Uridine kinase specificity

In summary, it is proposed that the most hopeful prospect for therapy of this condition would be to search for a specific inhibitor of orotate transport across cell membranes. This is likely to affect the metabolism of pyrimidines in only two tissues, liver and erythrocytes, and, unless it gives rise to toxic accumulations of orotate in the liver, could help to reduce pyrimidine nucleotide levels in pyrimidine 5 nucleotidase deficient erythrocytes. A trial of a specific inhibitor of uridine kinase might also be appropriate, and the final choice of a therapeutic approach will depend on the demonstration of which of these two alternative pathways amenable to therapeutic intervention contributes most to erythrocyte pyrimidine nucleotide accumulation. 

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. 

The answer is e. (Murray, pp 375-401. Scriver, pp 2663-2704. Sack, pp 121-138. Wilson, pp 287—320.) Orotic aciduria is the buildup of orotic acid due to a deficiency in one or both of the enzymes that convert it to UMP Either orotate phosphoribosyltransferase and orotidylate decarboxylase are both defective, or the decarboxylase alone is defective. UMP is the precursor of UTP, CTP, and TMP All of these end products normally act in some way to feedback-inhibit the initial reactions of pyrimidine synthesis. Specifically, the lack of CTP inhibition allows aspartate transcarbamoylase to remain highly active and ultimately results in a buildup of orotic acid and the resultant orotic aciduria. The lack of CTP, TMP, and UTP leads to a decreased erythrocyte formation and megaloblastic anemia. Uridine treatment is effective because uridine can easily be converted to UMP by omnipresent tissue kinases, thus allowing UTP, CTP, and TMP to be synthesized and feedback-inhibit further orotic acid production. 

FIG. 6.13 Mammalian pyrimidine salvage and interconversion pathways. Enzymes listed in Figs 6.13-6.17 are as follows (1) deoxyCMP deaminase (2) thymidylate synthase (3) ribonucleotide reductase (4) deoxyuridine triphosphatase (5) CTP synthetase (6) nucleotide kinase (7) deoxyTMP kinase (8) nucleotide diphosphokinase (9) non-specific phosphatase or nucleotidase (10) cytidine kinase (11) pyrimidine phos-phorylase or hydrolase (12) uracil PRTase (13) cytidine deaminase (14) thymidine kinase (15) cytidine phosphotransferase (16) uridine phosphotransferase (17) thymidine phosphotransferase (18) deoxyribo-nucleotide phosphotransferase (19) cytosine PRTase. 

S ATP + deoxyadenosine <1-10> (<1> transfers a phospho group from specific nucleoside 5 -triphosphate donors to 5 -position of deoxyadenosine [1] <1> no activity with adenosine and guanosine [1] <2> no activity with cytidine, uridine, guanosine, deoxyguanosine and thymidine [3] <4> enzyme bears two separate but interacting active sites for deoxyadenosine and deoxycytidine kinase activity [5] <4> enzyme exists in two heterodimeric complexes, complex 1 deoxycytidine/deoxyadenosine kinase and complex II deoxyguanosine/deoxyadenosine kinase [6] <3> enzyme has both adenosine kinase and deoxyadenosine kinase activity [14]) (Reversibility <1-10> [1-9,11,12,15]) [1-9, 11, 12, 14, 15]... 

Other halogenated uridine derivatives have been reported to exhibit antiviral activity. Fluorodeoxyuridine has in vitro antiviral activity but is not used in clinical practice. Bromodeoxyuridine is used in subacute sclerosing panencephalitis, a deadly, virus-induced CNS disease. This agent appears to interfere with DNA synthesis in the same way as idoxuridine. The 5 -amino analogue of idoxuridine (5-iodo-5 -amino-2, 5 -dideoxyuridine) is a better antiviral agent than idoxuridine, and it is less toxic. It is metabolized in herpesvirus-infected cells only by thymidine kinase to di- and triphosphoramidates. These metabolites inhibit HSV-specific late RNA transcription, causing reduction of less infective abnormal viral proteins. 5-Bromo-2 -deoxyuridine has an action similar to that of other iodinated compounds. None of these compounds are commercially available in the United States. 

Two routes are known by which the free base, uracU, can enter the ribonucleotide pool. One proceeds by the sequential actions of uridine phosphorylase and uridine-cytidine kinase (reactions 5 and 6, Fig. 12-1) this route is discussed below. The other route is by way of a single-step phosphoribosyltransferase reaction specific for uracil (reaction 4, Fig. 12-1) ... 

The kinase which converts deoxycytidine to its 5 -monophosphate has been studied most extensively in preparations from calf thymus 35, 36). The preferred substrate is deoxycytidine, for which the Michaelis constant (5 X 10 M) is much lower than that of two other substrates, deoxyadenosine and deoxyguanosine. Cytidine, uridine, and thymidine are not phosphorylated by this enzyme. Deoxycytidine kinase is subject to a complex pattern of allosteric regulation by nucleotides. The end product of deoxycytidine phosphorylation, dCTP, is a potent inhibitor this inhibition is reversed by dTTP. The enzyme has a rather broad specificity for the phosphate donor, with the triphosphates of the natural ribo- and deoxyribonucleosides being substrates the inactivity of dCTP is a notable exception. 

Our results could not conclusively explain the mechanisms for allopurinol- and oxipurinol-mediated inhibition of purine and pyrimidine biosynthesis. Most of our results are in accordance with the view that allopurinol-1-ribonucleotide and oxipurinol-7-ribonucleo-tide are to be accounted for the inhibitory effects on both purine and pyrimidine biosynthesis. Relatively high concentrations of allopurinol-1-ribonucleoside and oxipurinol-7-ribonucleoside have Inhibitory effects which are difficult to explain with the view that there exist no inosinic or uridine specific kinases in fibroblasts, catalyzing to the respective ribonucleotides. 

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