Differences between lymphoid organs with respect to the phosphorylation of deoxycytidine and thymidine

Cloning and expression of human deoxycytidine kinase cDNA

Deoxycytidine (dCyd) kinase is required for the phosphorylation of several deoxyribonucleosides and certain nucleoside analogs widely employed as antiviral and chemotherapeutic agents. Detailed analysis of this enzyme has been limited, however, by its low abundance and instability. Using oligonucleotides based on primary amino acid sequence derived from purified dCyd kinase, we have screened T-lymphoblast cDNA libraries and identified a cDNA sequence that encodes a 30.5-kDa protein corresponding to the subunit molecular mass of the purified protein. Expression of the cDNA in Escherichia coli results in a 40-fold increase in dCyd kinase activity over control levels. In dCyd kinase-deficient murine L cells, transfection with dCyd kinase cDNA in a mammalian expression vector produces a 400-fold increase over control in dCyd phosphorylating activity. The expressed enzyme has an apparent Km of 1.0 microM for dCyd and is also capable of phosphorylating dAdo and dGuo. Northern blot analysis reveals a single 2.8-kilobase mRNA expressed in T lymphoblasts at 5- to 10-fold higher levels than in B lymphoblasts, and decreased dCyd kinase mRNA levels are present in T-lymphoblast cell lines resistant to arabinofuranosylcytosine and dideoxycytidine. These findings document that this cDNA encodes the T-lymphoblast dCyd kinase responsible for the phosphorylation of dAdo and dGuo as well as dCyd and arabinofuranosylcytosine.

Follicular cells of tonsils metabolise more deoxycytidine than other cell populations

Nine subpopulations of tonsillar lymphocytes and the unseparated cells were compared in their utilization of exogenous deoxycytidine ([5-3H]CdR) and thymidine ([3H]TdR). Uptake phosphorylation and incorporation of labeled precursors were determined in B and T lymphocytes, in low density (LD; enriched in S phase cells) and in high density (HD; enriched in G0/G1 phase cells) cell fractions as well as in LD and HD subfractions of B and T lymphocytes, and in cells isolated from follicles of tonsils. As expected, LD cells and B lymphocytes were more active in TdR incorporation than HD cells and T lymphocytes. However, the ratio of [5-3H]CdR to [3H]TdR in their total phosphorylation and incorporation into DNA was much lower than the expected value of 1: about 0.5 for total phosphorylation and about 0.3 for incorporation in all subpopulations, except for the follicular cells, where these ratios were 1.0 and 0.7, respectively. These results show that the relative utilization of the two pyrimidine deoxyribonucleoside precursors varies among different lymphocyte subpopulations. However, this variation is not due to the different rate of DNA synthesis; rather, it depends on the differentiation stage of lymphocytes occurring in the germinal center of the follicles.

Deoxycytidine kinase is constitutively expressed in human lymphocytes: consequences for compartmentation effects, unscheduled DNA synthesis, and viral replication in resting cells

Deoxycytidine kinase specific activity was high in human peripheral lymphocytes and increased less than 2-fold when the lymphocytes were stimulated by phytohemagglutinin A. Ion-exchange chromatography showed the same profile of deoxycytidine kinase activity in resting and proliferating cells. This enzyme could also efficiently phosphorylate deoxyadenosine and deoxyguanosine. In contrast, the thymidine kinase activity was very low in resting peripheral lymphocytes and increased more than 40-fold upon stimulation. Similar relative changes in the activities of the two enzymes were observed in human T-lymphoblast cells (CCRF-CEM) separated by centrifugal elutriation into cells of different cell cycle phases. The ratio of deoxycytidine to thymidine kinase activities is 20:1 in extracts from resting human lymphocytes and 1:2 in PHA-stimulated cells. This drastic change in deoxyribonucleoside phosphorylating activities during the cell cycle in human lymphocytes is of importance for studies on unscheduled DNA synthesis, for the design of therapies to interfere with viral DNA metabolism, and for a correct interpretation of the compartmentation effects observed in DNA precursor metabolism.

Deoxycytidine is salvaged not only into DNA but also into phospholipid precursors

Deoxycytidine metabolism was investigated in light density human tonsillar lymphocytes using 5-3H-deoxycytidine as extracellular precursor. A significant portion of the deoxycytidine (more than 50% of ethanol soluble pool) was found to incorporate into dCDP-choline and dCDP-ethanolamine beside the well-known pathway i.e. incorporation into DNA in form of dCMP and dTMP. Hydroxyurea increased the labeling of the deoxyliponucleotides from 5-3H-deoxycytidine in spite of its inhibition of DNA synthesis.

DNA synthesis and nucleoside metabolism in human tonsillar lymphocyte subpopulations

High (HD) and low (LD) density cells were separated on 25% BSA gradient from tonsils of 3-6 years old children. Early B lymphocyte markers and sIg-s were found on the surface of 59-82% of the LD cells. This cell population was 5-6 times more active in DNA synthesis (3H-thymidine incorporation, DNA polymerase activity) than the HD cells. The total uptake of 3H-deoxycytidine was about the same as that of 3H-thymidine. As long as practically all thymidine taken up by the cells was immediately incorporated into DNA (90-95%), only 10-15% of deoxycytidine was incorporated into DNA under the same conditions, indicating different pool sizes for the DNA precursors. The majority of deoxycytidine (70%) was converted and incorporated as dTMP. A considerable part of labeled deoxycytidine could be detected in the soluble pool in form of nucleotides (3-8%), and in an unknown form, called substance X (8-14%). Substance X was purified by TL chromatography and identified by HPLC as deoxycytidine containing liponucleotides, probable precursors for plasmamembranes. The preferential utilisation of deoxycytidine for DNA and membrane synthesis in immature B lymphocytes draws the attention to its function in early events of lymphocyte maturation.

Preferential utilisation of deoxycytidine by undifferentiated (peanut positive) tonsillar lymphocytes

Peanut agglutinin (PNA), a D-galactose specific lectin, agglutinated 10-18% of lymphocytes isolated from tonsils of 3- to 6-yr-old children. PNA+ cells were found to be mainly B lymphocytes showing a 9.6 times higher specific activity of DNA polymerase alpha compared to the PNA- cells. The specific activity of deoxycytidine kinase as well as the incorporation of [5-3H]deoxycytidine were also much higher in PNA+ cell fraction than in PNA- fraction (7.7-fold and 6-fold, respectively). On the other hand, thymidine kinase activity and [5-3H]deoxythymidine incorporation were only 3.6 and 3.9 times higher, respectively. The data presented here show a high degree of DNA synthesis and preferential utilisation of [5-3H]deoxycytidine for DNA synthesis in this undifferentiated B lymphocyte fraction.

Effects of tri-n-butyltin chloride on energy metabolism, macromolecular synthesis, precursor uptake and cyclic AMP production in isolated rat thymocytes

The inhibitor of oxidative phosphorylation tri-n-butyltin chloride (TBTC) causes membrane damage and disintegration of isolated rat thymocytes at concentrations higher than 1 microM. From a concentration of 0.1 microM, TBTC disturbs energy metabolism as indicated by an increase in methylglucose uptake, glucose consumption and lactate production and by a decrease in cellular ATP levels. Over the same TBTC concentration range, the incorporation of DNA, RNA and protein precursors are markedly reduced. Moreover the production of cyclic AMP upon stimulation of the cells with prostaglandin E1 is effectively inhibited. These effects cannot be explained by an inhibition of nucleoside kinase activity, amino acid uptake or adenylate cyclase activity. The effects of TBTC on macromolecular synthesis and cyclic AMP production are possibly due to a disturbance of the cellular energy state.

Reversible permeabilization of lymphocytes destroys the incorporation of deoxythymidine but not of deoxycytidine

The total uptake, phosphorylation and incorporation of thymidine (dThd) and deoxycytidine (dCyd) were compared in intact and reversibly permeabilized human tonsillar lymphocytes. The total uptake of [3H]dThd was lower than that of [5-3H]dCyd, but almost all of [3H]dThd was incorporated into DNA. However, the main part of [5-3H]dCyd taken up by the lymphocytes was found in the pool as phosphorylated nucleoside (55%), and only a smaller part (13%) was incorporated into DNA. Phosphorylated nucleosides were determined by DEAE-cellulose sheets in the ethanol-soluble fraction of the cells. The reversible permeabilization of lymphocytes by Dextran T-150 destroys totally the [3H]dThd incorporation, while [5-3H]dCyd incorporation decreased only to 60% of intact cells. During permeabilization the phosphorylation of both nucleosides increased severalfold. After permeabilization all [3H]dThd was in dTMP form, while [5-3H]dCyd was also found in dCDP (3%) and dCTP (38%) form. In the meanwhile, 22% of thymidine kinase, 63% of deoxycytidine kinase and 98% of DNA polymerase activity were measured in permeabilized cells as compared to intact cells. The results suggest different relationships between the lymphocyte plasma membrane and the salvage pathways of the two pyrimidine nucleosides.