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PubMed1994

The roles of uridine-cytidine kinase and CTP synthetase in the synthesis of CTP in malignant human T-lymphocytic cells.

The pattern of incorporation of [14C]uridine showed that in MOLT-3 cells an increased proportion of CTP was synthesized via CTP synthetase, compared to proliferating normal human T lymphocytes at a physiological concentration of cytidine (< 0.5 microM). Furthermore, in the proliferating normal human T lymphocytes similar patterns of incorporation of [14C]uridine were observed in the presence of the physiological concentration of cytidine and after addition of 2 microM of cytidine. In contrast, in the MOLT-3 cells after addition of 2 microM of cytidine the proportion of CTP synthesized by conversion of UTP into CTP was substantially decreased, whereas the salvage of cytidine was proportionally increased. We conclude that the reutilization of uridine is a preferred route in the synthesis of CTP for MOLT-3 cells at physiological concentrations of uridine and cytidine, whereas in proliferating normal human T lymphocytes CTP is largely synthesized through reutilization of cytidine. This difference in salvage of pyrimidine ribonucleosides may be exploited for selective chemotherapy.

Carbon Radioisotopes↗

PubMed2005

Methylation protects cytidines from AID-mediated deamination.

Somatic hypermutation (SHM), class switch recombination (CSR), and gene conversion of immunoglobulin genes require activation-induced cytidine deaminase (AID). AID initiates these events by deaminating cytidines within antibody variable and switch regions. The mechanism that restricts mutation to antibody genes is not known. Although genes other than antibody genes have been found to mutate, not all highly transcribed genes mutate. Thus, somatic hypermutation does not target all genes and suggests a mechanism that either recruits AID to genes for mutation, and/or one that protects genes from promiscuous AID activity. Recent evidence suggests that AID deaminates methyl cytidines inefficiently. Methylation of cytidines could thus represent a means to protect the genome from potentially harmful AID activity that occurs outside of the immunoglobulin loci. To test this premise, we examined whether AID could deaminate methylated-CpG motifs in different sequence contexts. In agreement with a report that suggests that AID has processive-like properties in vitro, we found that AID could completely deaminate single-stranded DNA tracks in plasmid substrates that were greater than 300 nucleotides in length. In addition, methylated-CpG motifs, but not their unmethylated counterparts, were protected from AID-mediated deamination. However, methylation did not protect cytidines that neighbored CpG motifs indicating that methylation per se does not provide a more global safeguard against AID-mediated activity. These data also suggest that AID, and possibly other related cytidine deaminases, might represent a more rapid alternative to bisulfite sequencing for identifying methylated-CpG motifs.

Animals↗

PubMed1995

Reversal by cytidine of cyclopentenyl cytosine-induced toxicity in mice without compromise of antitumor activity.

Among nine compounds surveyed, cytidine was found to be the most effective in reversing the antiproliferative effects of cyclopentenyl cytosine (CPEC) on human T-lymphoblasts (MOLT-4) in culture. Cytidine, at concentrations of 1-25 microM, enabled cells to maintain normal logarithmic growth when added up to 12 hr after exposure to a 200 nM concentration of the oncolytic nucleoside, CPEC. The most abundant CPEC metabolite, CPEC-5'-triphosphate, is a potent [K1 approximately 6 microM] inhibitor of CTP synthetase (EC 6.3.4.2). Accumulation of this inhibitor resulted in a depletion of CTP levels to 17% of their original cellular concentration. Exogenous cytidine reversed CPEC-induced cellular cytotoxicity by suppressing the formation of CPEC-5'-triphosphate by 70%, and by partially replenishing intracellular CTP to at least 60-70% of its original concentration. In vivo, cytidine (500 mg/kg) administered intraperitoneally 4 hr after each daily dose of CPEC (LD10-LD100) for 9 days reduced the toxicity and abolished the lethality of CPEC to non-tumored mice. Of greater practical importance is the finding that, under these experimental conditions, cytidine did not curtail the antineoplastic properties of CPEC in L1210 tumor-bearing mice. Moreover, the concentration range over which CPEC exhibited antineoplastic activity was extended with cytidine administration.

Animals↗

PubMed1998

A comparison of the enantioselectivities of human deoxycytidine kinase and human cytidine deaminase.

The stereoselectivities of recombinant human deoxycytidine kinase (EC 2.7.1.74) (dCK) and of recombinant human cytidine deaminase (EC 3.5.4.5) (CDA) were investigated with respect to a series of cytidine analogs, most of them having the unnatural L-stereochemistry. The enantioselectivity of dCK was always low and generally favored the L-enantiomers in the case of beta-2',3'-dideoxycytidine (beta-ddC), 5-fluoro-beta-2',3'-dideoxycytidine (beta-FddC) and beta-cytidine (beta-riboC). Concerning beta-2'-deoxycytidine, dCK showed a preference for the D-enantiomer. All other examined beta-L-cytidine analogs, [1-beta-L-lyxofuranosyl cytosine (beta-L-lyxoC), l-beta-L-xylofuranosyl cytosine (beta-L-xyloC), and 5-fluoro-1-beta-L-xylofuranosyl cytosine (beta-L-Fxylo C)], were substrates of dCK regardless of the nature of the pentose. None of the studied alpha-L-anomers (alpha-L-riboC, alpha-L-araC, alpha-L-lyxoC, or alpha-L-xyloC) was a substrate of dCK. Contrasting with the relaxed enantioselectivity of dCK, CDA had a strict requirement for D-cytidine analogs since none of the already listed beta-L- or alpha-L analogs was a substrate or an inhibitor of the enzyme. The conjunction of the preceding stereochemical properties of dCK and CDA confers to L-cytidine analogs important potentialities in antiviral and anticancer therapies.

Antiviral Agents↗

PubMed2006

The 1.48 A resolution crystal structure of the homotetrameric cytidine deaminase from mouse.

Cytidine deaminase (CDA) is a zinc-dependent enzyme that catalyzes the deamination of cytidine or deoxycytidine to form uridine or deoxyuridine. Here we present the crystal structure of mouse CDA (MmCDA), complexed with either tetrahydrouridine (THU), 3-deazauridine (DAU), or cytidine. In the MmCDA-DAU complex, it clearly demonstrates that cytidine is distinguished from uridine by its 4-NH(2) group that acts as a hydrogen bond donor. In the MmCDA-cytidine complex, cytidine, unexpectedly, binds as the substrate instead of the deaminated product in three of the four subunits, and in the remaining subunit it binds as the product uridine. Furthermore, the charge-neutralizing Arg68 of MmCDA has also exhibited two alternate conformations, I and II. In conformation I, the only conformation observed in the other structurally known homotetrameric CDAs, Arg68 hydrogen bonds Cys65 and Cys102 to modulate part of their negative charges. However, in conformation II the side chain of Arg68 rotates about 130 degrees around the Cgamma-Cdelta bond and abolishes these hydrogen bonds. The lack of hydrogen bonding may indirectly weaken the zinc-product interaction by increased electron donation from cysteine to the zinc ion, suggesting a novel product-expelling mechanism. On the basis of known structures, structural analysis further reveals two subclasses of homotetrameric CDAs that can be identified according to the position of the charge-neutralizing arginine residue. Implications for CDA-RNA interaction have also been considered.

3-Deazauridine↗

PubMed1983

Cytidine metabolism in photoreceptor cells of the rat.

During brief (30-min) incubations, isolated rat retinas accumulated [3H]cytidine, converted it to cytidine triphosphate (CTP), and incorporated it into RNA and cytidine diphosphate-diacylglyceride (CDG), a phospholipid precursor of phosphatidylinositol (Pl). Labeled CTP, RNA, and CDG contents were found to be two- to three-fold higher in photoreceptor cells than in cells of the inner retina. Autoradiograms showed that, within photoreceptor cells, silver grains representing RNA were concentrated over the nuclei in dark and light, while silver grains representing CDG were concentrated over the inner segments only after incubation in dark. The formation of labeled CTP and the synthesis of RNA were enhanced in light, while labeled CDG levels became reduced in light concurrent with an increase in the incorporation of labeled inositol into Pl. The 3H-labeled CDG content, however, was increased two- to fourfold in light in the presence of actinomycin D, and autoradiograms show a heavy concentration of silver grains over the inner segments of photoreceptor cells. These findings establish a role for cytidine nucleotides in photoreceptor cell metabolism and in light-dependent increases in RNA and Pl synthesis. Furthermore, the observations indicate that a competition may exist in light for cytidine or CTP and suggest that availability of cytidine for CDG synthesis may have a regulatory role in Pl metabolism within the photoreceptor cells.

Animals↗

PubMedDate not supplied

In-situ cytidine-deaminase activity and chromosome 1P deletion in human neuroblastoma cells.

Cytidine deaminase can cause the deamination of cytotoxic analogues of cytidine or rescue cells from the cytotoxicity of uracil analogues. Therefore, cytidine deaminase influences the cytotoxicity exerted by these compounds. We investigated the activity of this enzyme in situ in neuroblastoma cell-line cells. The in-situ cytidine deaminase activity in human neuroblastoma cell-line SK-N-SH cells appeared to be two-fold higher than in the human neuroblastoma N-myc amplified cell-line SK-N-BE(2)-C cells. The observed activity correlated with the presence of both alleles in SK-N-SH cells versus one allele of cytidine deaminase in SK-N-BE(2)-C cells. This observation may be exploited for the treatment of neuroblastoma patients having a tumour with a chromosome Ip deletion including the domain containing the gene encoding cytidine deaminase.

Chromosome Deletion↗

PubMed1984

The influence of cytidine on the endogenous pool of CDP-choline, CDP-ethanolamine, and CMP of the rat brain.

Cold cytidine was intraventricularly administered into the brain of young rats, and its effect on CDP-choline, CDP-ethanolamine, and CMP pools followed for different time intervals and with various amounts of administered cytidine. The injected nucleoside produces a measureable increase of th concentrations of all three nucleotides. The increase produced by injecting 2.5 mumol of cytidine for brain does not essentially change with higher doses of injected nucleoside, except for CMP, whose increase reaches a maximum with 5 mumol of cytidine. A clear time dependence on cytidine administration was shown. The increases of the three nucleotide concentrations do not show a maximum till 60 min from administration into CMP and CDP-bases and measurably increases their endogenous brain pools. The compound is likely to enter metabolic events connected with phospholipid metabolism in brain.

Animals↗

PubMed1990

Modulation of the effect of 1-beta-D-arabinofuranosylcytosine based on changes of cytidine deaminase activity in HL60 cells.

The effect of changes in cytidine deaminase activity on the growth inhibitory effect of 1-beta-D-arabinofuranosylcytosine (Ara-C) has been investigated in the human promyelocytic cell line, HL60. 1,25 Dihydroxy vitamin D3 (vit. D3), which is an inducer of differentiation, caused an increase of cytidine deaminase in HL60 cells simultaneous with an inhibition of the effect of Ara-C. On the other hand, retinoic acid, which also induces differentiation had no influence on either the activity of cytidine deaminase or the effect of Ara-C. An atoxic inhibitor of cytidine deaminase tetrahydrouridine, THU, enhanced the effect of Ara-C. THU also enhanced the effect of Ara-C, even if the Ara-C effect was inhibited by deoxycytidine. These results show that it is possible to modulate the effect of Ara-C by changing the activity of cytidine deaminase. Furthermore, changes in the activity of cytidine deaminase and the effect on the Ara-C growth inhibition vary dependent on the differentiating inducer used.

Cell Division↗

PubMed1976

Nuclear magnetic relaxation studies of cytidine and its complexing dimethyl sulfoxide solution.

13C spin-lattice relazation times (T1 values) in cytidine were determined experimentally to investigate molecular motions of both metal-free and ion-complexed cytidines in dimethylsulfoxide solutions. It was found that the correlation times of the protonated carbons were equal within experimental error, and this equality of correlation times of different sites of the molecule suggests strongly isotropic random motion of the molecule. Correlation times for internal motion of the amino group obtained from the observed T1 of the amino protons are 4.6 -10(-11) S, 2.0 - 10(-9) S and 1.1 - 10(-10) S for the metal-free cytidine and the cytidine complexed with either CaCl2 or ZnCl2, respectively. An experimental value of T1 of the H6 proton of the cytidine base agrees very well with the value estimated from a conformation determined by the nuclear "Overhauser" effect. Spin-lattice relaxation time measurements of the 7Li nucleus in the LiCl/cytidine system strongly suggested that the 7Li cation is directly coordinated with cytidine.

Binding Sites↗

PubMed1995

Pyrimidine nucleoside catabolism in Sphingomonas paucimobilis: role of cytidine deaminase and uridine phosphorylase.

Pyrimidine nucleoside catabolism in the human pathogen Sphingomonas paucimobilis was studied. It was observed that S. paucimobilis was only capable of utilizing cytidine or deoxycytidine as a sole nitrogen source when glucose served as the carbon source. Thinlayer chromatographic analyses of cytidine and uridine catabolic products revealed that the enzymes cytidine deaminase and uridine phosphorylase were active in the extracts prepared from S. paucimobilis cells. The levels of cytidine deaminase and cytosine deaminase activities were lowered after growth on cytidine or deoxycytidine as a nitrogen source instead of ammonium sulfate. Uridine phosphorylase activity increased more than 4-fold after growth on deoxycytidine as a nitrogen source while growth on the nitrogen source cytidine caused a depression in phosphorylase activity.

Cytidine Deaminase↗

PubMed2001

The effect of cytidine on the structure and function of an RNA ligase ribozyme.

A cytidine-free ribozyme with RNA ligase activity was obtained by in vitro evolution, starting from a pool of random-sequence RNAs that contained only guanosine, adenosine, and uridine. This ribozyme contains 74 nt and catalyzes formation of a 3',5'-phosphodiester linkage with a catalytic rate of 0.016 min(-1). The RNA adopts a simple secondary structure based on a three-way junction motif, with ligation occurring at the end of a stem region located several nucleotides away from the junction. Cytidine was introduced to the cytidine-free ribozyme in a combinatorial fashion and additional rounds of in vitro evolution were carried out to allow the molecule to adapt to this added component. The resulting cytidine-containing ribozyme formed a 3',5' linkage with a catalytic rate of 0.32 min(-1). The improved rate of the cytidine-containing ribozyme was the result of 12 mutations, including seven added cytidines, that remodeled the internal bulge loops located adjacent to the three-way junction and stabilized the peripheral stem regions.

Base Sequence↗

PubMed1984

Attachment of reporter groups to specific, selected cytidine residues in RNA using a bisulfite-catalyzed transamination reaction.

Bisulfite catalyzes transamination of cytidine at the N4 position; the suitability of this reaction for attaching reporter groups to selected cytidine residues in RNA molecules has been investigated. Poly(C) is nearly quantitatively converted to the poly (N4 aminoethyl-C) derivative after 3 hrs at 42 degrees C with ethylene diamine (pK1 = 7.6) and bisulfite. This derivative reacts quantitatively with N-hydroxysuccinimide esters; the linkage of a fluorescent dye, nitrobenzofurazan, to cytidine by this reaction is demonstrated. To direct the bisulfite reaction to selected cytidines within a large RNA molecule, the RNA is hybridized to complementary DNA containing a deletion. Only the cytidines in the single strand RNA loop (corresponding to the DNA deletion) are reactive. Two cytidines in the middle of a 340 base RNA fragment from 16S ribosomal RNA have been modified by this technique.

Amines↗

PubMed1990

Cytidine deaminase activity in synovial fluid of patients with rheumatoid arthritis: relation to lactoferrin, acidosis, and cartilage proteoglycan release.

It is claimed that cytidine deaminase activity reflects local granulocyte turnover or activity in the synovial fluid of patients with rheumatoid arthritis, but cytidine deaminase is not a granulocyte specific enzyme. Lactoferrin is a granulocyte specific protein that is released from the secondary granulae during activation. We measured cytidine deaminase activity and lactoferrin concentrations in 33 rheumatic synovial fluid samples. Cytidine deaminase activity and lactoferrin concentrations correlated closely, indicating that both analyses reflect similar events in the joint-that is, result in their release from granulocytes. Cytidine deaminase activity and granulocyte concentrations correlated less closely, suggesting that there are additional factors besides the cell number which contribute to this release. Joint acidosis may be one such factor, as pH and cytidine deaminase activity correlated inversely. There was no association with synovial fluid proteoglycan concentrations, a marker of cartilage degradation.

Acidosis↗

PubMed1977

Factors affecting cytidine uptake and utilization by the rat uterus.

The influence of cytidine dosage on uptake and distribution of [3H]cytidine by the uterus of the adult ovariectomized control rat was examined 20 min after injecting the nucleoside. Intracellular accumulation of free cytidine was observed only at a dosage of 4.2 mumol/100g. At lower dosages, the nucleoside was phosphorylated as rapidly as it entered uterine cells, thereby preventing its accumulation within cells. [3H]Cytidine (4.2 mumol/100 g) was used in time-course studies in ovariectomized control and estradiol-treated rats. From this study the following differences between uteri of control and estradiol-treated animals were noted: 1) both nucleotides and RNA were five to six times more highly labeled in treated than control animals after a five minute labeling period, 2) this difference between control and treated animals was reduced to twofold when the labeling period was extended to 20 min, 3) free nucleoside had begun to accumulate intracellularly in treated animals, but had not equilibrated with uterine extracellular space in controls 5 min after [3H]cytidine injection, and 4) at 20 min, the free nucleoside of the uterus was labeled about equally in control and treated rats. It is concluded that estradiol causes a rapid equilibration of circulating cytidine with the uterus and that increases in labeling of RNA and precursor nucleotides 2 h after hormone treatment are largely a consequence of this rapid equilibration.

Animals↗

PubMed1998

Some kinetic studies on cytidine aminohydrolase activity from Aspergillus niger NRRL3.

Cell free extract of nitrate-grown Aspergillus niger NRRL3 catalyzed the hydrolytic deamination of cytidine out of the tested bases, their nucleosides and nucleotides to uridine maximally at pH 7 and at 50 degrees C. The deaminating activity seems to be specific for cytidine, as the extracts could not deaminate AMP, GMP, CMP, adenosine, guanosine, adenine, guanine, and cytosine. Maximum activity of cytidine deaminase was achieved in Tris-HCl buffer at concentration of 0.15 M. Incubation of the extracts at 70 degrees C for 30 minutes in absence of cytidine caused about 70% loss in its activity, while dialysis, freezing and thawing has no effect on the activity. Results indicated the absence of the involvement of SH group(s) in the catalytic site of cytidine deaminase. Uridine competitively inhibited the enzyme activity, while ammonia had no effect. The apparent K(m) value of this enzyme for cytidine was 2.6 x 10(-3) and the Ki value for uridine was 10.06 x 10(-3).

Aspergillus niger↗

PubMed1980

Inhibition of urethan induced lung tumour growth in strain A mice by 5-methyl cytidine.

In two in vivo experiments, low doses of 5-methyl cytidine significantly suppressed the growth of urethan-induced lung tumours in strain A mice while higher doses were less effective. This inverse dose-response relationship mimics the previously reported effect of 5-methyl cytidine on mammary tumour growth in C3H mice. The finding that 5-methyl cytidine inhibits lung tumour growth as well as mammary tumour growth suggests this nucleoside may be an effective inhibitor of solid tumour growth in general. A study of the effect of 5-methyl cytidine on mouse lung tumour cell viability in vitro suggests that the direct cytotoxic effect of 5-methyl cytidine is dependent on the cell cycle. Since cell-cycle dependent agents do not inhibit lung tumour growth in vivo, 5-methyl cytidine probably inhibits mouse lung tumour growth indirectly by some effect on the host rather than by a direct cytotoxic effect on the lung tumour.

Adenoma↗

PubMedDate not supplied

Resistance to cytosine arabinoside by retrovirally mediated gene transfer of human cytidine deaminase into murine fibroblast and hematopoietic cells.

Dose-limiting hematopoietic toxicity produced by the cytosine nucleoside analogue cytosine arabinoside (ARA-C) is one of the major factors that limit its use in the treatment of neoplastic diseases. An interesting approach to overcome this problem would be to insert a gene for drug resistance to ARA-C in normal hematopoietic cells to protect them from drug toxicity. The deamination of ARA-C by cytidine deaminase results in a loss of its antineoplastic activity. The objective of this study was to determine if gene transfer of human cytidine deaminase into murine fibroblast and hematopoietic cells would confer drug resistance to ARA-C. Retrovirally mediated transfer of the human cytidine deaminase gene into 3T3 fibroblasts resulted in efficient expression of the proviral RNA for this gene and in increased cytidine deaminase activity in cytoplasmic extracts. These cells showed marked resistance to ARA-C as determined by the effects of this drug on colony formation, cell growth, and DNA synthesis. The transfer of the human cytidine deaminase gene into murine bone marrow cells by the retroviral vector conferred a high level of drug resistance to ARA-C in clonogenic assays. These studies indicate that the cytidine deaminase gene could be used in cancer gene therapy by protecting normal hematopoietic cells against the cytotoxic effects of ARA-C and related cytosine nucleoside analogues.

Animals↗