The Role of Deoxycytidine Kinase in DNA Synthesis and Nucleoside Analog Activation

Deoxycytidine kinase (dCK) is the main enzyme in the salvage of deoxyribonucleosides as a consequence of its broad substrate specificity. dCK is the only enzyme that can supply cells with all four precursors of DNA; is capable of 5′-phosphorylation of the natural substrates deoxycytidine (dCyt), deoxyadenosine, and deoxyguanosine; and can be interconverted into thymine nucleotides. The deoxycytidine triphosphate (dCTP), in addition to DNA, can be utilized for special processes, such as for synthesis of “Cliponucleotides, ”which are precursors of membrane phospholipids. The expression of dCK is highest in lymphoid cells/tissues (e.g., such as thymus, spleen, lymph nodes, stimulated peripheral blood mononuclear and bone marrow cells) and in all malignancies of these cells. The cell cycle dependence of the expression of dCK has been a matter of discussion; even higher dCK activity and dCyt metabolism were found in undifferentiated rather than in differentiated human lymphocytes. An enhancement of dCK activity occurred on preincubation of cells with a variety of nucleoside derivatives and nonnucleoside genotoxic agents, such as aphidicolin, etoposide (VP16), taxol, and even the G protein modulator sodium fluoride. γ-Irradiation and ultraviolet (UV) C irradiation also augmented dCK activity in different cells. The decrease of dCK activity was observed with protein phosphatase inhibitors, suggesting a regulatory role for reversible protein phosphorylation in the activation process. Cytosolic Ca 2+ ion and p53 protein are necessary for the increase of dCK activity in cells after toxic treatments. The reason for the increase of dCK activity after toxic treatment of cells seems to be a compensatory mechanism induced by “metabolic stress” signals; cells need deoxynucleotides to repair damaged DNA. A positive correlation was found between dCK activity and the sensitivity of malignant cells to chemotherapy; thus, dCK has an outstanding importance in human chemotherapy. dCK is often the rate-limiting enzyme in the activation of these analogs. L-2′3′-dideoxy-3′-thiacytidine (lamivudine, 3TC); arabinosylcytosine (Cytosar, ara-C); 2-chlorodeoxyadenosine (cladribine, CdA); and 2′,2′-difluorodeoxycytidine (gemcitabine, dFdC), the first a human immunodeficiency virus drug and the last three valuable anticancer agents, are all substrates for dCK, and they are between 5% and 50% as efficient as dCyt as substrates for the enzyme. dCK prefers nucleoside sugars in the S-conformation (C2′-endo-C3′-exo) because α-2′,3′-dideoxycytidine adopts that conformation preferentially. dCK is composed by two identical polypeptides of 261 amino acids ( 54 ), and it shows some significant sequence similarity with the herpes simplex type 1 virus thymidine kinase, as well as about 40% sequence identity to the mitochondrial thymidine kinase 2. In 2003, the structure of dCK in complex with dCyt and ADP-Mg 2+ was solved.