Nucleoside kinases adenosine kinase

Regardless of the physiological role of the purine nucleoside kinases, adenosine kinase is very important in cancer chemotherapy with purine nucleoside analogues. Attempts are being made to treat tumor cells with purine nucleosides in which the base and sugar moieties have been altered from the normal structures. Conversion to nucleotides is necessary for the expression of the pharmacological activity of many of these compounds, and this is accomplished through the broad substrate specificity of adenosine kinase. 

Adenosine metabolism (Fig. 12.2) is reviewed in Dunwiddie Masino (2001) and Ribeiro et al. (2002). The phosphorylation of intracellular adenosine to AMP is catalyzed by adenosine kinase. Intracellularly, adenosine can also be deami-nated to inosine by adenosine deaminase. Free intracellular adenosine is normally low. Excess adenosine, which cannot be regenerated to ATP, is extruded to the extracellular space by equilibrative nucleoside transporters (ENTs) in the cell membrane. During electrical stimulation or energy depletion, adenosine is... 

Sinciair, C. J., et al. Nucleoside transporter subtype expression effects on potency of adenosine kinase inhibitors. Br. J. Pharmacol. 2001, 134, 1037-1044. 

ATP diphosphohydrolase Diadenosine polyphosphatase 5 nucleotidase Nucleoside transporter Adenosine deaminase Adenosine kinase Xanthine oxidase Nucleoside phosphorylase... 

Adenosine is not a classical neurotransmitter because it is not stored in neuronal synaptic granules or released in quanta. It is generally thought of as a neuromodulator that gains access to the extracellular space in part from the breakdown of extracellular adenine nucleotides and in part by translocation from the cytoplasm of cells by nucleoside transport proteins, particularly in stressed or ischemic tissues (Fig. 17-2C). Extracellular adenosine is rapidly removed in part by reuptake into cells and conversion to AMP by adenosine kinase and in part by degradation to inosine by adenosine deaminases. Adenosine deaminase is mainly cytosolic but it also occurs as a cell surface ectoenzyme. 

The purine and pyrimidine bases can be converted to then-respective nncleotides by reaction with 5-phosphoribosyl 1-pyrophosphate. Since these bases are not very soluble, they are not transported in the blood, so that the reactions are only of qnantitative significance in the intestine, where the bases are produced by degradation of nucleotides. In contrast, in some cells, nucleosides are converted back to nucleotides by the activity of kinase enzymes. In particular, adenosine is converted to AMP, by the action of adenosine kinase, and uridine is converted to UMP by a uridine kinase... 

Although purine nucleosides are intermediates in the catabolism of nucleotides and nucleic acids in higher animals and humans, these nucleosides do not accumulate and are normally present in blood and tissues only in trace amounts. Nevertheless, cells of many vertebrate tissues contain kinases capable of converting purine nucleosides to nucleotides. Typical of these is adenosine kinase, which catalyzes the reaction... 

Fig. 9.1 Schematic diagram illustrating A3 adenosine receptor localization in the brain. ADO adenosine ADA adenosine deaminase ATP adenosine triphosphate, AMP adenosine mono-phospate AKA adenosine kinase T bidirezional nucleoside transporter NPTDase family of ecto-nucleotidases, including NPTDase 1,2,3. During cerebral ischemia, extracellular ADO concentration increases acting on A3 adenosine receptors located on different cell type...

Thus, uridine-cytidine kinase converts uridine and cytidine to UMP and CMP, respectively thymidine kinase converts thymidine to dTMP and adenosine kinase converts adenosine to AMP. Specific kinases convert monophospho-nucleotides to dinucleotides using ATP as a phosphate donor. The conversion of diphosphonucleotides to triphosphonucleotides is carried out by a nonspecific nucleoside diphosphate kinase. This includes both the ribo- and deoxy-ribonucleotides. Cytosine and its nucleoside and nucleotide transformations are often associated with the metabolism of uracil and its nucleosides and nucleotides. Note that UTP can give rise to CTP (Figure 10.9), and also that, in the presence of cytidine deaminase, cytidine can be converted to uridine. 

Once inside the cell, the nucleoside is converted to the corresponding NMP, adenosine by adenosine kinase and uridine by uridine kinase ... 

Srikanth K, Debnath B, Jha T. QSAR study on adenosine kinase inhibition of pyrrolo[2,3-d]pyrimidine nucleoside analogues using the Hansch approach. Bioorg Med Chem Lett 2002 12 899-902. 

Nucleoside kinases specific for inosine or guanosine have been described. Adenosine kinase is widely distributed in mammalian tissues. 

Adenosine kinase is one of a family of nucleoside kinases that are widely found in animal tissues, microorganisms, and plants. This enzyme catalyzes reaction (22), the phosphorylation of the 5 -hydroxyl group of adenosine by MgATP. 

As is known for other nucleoside kinases, the steady-state kinetics of the adenosine kinase reaction is complex owing to regulatory effects. Adenosine and AMP apparently bind at a regulatory site, where they modulate activity, as well as at the active site, where they act as substrates. These interactions complicate the kinetics, but a careful analysis shows that the basic kinetic pathway is sequential and involves the compulsory formation of ternary complexes (79). Thus, the kinetics is consistent with the stereochemistry and suggests that the phospho transfer is a direct, one-step displacement between substrates bound at the active site in a ternary complex. Complications introduced into the mechanistic analysis of this enzyme by the adventitious phosphorylation of the protein by ATP have been discussed elsewhere (7). 

Reactions (35a) and (35b) are catalyzed by galactose-1-P uridylyltransferase and UDPglucose pyrophosphorylase, respectively reactions (36a) and (36b) are catalyzed by nucleoside diphosphate kinase and adenylate kinase, respectively and reactions (37a) and (37b) are catalyzed by nucleoside phosphotransferase and adenosine kinase, respectively. 

Of all the nucleoside kinases, the development of adenosine kinase inhibitors has received the most attention. This can be ascribed to the unique role that adenosine plays as a secondary messenger, modulating neuronal activity... 

Figure 6.5 Overlay of the co-crystal structures of Abbott s alkynylpyrimidine 15 (brown) and the prototypical nucleoside inhibitor 5-IT (20, cyan) bound to adenosine kinase. The binding mode of 5-IT shows H-bonds from the ribose 2 -OH and 3 -OH to Asp-18. The exocyclic amine is engaged in a water-mediated H-bond to the main chain of Phe-170 and the side chain of Ser-173. One of the pyrimidine nitrogens interacts with the nitrogen of the side chain of Asn-14, while the other bonds to the main-chain nitrogen of Ser-65. The Abbott inhibitor (15) is engaged in the same interactions with Ser-65 and Asn-14. The morpholinopyridine intersects the space between the Phe-201 and Leu-40 side chains, causing a significant conformational change of the enzyme (compare brown and cyan ribbons).

Figure 6.7 Nucleoside derivatives developed as adenosine kinase inhibitors by Gensia Sicor / Metabasis.

Nucleoside kinase inhibitors hold promise for the development of anticancer drugs by inhibition of the salvage pathway. Although previous attempts to develop these types of inhibitors into drugs have failed, the majority of these efforts focused on nucleoside analogs with poor physicochemical properties. Several non-nucleoside inhibitors have been reported for adenosine kinase and deoxycytidine kinase, with promising initial properties. It is possible that future efforts may result in the identification of non-nucleoside inhibitors that possess an appropriate efficacy-side effect profile. 

In the previous sections, we have discussed examples of inhibitors of nonprotein kinases for the development of novel therapeutics. Due to their important role in sugar metabolism, sugar kinase inhibitors have potential utility for the treatment of cancer, metabolic disorders diabetes and cardiovascular disease. Nucleoside kinase inhibitors are established oncology targets that operate by inhibition of the salvage pathway. Adenosine kinase inhibitors and lipid kinase inhibitors are highly significant in the development of cancer therapeutics. 

Leishmania adenylosuccinate synthetase has a narrow substrate specificity but accepts several IMP analogs which include allopurinol ribonucleotide (34). The GMP reductase from L. donovani is quite different from the human GMP reductase (35) and IMP analogs are more potent inhibitors for it. Other leishmanial enzymes that have been investigated include IMP dehydrogenase (36), nucleoside hydrolase and phos-phorylase activities (37,38), adenosine kinase (39), nucleotidases (40) and the adenylosuccinate lyase (34). 

Individual enzymes of purine salvage are similar to those of Leishmania. PRTase activities were found for adenine, hypoxanthine, and guanine in the three forms (43). As in Leishmania, there is also a separate xanthine PRTase. Nucleoside kinase activities were found for adenosine, inosine, and guanosine (43), nucleoside hydrolase activities for inosine and guanosine and a nucleoside phosphorylase activity for adenosine. There are both nucleoside hydrolase and phosphorylase activities in epimastigotes (44,45). The adenylosuccinate synthetase and adenylosuccinate lyase are essentially identical to those found in L. donovani (46). 

Few detailed studies have been done on the purine salvage enzymes of procyclic African trypanosomes. Tb. gambiense has high levels of guanine deaminase and lacks adenine and adenosine deaminase activities (8). Tb. brucei, T.b. gambiense and T.b. rhodesiense convert allopurinol into aminopyrazolopyrimidine nucleotides and incorporates these into RNA (49). This indicates that HPRTase, succino-AMP synthetase, and succino-AMP lyase are present. At least three nucleoside cleavage activities are present (Berens, unpublished results) two are hydrolases, of which one is specific for purine ribonucleosides and the other is specific for purine deoxyribonucleosides. The third nucleoside cleavage activity is a methylthioadenosine/adenosine phosphorylase. The adenosine kinase is similar to that of L. donovani (Berens, unpublished results). 

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