Quantum chemical study of the electronic and conformational characteristics of adenosine and 8-substituted derivatives: functional implications in the mechanism of reaction of adenosine deaminase

Synthesis, conformational analysis and antiviral and antitumoral activity of new 1,2-disubstituted carbocyclic nucleosides

New 1',2'-cis-disubstituted 8-azapurine-based carbocyclic analogues of nucleosides with or without a methylene between the carbocycle and the base were synthesised, starting from appropriate amino alcohols, via 6-chloro-8-azapurines; their antiviral and antitumoral activities were evaluated; and their structures were compared with that of 2',3'-dideoxyadenosine (ddA) on the basis of AM1 calculations. No new compound had antiviral activity. The one with the best overall antitumoral activity against L1210, Molt4/C8 and CEM/0 cells, compound 10, was that in which the position of the hydroxymethyl group on the carbocycle relative to the heterocyclic base was closest to that found in the best-fitting low-energy conformer of ddA.

Adenosine deaminase: functional implications and different classes of inhibitors

Adenosine deaminase (ADA) is an enzyme of the purine metabolism which catalyzes the irreversible deamination of adenosine and deoxyadenosine to inosine and deoxyinosine, respectively. This ubiquitous enzyme has been found in a wide variety of microorganisms, plants, and invertebrates. In addition, it is present in all mammalian cells that play a central role in the differentiation and maturation of the lymphoid system. However, despite a number of studies performed to date, the physiological role played by ADA in the different tissues is not clear. Inherited ADA deficiency causes severe combined immunodeficiency disease (ADA-SCID), in which both B-cell and T-cell development is impaired. ADA-SCID has been the first disorder to be treated by gene therapy, using polyethylene glycol-modified bovine ADA (PEG-ADA). Conversely, there are several diseases in which the level of ADA is above normal. A number of ADA inhibitors have been designed and synthesized, classified as ground-state and transition-state inhibitors. They may be used to mimic the genetic deficiency of the enzyme, in lymphoproliferative disorders or immunosuppressive therapy (i.e., in graft rejection), to potentiate the effect of antileukemic or antiviral nucleosides, and, together with adenosine kinase, to reduce breakdown of adenosine in inflammation, hypertension, and ischemic injury.

Effect of a chemical modification on the hydrated adenosine intermediate produced by adenosine deaminase and a model reaction for a potential mechanism of action of 5-aminoimidazole ribonucleotide carboxylase

Using the hydrated adenosine intermediate (6R)-6-amino-1, 6-dihydro-6-hydroxy-9-(beta-D-ribofuranosyl)purine (2) produced by adenosine deaminase (ADA, EC 3.5.4.4) as a starting point, the active site probe and inhibitor platform 5-(formylamino)imidazole riboside (FAIRs, 4) was designed by removal of the-C6(OH)(NH2)-molecular fragment of 2 generated by the early events of the enzyme-catalyzed hydrolysis. FAIRs was synthesized directly from the sodium salt of 5-amino-1-(beta-D-ribofuranosyl)imidazole-4-carboxylic acid (CAIR) along a reaction sequence involving a tandem N-formylation/decarboxylation that may have a mechanistic connection to the Escherichia coli purE-catalyzed constitutional isomerization of N5-CAIR to CAIR. The physical and spectral properties of FAIRs were elucidated, its X-ray crystal and NMR solution structures were determined, and its interaction with ADA was investigated. Crystalline FAIRs exists solely as the Z-formamide rotamer and exhibits many of the same intramolecular hydrogen bonding events known to contribute to the association of Ado to ADA. In water and various organic solvents, however, FAIRs exists as NMR-distinct, slowly interconverting Z and E rotamers. This truncated enzymatic tetrahedral intermediate analog was determined to be a competitive inhibitor of ADA with an apparent Ki binding constant of 40 microM, a value quite close to that (33 microM) of the natural substrate's K(m). The actual species selected for binding by ADA, though, is likely the minor hydroxyimino prototropic form of Z-FAIRs possessing a far lower true Ki value. As the structural features of FAIRs appear well-suited to support its use as a template for constructing active site probes of both ADA and AIR carboxylases, a variety of carbohydrate-protected versions of FAIRs suitable for facile aglycon elaborations were synthesized. The N3-alkylation, N3-borane complexation, and C4-iodination of some of these were investigated in order to assess physicochemical properties that may assist in the elucidation of mechanisms for the AIR carboxylases. The survey of these properties taken together with a reasonable mechanism for the model CAIRs-->FAIRs synthetic transformation is interpreted to support a mechanism for the purE-catalyzed N5-CAIR-->CAIR biosynthetic one that involves a carboxylative sp3-rehybridization of the imidazole C4 atom rather than one possessing a dipole-stabilized C4 sp2 carbanionic intermediate.

Site-directed mutagenesis of active site glutamate-217 in mouse adenosine deaminase

Mouse adenosine deaminase (ADA) contains an active site glutamate residue at position-217 that is highly conserved in other adenosine and AMP deaminases. Previous research has suggested that proton donation to N-1 of the adenosine ring occurs prior to catalysis and supports the mechanism as proceeding via formation of a tetrahedral intermediate at C-6 of adenosine. The proposed catalytic mechanism of ADA based on the recent elucidations of the crystal structure of this enzyme with transition- and ground-state analogs hypothesized that Glu217 was involved in this proton donation step [Wilson, D. K., Rudolph, F. B., & Quiocho, F. A. (1991) Science 252, 1278-1284; Wilson, D. K., & Quiocho, F. A. (1993) Biochemistry 32, 1689-1693]. Site-directed mutagenesis of the equivalent glutamate in human ADA resulted in a dramatic loss of enzyme activity [Bhaumik, D., Medin, J., Gathy, K., & Coleman, M. (1993) J. Biol. Chem. 268, 5464-5470]. To further study the importance of this residue, site-directed mutagenesis was used to create mouse ADA mutants. Glu217 was mutated to Asp, Gly, Gln, and Ser, and all mutants were successfully expressed and purified. Circular dichroism and zinc analysis showed no significant changes in secondary structure or zinc content, respectively, compared to the native protein. The mutants showed only a slight variation in Km but dramatically reduced kcat, less than 0.2% of wild-type activity. UV difference and 13C NMR spectra conclusively demonstrated the failure of any of these mutants to hydrate purine riboside, a reaction carried out by the wild-type enzyme that results in formation of an enzyme-inhibitor complex. Surprisingly, Ki values for binding of the inhibitor to the mutants and to wild-type protein are similar, irrespective of whether the inhibitor is hydrated upon binding. These data confirm the importance of Glu217 in catalysis as suggested by the crystal structure of mouse ADA.

8-substituted adenosine and theophylline-7-riboside analogues as potential partial agonists for the adenosine A1 receptor

A series of 8-substituted adenosine and theophylline-7-riboside analogues (28 and 9 compounds, respectively) was tested on adenosine A1 and A2A receptors as an extensive exploration of the adenosine C8-region. Alkylamino substituents at the 8-position cause an affinity decrease for adenosine analogues, but an affinity increase for theophylline-7-riboside derivatives. The affinity decrease is probably due to a direct steric hindrance between the C8-substituent and the binding site as well as to electronic effects, not to a steric influence on the ribose moiety to adopt the anti conformation. The 8-substituents increase the affinity of theophylline-7-riboside analogues probably by binding to a lipophilic binding site. The intrinsic activity was tested in vitro for some 8-substituted adenosine analogues, by determining the GTP shift in receptor binding studies and the inhibition of adenylate cyclase in a culture of rat thyroid FRTL-5 cells, and in vivo in the rat cardiovascular system for 8-butylaminoadenosine. Thus, it was shown that 8-ethyl-, 8-butyl-, and 8-pentylamino substituted analogues of adenosine may be partial agonists in vitro, and that 8-butylaminoadenosine is a partial agonist for the rat cardiovascular A1 receptor in vivo.