Pyrazolo[3,4-d]pyrimidine ribonucleosides as anticoccidials. 1. Synthesis and activity of some nucleosides of purines and 4-(alkylthio)pyrazolo[3,4-d]pyrimidines

Thermus thermophilus nucleoside phosphorylases active in the synthesis of nucleoside analogues

Cells extracts from Thermus thermophilus HB27 express phosphorolytic activities on purines and pyrimidine nucleosides. Five putative encoding genes were cloned and expressed in Escherichia coli, and the corresponding recombinant proteins were purified and studied. Two of these showed phosphorolytic activities against purine nucleosides, and third one showed phosphorolytic activity against pyrimidine nucleosides in vitro, and the three were named TtPNPI, TtPNPII, and TtPyNP, respectively. The optimal temperature for the activity of the three enzymes was beyond the water boiling point and could not be measured accurately, whereas all of them exhibited a wide plateau of optimal pHs that ranged from 5.0 to 7.0. Analytical ultracentrifugation experiments revealed that TtPNPI was a homohexamer, TtPNPII was a monomer, and TtPyNP was a homodimer. Kinetic constants were determined for the phosphorolysis of the natural substrates of each enzyme. Reaction tests with nucleoside analogues revealed critical positions in the nucleoside for its recognition. Activities with synthetic nucleobase analogues, such as 5-iodouracil or 2,6-diaminopurine, and arabinosides were detected, supporting that these enzymes could be applied for the synthesis of new nucleoside analogs with pharmacological activities.

Pyrazolo[3,4-d]pyrimidine ribonucleosides related to 2-aminoadenosine and isoguanosine: synthesis, deamination and tautomerism

The syntheses and properties of 8-aza-7-deazapurine (pyrazolo[3,4-d]pyrimidine) ribonucleosides related to 2-aminoadenosine and isoguanosine are described. Glycosylation of 8-aza-7-deazapurine-2,6-diamine 5 with 1-O-acetyl-2,3,5-tri-O-benzoyl-beta-D-ribofuranose (12) in the presence of BF(3) x Et(2)O as a catalyst gave the N(8) isomer 14 (73%) with a trace amount of the N(9) isomer 13a (4.8%). Under the same reaction conditions, the 7-halogenated 8-aza-7-deazapurine-2,6-diamines 6-8 afforded the thermodynamically more stable N(9) nucleosides 13b-d as the only products (53-70%). Thus, a halogen in position 7 shifts the glycosylation from N(8) to N(9). The 8-aza-7-deazapurine-4,6-diamine ribonucleosides 1a-d were converted to the isoguanosine derivatives 3a-d by diazotization of the 2-amino group. Although compounds 1a,b do not contain a nitrogen at position 7 (the enzyme binding site), they were deaminated by adenosine deaminase; however, their deamination occurred with a much slower velocity than that of the related purines. The pK(a) values indicate that the 7-non-functionalized nucleosides 1a (pK(a) 5.8) and 15 (pK(a) 6.4) are possibly protonated in neutral conditions when incorporated into RNA. The nucleosides 3a-d exist predominantly in the keto (lactam) form with K(TAUT) (keto/enol) values of 400-1200 compared to 10(3)-10(4) for pyrrolo[2,3-d]pyrimidine isoguanosine derivatives 4a-c and 10 for isoguanosine itself, which will reduce RNA mispairing with U.

Synthesis and biological evaluation of some acyclic 4,6-disubstituted 1H-pyrazolo[3,4-d]pyrimidine nucleosides

The chemical synthesis and biological evaluation of some acyclic alpha-[6-(1'-carbamoylalkylthio)-1H-pyrazolo[3,4-d]pyrimidin-4-yl]thioalkylamide nucleosides are described.

Synthesis and biological evaluation of some acyclic alpha-(1H-pyrazolo-[3,4-d]pyrimidin-4-yl)thioalkylamide nucleosides

The chemical synthesis of some acyclic alpha-(1H-pyrazolo[3,4-d]pyrimidin-4-yl)thioalkylamide nucleosides (10-12)a-c is described. The treatment of IH-pyrazolo[3,4-d]pyrimidin-4-thione 1 with compounds 2a-c gave, regioselectively, ethyl alpha-(1H-pyrazolo[3,4-d]pyrimidin-4-yl)thioalkylates 3a-c, respectively. These heterocycles were alkylated, separately, with alkylating agents 4, 5 and 6 to give, regioselectively, the N1-acyclic nucleosides (7-9)a-c which were deprotected to afford the desired products (10-12)a-c. All synthetic compounds were characterized on the basis of their physical and spectroscopic properties. The products (10-12)a-c were evaluated for their inhibitory effects against the replication of HIV-1 (III(B)), HIV-2 (ROD), various DNA viruses, a variety of tumor-cell lines and M. tuberculosis. No marked biological activity was found.

Three enzymes newly identified from the genus Eimeria and two more newly identified from E. maxima, leading to the discovery of some aliphatic acids with activity against coccidia of the domesticated fowl

Nine enzymes were detected in sporulated oocysts of Eimeria tenella and E. maxima, parasites of the domesticated fowl (Gallus gallus). Three enzymes, hydroxybutyrate dehydrogenase, alanine aminotransferase and gamma-glutamyltransferase, all identified for the first time in Eimeria of fowl, occurred both in E. tenella and in E. maxima. The remaining enzymes assayed had previously been found in various Eimeria species of fowl, although creatine kinase and glutamate dehydrogenase were hitherto unknown from E. maxima. The three enzymes newly recorded from Eimeria of fowl are of interest as potential genetic markers, and also as potential chemotherapeutic targets. The discovery of hydroxybutyrate dehydrogenase led to the demonstration of anticoccidial activity by some aliphatic acids. The paper also includes a list of the enzymes detected in Eimeria of fowl in previous studies.

Identification, partial purification and inhibition by guanine analogues of a novel enzymic activity which phosphorylates guanosine to GMP in the protozoan parasite Eimeria tenella

From oocysts of the protozoan parasite Eimeria tenella, responsible for avian coccidiosis, we have partially purified and characterized a novel enzymic activity which specifically phosphorylates guanosine to GMP. The enzyme is able to use several phosphate donors, in the order: acetyl phosphate (Ac-P) > ATP > UTP > CTP > phosphoribosyl pyrophosphate (PRPP) > dUTP > or = dATP. The low specificity of this enzyme for the phosphate donor suggested that it be named guanosine phosphotransferase (GPTase). This enzyme is biochemically distinct from the previously described adenosine kinase (AK) and hypoxanthine/xanthine/guanine phosphoribosyltransferase (HXGPRTase), and may enable the parasite to synthesize guanine nucleotides under conditions of imbalance between adenine and guanine nucleotides. Because of its possible role in the purine salvage pathways, we have studied the effect of several guanine and guanosine analogues, recently synthesized in our laboratory, on the activity of GPTase in vitro. GPTase is specifically inhibited in the micromolar range by several substituted N2-phenylguanine bases. These results indicate that, as previously found for AK and HXGPRTase, GPTase could be a potential target for antiparasitic chemotherapy.

Aldehyde oxidase from rabbit liver: specificity toward purines and their analogs

Aldehyde oxidase (EC 1.2.3.1) plays a role in the oxidation of aromatic heterocyclic compounds ingested by some higher vertebrates. To better understand this function, the specificity of the rabbit liver enzyme toward purines and their analogs was quantitatively studied. The chemical nature of the 6-substituent of purine markedly influenced substrate efficiency (Vmax/Km). Substituents that were hydrophobic were generally favorable. There was a correlation between the degree of hydrophobicity and the tightness of binding. 6-Substituents that were strongly electron-withdrawing also enhanced substrate efficiency. 6-Hydroxy and 6-amino substituents virtually obliterated substrate activity. In contrast, 2-hydroxypurine and 2-aminopurine were efficiently oxidized. 2,6-Disubstitution of purine was much less favorable than either 2- or 6-monosubstitution. N-Substitution of purines enhanced substrate efficiency in many cases. The typical order of preference for 9-substituents was 2'-deoxyribofuranosyl greater than ribofuranosyl greater than arabinofuranosyl greater than H. Acyclic nucleosides (9-[(hydroxy-alkyloxy)methyl]purines) were usually more efficient substrates than were 2'-deoxyribonucleosides. The kinetic constants of a variety of purine analogs revealed that the pyrimidine portion of the purine ring system was a more important determinant of substrate activity than the imidazole portion. The efficient oxidation of a variety of nucleosides suggests that detoxification of naturally occurring nucleoside analogs might be an important aspect of the physiological role of this enzyme. Overall, the data presented serve as a guide for predicting the susceptibility of heterocycles to oxidation by this enzyme.

Correlation of substrate-stabilization patterns with proposed mechanisms for three nucleoside phosphorylases

Substrate-stabilization of uridine phosphorylase (uridine:orthophosphate ribosyltransferase, EC 2.4.2.3), thymidine phosphorylase (thymidine:orthophosphate deoxyribosyltransferase, EC 2.4.2.4) and purine-nucleoside phosphorylase (purine-nucleoside:orthophosphate ribosyltransferase, EC 2.4.2.1) from Escherichia coli was investigated by heat-inactivation experiments. Nucleoside substrates stabilized uridine phosphorylase and purine-nucleoside phosphorylase, but not thymidine phosphorylase. Aglycone substrates stabilized only uridine phosphorylase. Phosphate or pentose-1-phosphate ester substrates stabilized all three enzymes. The appropriate pentose-1-phosphate ester was a more effective stabilizer than was phosphate with all three enzymes. In previous reports dealing with the kinetic analysis of these phosphorylases, sequential mechanisms were proposed. Each enzyme appeared to have different sequence of substrate addition. The substrate-stabilization patterns reported here are consistent with the proposed mechanisms.