and 3.30 micrograms/ml, respectively. ZDDCA did not inhibit MoLV infection at the concentrations tested. Antiviral activity correlated with the ability of the individual compounds to maintain sustained elevations in intracellular S-adenosylhomocysteine (AdoHcy) concentrations in the SC-1 cells. ZDDFA, the most potent inhibitor of AdoHcy-nase and MoLV was also the most active in maintaining sustained elevations in intracellular AdoHcy levels. The antiviral activity of ZDDFA was also examined in murine C3H1OT1/2 fibroblasts which constitutively produce MoLV. Pretreatment with ZDDFA (1.0 microgram/ml) for 24 hr inhibited virus production by 88%. Similar to the SC-1 cells, and concomitant with enzyme inhibition, there was a 300-fold increase in AdoHcy levels in ZDDFA (1.0 microgram/ml) treated C3H1OT1/2 cells. Incorporation of a [3H]methyl group from tritiated S-adenosylmethionine into total RNA in C3H1OT1/2 cells was inhibited by ZDDFA without affecting cell viability. These results suggest that mechanism-based inhibitors of AdoHcy-nase, such as ZDDFA, may have potential as antiretroviral agents.

Influence of S-adenosylhomocysteine hydrolase inhibitors on S-adenosylhomocysteine and S-adenosylmethionine pool levels in L929 cells

S-Adenosylhomocysteine hydrolase has been recognized as the target enzyme for the antiviral activity of several carbocyclic and acyclic adenosine analogues. In a previous study [Cools M and De Clercq E, Biochem Pharmacol 38: 1061-1067, 1989], we found a close correlation between the antiviral activity of six adenosine analogues [S)-9-(2,3-dihydroxypropyl)adenine [(S)-DHPA], (RS)-3-adenin-9-yl-2-hydroxypropanoic acid [(RS)-AHPA] (isobutyl ester), 3-deazaneplanocin A, carbocyclic 3-deazaadenosine (C-c3 Ado), adenosine dialdehyde and neplanocin A) against vaccinia virus and vesicular stomatitis virus and the inhibitory effect of these compounds on purified AdoHcy hydrolase isolated from murine L929 cells. We have now examined the effects of the different adenosine analogues on the intracellular pool levels of S-adenosylhomocysteine (AdoHcy) and S-adenosylmethionine (AdoMet). Treatment of vaccinia virus-infected L929 cells for 24 hr with the adenosine analogues at a dose that reduced vaccinia virus growth by 90% (ID90) increased the average AdoHcy pool levels from 0.027 nmol/mg protein to approximately 0.3 nmol/mg protein and the AdoHcy/AdoMet ratio from 0.038 to approximately 0.3. Moreover, the AdoHcy/AdoMet ratio correlated closely with the vaccinia virus yield reduction, both determined over the 24-hr post infection period (correlation coefficient of 0.972). These findings indicate that the activity of the AdoHcy hydrolase inhibitors against vaccinia virus may be related to the raise in intracellular AdoHcy pool levels and AdoHcy/AdoMet ratio.

2-Methylpropyl ester of 3-(adenin-9-yl)-2-hydroxypropanoic acid. Mechanism of antiviral action in vaccinia virus-infected L929 cells

Alkyl esters of (RS)-3-(adenin-9-yl)-2-hydroxypropanoic acid (AHPA) were shown recently to be broad spectrum antiviral agents (De Clercq E and Holy A, J Med Chem 28: 282-287, 1985). It was postulated that these alkyl esters function as prodrugs by undergoing hydrolysis catalyzed by cellular esters to AHPA, a known inhibitor of S-adenosyl-L-homocysteine (AdoHcy) hydrolase. In this study, we describe the metabolic fate of the 2-methylpropyl ester of AHPA (AHPA-iBu) in murine L929 cells. When AHPA-iBu was included in the culture medium, it was taken up rapidly by murine L929 cells. The uptake was time- and concentration-dependent, resulting in the intracellular accumulation of the free acid, AHPA. Treatment with AHPA-iBu caused inhibition of cellular AdoHcy hydrolase in both a time- and a dose-dependent manner. Complete inhibition of the enzyme was achieved after a 1-hr incubation in culture medium containing 50 microM AHPA-iBu. The inhibition of the enzyme caused cellular accumulation of AdoHcy and a significant increase in the ratio of AdoHcy/S-adenosyl-L-methionine (AdoMet). Partial recovery of the AdoHcy hydrolase activity in L929 cells treated with 50 microM AHPA-iBu was observed after 24 hr. This recovery of enzyme activity was paralleled by a significant decrease in the cellular levels of AdoHcy and the ratio of AdoHcy/AdoMet. AHPA-iBu also exerted an inhibition (IC50 = 0.17 microM) of vaccinia virus plaque formation in monolayers of L929 cells. A 1 microM concentration of AHPA-iBu, which caused 80% inhibition of plaque formation, produced a 17-fold increase in AdoHcy content in drug-treated, virus-infected cells versus non-drug-treated, virus-infected cells and a 15% undermethylation of the poly(A)+ RNA. These data show that AHPA-iBu is a prodrug for AHPA which inhibits cellular AdoHcy hydrolase. The inhibition of this enzyme elevates cellular levels of AdoHcy, creating an unfavorable environment which suppresses replication of vaccinia virus.

Mechanism of the synergistic antiviral and cytostatic activity of (RS)-3-(adenin-9-yl)-2-hydroxypropanoic acid isobutyl ester and D,L-homocysteine

In a previous report (De Clercq E, Cools M and Balzarini J, Biochem Pharmacol 38: 1771-1778, 1989) we showed that homocysteine (Hcy) enhanced the antiviral and cytostatic activity of S-adenosylhomocysteine (AdoHcy) hydrolase inhibitors. The mechanism of synergistic action between Hcy and the isobutyl ester of (RS)-3-(adenin-9-yl)-2-hydroxypropanoic acid [(RS)-AHPA] has been the subject of the present study. The selectivity index of (RS)-AHPA against vaccinia virus in murine L929 cells was significantly increased if the drug was combined with 1 or 3 mM Hcy. Even if Hcy was added as late as 12 hr after (RS)-AHPA, a synergistic antiviral activity was noted. Treatment of the L929 cells with (RS)-AHPA caused a significant increase in AdoHcy levels, and these levels were further increased if, in addition to (RS)-AHPA, Hcy (1 mM) was added to the cell cultures. Double-pulse label experiments showed that the additional AdoHcy built up after the combined treatment of (RS)-AHPA with Hcy did not originate from S-adenosylmethionine (via transmethylation reactions), but resulted from residual AdoHcy hydrolase activity (in the synthetic direction). To maintain sufficient levels of AdoHcy, AdoHcy hydrolase activity must be inhibited in the hydrolytic direction.