Comparison of the substrate specificities of human thymidine kinase 1 and 2 and deoxycytidine kinase toward antiviral and cytostatic nucleoside analogs

Nucleoside transport and metabolism in lymphocytes, polymorphonuclear cells and cerebral synaptosomes

The salvage of nucleosides dominates over de novo biosynthesis in lymphocytes, polymorphonuclear cells (PMN) and in central neutral nervous system (CNS) in higher organisms. Earlier works in our laboratory have shown that the salvage of deoxycytidine (dCyd) did not correlate with DNA synthesis. The uptake and metabolism of dCyd was higher in undifferentiated germinal center lymphocytes and in follicles comparing to more differentiated cells. Recently we have compared the transport of thymidine (dThd), dCyd, uridine (Urd) and adenosine (Ado) in the three cell systems in which the salvage of nucleosides is dominating. It was found that dCyd was transported 30 times more effectively into lymphocytes than into PMN and synaptosomes, while Urd was transported about the same rate into the two cells and into synaptosomes. All transport processes could be inhibited by dipyridamole, NBRPR, papaverine and dilazep. The dCyd and dThd was phosphorylated even at 0 degrees C up to TTP and dCTP without incorporation into DNA and into liponucleotides. Our results show that the processes of transport-phosphorylation, as well as the processes of DNA-CDP-phospholipid synthesis are tightly coupled to each other in intact cells and organelles.

Effects of antiviral nucleoside analogs on human DNA polymerases and mitochondrial DNA synthesis

Inhibition constants were determined for 16 nucleoside analog triphosphates against human DNA polymerases alpha, beta, gamma, and epsilon, and 7 nucleoside analogs were examined as inhibitors of mitochondrial DNA synthesis in human Molt-4 cells in culture. The results demonstrate no clear quantitative or qualitative correlation between inhibition of DNA polymerases, particularly mitochondrial DNA polymerase gamma, and the inhibition of mitochondrial DNA synthesis in Molt-4 cell culture. Furthermore, the data indicate that inhibition of isolated DNA polymerases may not be predictive of in vitro or in vivo toxicity. Finally, it is not clear whether inhibition of mitochondrial DNA synthesis will be an accurate predictor of the potential in vivo toxicity of antiviral nucleoside analogs.

New targets for pyrimidine antimetabolites for the treatment of solid tumours. 2: Deoxycytidine kinase

Deoxycytidine kinase is an enzyme required for the activation of, for example, cytarabine, the most widely used agent for the chemotherapy of haematological malignancies. However, deoxycytidine kinase also plays an important role in the activation of several new agents used in the treatment of leukaemia, such as cladribine. Recently, a new cytidine analogue, gemcitabine, has shown impressive activity as a single agent against several solid malignancies (ovarian cancer, non-small cell lung cancer), demonstrating that in solid tumours deoxycytidine kinase can be an important target for the activation of antimetabolites. Studies on the regulation of deoxycytidine kinase have shown that the enzyme has a complicated regulation (feedback inhibition by the product and regulation by ribonucleotides). Modulation of deoxycytidine kinase activity has already been shown to be an effective way to improve the effect of cytarabine and will probably be a target for new therapies.

Properties and levels of deoxynucleoside kinases in normal and tumor cells; implications for chemotherapy

Deoxynucleoside kinases are key enzymes in deoxyribonucleoside salvage, activating several clinically important chemotherapeutic drugs. The four known kinases, cytosolic thymidine kinase (TK1) and deoxycytidine kinase (dCK) and the mitochondrial thymidine kinase (TK2) and deoxyguanosine kinase (dGK), have been purified and characterized as to the subunit structure as well as specificity with a large number of analogs. These results are summarized and used to establish selective assays for the four enzymes in crude extracts of normal and malignant human peripheral blood mononuclear cells, gastrointestinal tissues and sarcomas. TK2 and dGK activities were found at low levels in all tissues, possibly correlated to the content of mitochondria. TK1 activity was detected only in samples containing a significant number of S phase cells. We have measured dCK activity as well as dCK polypeptide level by immuno blotting in these extracts. High levels of dCK were found in normal mononuclear leukocytes (91-145 ng dCK/mg protein) and in B-cell chronic lymphocytic leukemia (80 +/- 30 ng/mg, n = 23). Hairy cell leukemia contained lower levels (28 +/- 23 ng/mg, n = 7), as did unexpectedly three samples of T-cell chronic lymphocytic leukemia (18 +/- 14 ng/mg). Phytohemaglutinine stimulation of normal lymphocytes did not lead to any substantial increase in either dCK activity or expression (less than 2.5-fold). In colon adenocarcinomas, the dCK content was significantly higher (21 +/- 9.3 ng/mg, n = 20) than in normal colon mucosa (8.2 +/- 3.7 ng/mg, n = 19, p < 0.05). A similar pattern of dCK expression was found in gastric adenocarcinomas (21 +/- 13 ng/mg, n = 5) and normal ventricular mucosa (6.2 +/- 5.4 ng/mg, n = 5, p < 0.15). One leiomyosarcoma and one extra-skeletal osteosarcoma showed a dCK levels comparable to those found in normal lymphocytes (84 +/- 6 and 109 +/- 4 ng/mg), while other sarcoma samples contained levels comparable to the gastrointestinal adenocarcinomas (20 +/- 7 ng/mg, n = 12). We confirm that dCK is expressed constitutively and predominantly in lymphoid cells, but conclude that a significant expression may be found in non-lymphoid tissues as well, with increased levels in the corresponding tumor tissue. 2-Chlorodeoxyadenosine (CdA), an antileukemic agent used in treatment of hairy cell leukemia and chronic lymphocytic leukemias (B-CLL), is phosphorylated by dCK which was used as the selective substrate for this enzyme. A study was performed to investigate if there was a correlation between the dCK levels and the response to CdA treatment.(ABSTRACT TRUNCATED AT 400 WORDS)

Varicella-zoster virus thymidine kinase. Characterization and substrate specificity

The varicella-zoster virus (VZV) thymidine kinase (TK) EC 2.7.2.21) catalyzes the phosphorylation of many anti-VZV nucleosides. Purified, bacterially expressed VZV TK was characterized with regard to N-terminal amino acid sequence, pI value, pH optimum, metal ion requirement, phosphate donor and acceptor specificity, and inhibition by dTTP. Initial velocities of thymidine phosphorylation with variable MgATP concentrations fit a two-site model with apparent Km values for MgATP of 0.10 and 900 microM. dTTP was a noncompetitive inhibitor of thymidine phosphorylation but was competitive with MgATP. Phosphate donor and acceptor specificities of the bacterially expressed enzyme were indistinguishable from those of VZV TK purified from infected cells. Detailed studies of the nucleoside specificity with the bacterially expressed enzyme showed that, for a given sugar moiety, thymine nucleosides were the most efficient substrates followed by nucleosides of cytosine, uracil, adenine, and with some exceptions, guanine. For a given pyrimidine or purine (except guanine), 2'-deoxyribonucleosides were the most efficient substrates, followed by arabinosides, ribonucleosides, 2',3'-dideoxyribonucleosides, and the acyclic moiety of acyclovir.

Cell cycle regulation of deoxycytidine kinase. Evidence for post-transcriptional control

Deoxycytidine kinase enzyme activity and deoxycytidine kinase mRNA content were determined at different positions of the cell cycle in permanent cell lines. There was no variation of deoxycytidine kinase activity during the cell cycle in two cell lines, whereas in two other lines the enzyme activity was induced (10- and 15-fold) at the G1/S boundary. In contrast, the level of deoxycytidine kinase mRNA never displayed any cell cycle-dependent changes. The decay of enzymatic activity was measured after addition of cycloheximide in different phases of the cell cycle; the enzyme was much more stable in cells with constant activity. We suggest that post-transcriptional mechanisms account for the periodic behaviour of the enzyme activity, and that whether this regulation can be detected depends on the half-life of deoxycytidine kinase.

Binding of substrates to human deoxycytidine kinase studied with ligand-dependent quenching of enzyme intrinsic fluorescence

Deoxycytidine kinase is a key enzyme in the salvage pathway, and its activity is required for 5'-phosphorylation of several important antiviral and cytostatic nucleoside analogues. It has recently been purified completely from human sources. Steady-state and time-resolved fluorescence of human deoxycytidine kinase was used to study its interaction with the substrates dCyd, dAdo, dUrd, dTTP, and the feedback inhibitor dCTP. Enzyme fluorescence quenching by dCTP, dCyd, dTTP, and dAdo was bimodal, and the best fits of the quenching patterns were obtained using two modified Stern-Volmer equations with two sets of quenching constants (Ksv) and accessibility values (fa) fitted independently for "low" and "high" concentration ranges of ligands. The transition between these occurred at about 20 microM dCTP, 50 microM dCyd, 30 microM dTTP, and 180 microM dAdo. Enzyme fluorescence showed unimodal quenching by dAdo and 30% reduced accessibility of the binding site in the presence of dCyd. dUrd quenching was also unimodal with Ksv = 0.0047 +/- 0.0007 microM-1 and fa = 0.75 +/- 0.05, hence in the same range as for the "high" concentration range of dAdo in the absence of dCyd, where they are 0.0025 +/- 0.0003 microM-1 and 0.73 +/- 0.03, respectively. Fluorescence quenching was used to directly determine enzyme-ligand binding and revealed bimodal binding of dCTP, dCyd, dTTP, and dAdo and unimodal binding of dUrd, and of dAdo in the presence of 0.1 microM dCyd. Transition between these two modes of binding occurred at the concentrations described above.(ABSTRACT TRUNCATED AT 250 WORDS)

Deoxycytidine kinase and deoxycytidine deaminase activities in human tumour xenografts

Deoxycytidine kinase (dCK) and deaminase (dCDA) are both key enzymes in the activation and inactivation, respectively, of several deoxycytidine antimetabolites. We determined the total dCK and dCDA activities using standard assays, in 28 human solid tumours grown as xenografts in nude mice, and four corresponding cell lines. dCK activities in colon tumours varied from 11 to 12 nmol/h/mg protein, in ovarian tumours from 3 to 10 nmol/h/mg protein, in soft tissue sarcomas from 2 to 7 nmol/h/mg protein and in squamous cell carcinomas of the head and neck about 45-fold, between 0.4 and 18 nmol/h/mg protein. The dCDA activities showed a larger variation, from 243 to 483, 14 to 1231, 3 to 7 and 1 to 222 nmol/h/mg protein, respectively. The ratios of dCK vs. dCDA activities in these tumours varied from 0.025 to 0.046, 0.004 to 0.240, 0.581 to 1.123 and from 0.012 to 4.227, respectively. In four cell lines (A2780, OVCAR-3, WiDr and UM-SCC-14C), sources for some of the above mentioned tumours, a different pattern in dCK and dCDA was observed than in the corresponding tumours. The variation in dCDA activities was in a smaller range (20-fold) than in the tumours (40-fold). In all cell lines dCK activity was higher than dCDA activity, in contrast to the corresponding tumours, in which the reverse pattern was observed. Previously, some of the tumours were tested for sensitivity to the deoxycytidine analogues 5-aza-deoxycytidine and 2',2'-difluorodeoxycytidine. In the sensitive tumours, both the highest and lowest dCK activity was observed, indicating that dCK activity in solid tumours is high enough to activate deoxycytidine analogues.

Cytofluorometric assay for the determination of thymidine uptake and phosphorylation in living cells

Thymidine kinase is a key enzyme for the application of drugs in chemotherapy and for diagnosis. Although of great interest, its regulation during cell cycle and differentiation is difficult to study, as current techniques for isolation of cells in different phases of growth are unsatisfactory. An assay that allows the determination of enzymatic activity in situ in single cells would be much faster than present methods and would elegantly avoid synchronization procedures. We synthesized different analogues of thymidine with the 5-methyl group substituted by a fluorochrome. At least three of these compounds were phosphorylated by thymidine kinase in cell free extracts and were taken up and phosphorylated by cells in culture. The cytofluorometric signal of the accumulated fluorochrome in any given cell reflected the thymidine kinase activity of this cell. Simultaneous measurement of cell-cycle dependent parameters allowed the correlation of thymidine kinase activity with the phase of growth in mixed cell populations.

Selective assays for thymidine kinase 1 and 2 and deoxycytidine kinase and their activities in extracts from human cells and tissues

Human cells salvage pyrimidine deoxyribonucleosides via 5'-phosphorylation which is also the route of activation of many chemotherapeutically used nucleoside analogs. Key enzymes in this metabolism are the cytosolic thymidine kinase (TK1), the mitochondrial thymidine kinase (TK2) and the cytosolic deoxycytidine kinase (dCK). These enzymes are expressed differently in different tissues and cell cycle phases, and they display overlapping substrate specificities. Thymidine is phosphorylated by both thymidine kinases, and deoxycytidine is phosphorylated by both dCK and TK2. The enzymes also phosphorylate nucleoside analogs with very different efficiencies. Here we present specific radiochemical assays for the three kinase activities utilizing analogs as substrates that are by more than 90 percent phosphorylated solely by one of the kinases; i.e. 3'-azido-2',3'-dideoxythymidine (AZT) as substrate for TK1, 1-beta-D-arabinofuranosylthymidine (AraT) for TK2 and 2-chlorodeoxyadenosine (CdA) for dCK. We determined the fraction of the total deoxycytidine and thymidine phosphorylating activity that was provided by each of the three enzymes in different human cells and tissues, such as resting and proliferating lymphocytes, lymphocytic cells of leukemia patients (chronic lymphocytic, chronic myeloic and hairy cell leukemia), muscle, brain and gastrointestinal tissue. The detailed knowledge of the pyrimidine deoxyribonucleoside kinase activities and substrate specificities are of importance for studies on chemotherapeutically active nucleoside analogs, and the assays and data presented here should be valuable tools in that research.

Mammalian thymidine kinase 2. Direct photoaffinity labeling with [32P]dTTP of the enzyme from spleen, liver, heart and brain

Thymidine kinase 2 (TK2), also called mitochondrial thymidine kinase, is a pyrimidine deoxyribonucleoside kinase expressed in all cells and tissues. It was recently purified to apparent homogeneity from human leukemic spleen and the active enzyme was shown to be a monomer of a 29-kDa polypeptide. The enzyme is feedback-inhibited by both end products, dCTP and dTTP. Here we show that TK2 purified from several different sources, including purified beef heart mitochondria, could be directly photoaffinity labeled with radioactive dTTP (approximately 18% of all TK2 molecules were cross-linked to dTTP after 20 min of ultraviolet irradiation) or to a lower extent with dCTP. Photo-incorporation was inhibited by the presence of the other effector but also the phosphate donor ATP blocked photolabeling, with dTTP. Addition of nucleoside substrates gave only a marginal inhibition of photo-incorporation. There were no detectable difference in the molecular size of photolabeled TK2 isolated from human spleen, brain or placenta, monkey liver, beef heart and beef heart mitochondria. Nor was there any significant differences in the enzyme kinetic properties of these enzymes. Cleavage of labeled TK2 with cyanogen bromide showed that dTTP was incorporated into a single 3-kDa peptide. TK2 was the only pyrimidine deoxynucleoside kinase expressed in liver, heart and brain. A detailed characterization of the subunit structure and substrate specificity of this enzyme is of importance for the design of new antiviral and cytostatic therapies based on nucleoside analogs.

Substrate specificity of human deoxycytidine kinase toward antiviral 2',3'-dideoxynucleoside analogs

Deoxycytidine (dCyd) kinase has been purified to homogeneity from human leukemic spleen, and the capacity of the enzyme to phosphorylate 2',3'-dideoxynucleoside (ddN) analogs that are clinically effective inhibitors of human immunodeficiency virus (HIV) replication was evaluated. Cytosine-containing ddN analogs, such as 2',3'-dideoxycytidine, 2',3'-dideoxy-2',3'-dehydrocytidine, and cytallene, were efficiently phosphorylated by dCyd kinase, while no phosphorylation of purine-containing ddN analogs was detected. dCyd kinase was completely inactive toward 2',3'-dideoxyadenosine (ddAdo), 2',3'-dideoxyinosine, 2',3'-dideoxyguanosine, and adenallene, although it was capable of phosphorylating both 2'-deoxyadenosine (dAdo) and 2'-deoxyguanosine (dGuo). The abilities of wild type and mutant human T lymphoblastoid CEM cells to accumulate ddAdo in situ and in vitro were also ascertained. Comparison of the abilities of intact wild type CEM cells and derivatives deficient in nucleoside transport, dCyd kinase, and/or adenosine (Ado) kinase to accumulate [3H]ddAdo-derived radioactivity revealed no significant differences among the wild type and mutant strains. However, ddAdo phosphorylating activity was decreased in extracts from Ado kinase-deficient cells but not in lysates prepared from cells genetically deficient in dCyd kinase activity. In comparative growth rate experiments, wild type, nucleoside transport-deficient, and dCyd kinase-deficient CEM cells were equally sensitive to ddAdo toxicity, while, interestingly, a deficiency in Ado kinase correlated with a 5-fold decreased growth sensitivity to the purine ddN. Insertion of an adenine phosphoribosyltransferase deficiency into the CEM cell lines did not influence ddAdo toxicity or incorporation rate. These results imply that Ado kinase may be an important factor in ddAdo phosphorylation by CEM cells. Furthermore, these studies demonstrate that cytosine- and purine-containing ddNs are transported and activated by independent pathways and, therefore, have important implications for anti-HIV therapy in that pyrimidine and purine ddNs might be used in combination for the treatment of acquired immunodeficiency syndrome.

Deoxynucleoside phosphorylating enzymes in monkey and human tissues show great similarities, while mouse deoxycytidine kinase has a different substrate specificity

Three key enzymes in the anabolic phosphorylation of deoxyribonucleosides and deoxyribonucleoside analogs were purified i.e. cytoplasmic thymidine kinase (TK1), mitochondrial thymidine kinase (TK2) and cytoplasmic deoxycytidine kinase (dCK) from human, mouse and monkey liver and spleen. Their subunit structure and substrate specificities were compared. Extensive purification of TK1 and dCK from mouse spleen and TK2 from mouse and monkey livers revealed major polypeptide bands of 25, 30 and 28 kD, respectively, on sodium dodecyl sulphate-polyacrylamide gel electrophoresis which are very similar to the subunit molecular weights of the corresponding human enzymes. Affinity purified polyclonal antibodies against human dCK also cross-reacted with 30 kD bands in extracts from both mouse and monkey spleen. Thus, the molecular weights of the subunits of these three enzymes appeared to be very similar in all three species. TK1 and TK2 from these different sources appeared to have similar substrate specificities against several deoxyribonucleoside analogs. However, mouse dCK differed significantly from monkey and human dCK in its capacity to phosphorylate dAdo and 2',3'-dideoxycytidine (ddCyd) with a Vmax approximately 10-fold lower than that of the two latter enzymes. The Km and Vmax values for dCyd and arabinocytosine appeared to be very similar with the enzymes from all three species. The fact that mouse dCK shows low activity with dAdo and ddCyd explains differences reported previously in the metabolism of dAdo and ddCyd in mouse compared to that in human lymphocytes. These results argue against the use of mice as model systems for human deoxynucleoside metabolism.