Staub Mária
Orvosi Vegytani, Molekuláris Biológiai és Pathobiokémiai Intézet, Semmelweis Egyetem, Budapest 1088, Hungary.
Magy Onkol. 2004;48(3):229-34. Epub 2004 Nov 1.
Deoxycytidine kinase (dCK) plays a central role in the deoxynucleoside salvage processes, phosphorylating dC, dA, and dG to their monophosphates. In mammalian cells, the major source of dTTP comes also from dC via dCMP deaminase. Moreover, based on its broad substrate specificity, this enzyme is responsible for the activation of several nucleoside analogues of therapeutical importance, influencing the sensitivity of malignant tissues towards chemotherapy. The expression of dCK is highest in different lymphoid cells/tissues, in embryonic cells and in most malignant cells (2, 7, 13-15, 18). The activity of dCK is not cell cycle-regulated. In contrast to this, dCK activity was found to be elevated several fold upon short-term treatments of normal human lymphocytes with therapeutic nucleoside analogs, and other genotoxic agents as well as by DNA damaging agents including the DNA polymerase inhibitor aphidicolin, the topoisomerase II inhibitor etoposide and gamma-irradiation, which might be a potentially important phenomenon with respect to the clinical practice, too. These findings indicated that the main trigger of activation could be the damaged DNA itself, and the biological relevance might be to supply the dNTPs for the enhanced DNA repair. Activation of dCK was paralleled by elevated levels of intracellular dATP, raising the possibility that dCK activation is linked to the induction of apoptosis. With regard to the mechanism of enzyme activation, no changes were found in the protein and mRNA levels of dCK upon stimulation, while the activation process was calcium dependent and comprised a protein phosphorylation step. A positive correlation was found between the enzymatic activity and the native immunoreactivity of dCK, strongly arguing that dCK undergoes a conformational change during activation, which results in the formation of a catalytically more active steric structure (8-11, 22, 26, 32-34, 35, 36).
脱氧胞苷激酶（dCK）在脱氧核苷补救过程中起核心作用，将脱氧胞苷（dC）、脱氧腺苷（dA）和脱氧鸟苷（dG）磷酸化为它们的单磷酸形式。在哺乳动物细胞中，脱氧胸苷三磷酸（dTTP）的主要来源也通过dCMP脱氨酶来自dC。此外，基于其广泛的底物特异性，该酶负责激活几种具有治疗重要性的核苷类似物，影响恶性组织对化疗的敏感性。dCK在不同的淋巴细胞/组织、胚胎细胞和大多数恶性细胞中表达最高（2, 7, 13 - 15, 18）。dCK的活性不受细胞周期调节。与此相反，发现用治疗性核苷类似物、其他基因毒性剂以及包括DNA聚合酶抑制剂阿非迪霉素、拓扑异构酶II抑制剂依托泊苷和γ射线在内的DNA损伤剂对正常人淋巴细胞进行短期处理后，dCK活性会升高几倍，这在临床实践中可能也是一个潜在的重要现象。这些发现表明激活的主要触发因素可能是受损的DNA本身，其生物学意义可能是为增强的DNA修复提供脱氧核苷三磷酸（dNTPs）。dCK的激活与细胞内dATP水平升高同时发生，这增加了dCK激活与细胞凋亡诱导相关的可能性。关于酶激活的机制，刺激后dCK的蛋白质和mRNA水平未发现变化，而激活过程是钙依赖性的，并且包括一个蛋白质磷酸化步骤。发现酶活性与dCK的天然免疫反应性之间存在正相关，有力地证明dCK在激活过程中发生构象变化，从而导致形成催化活性更高的空间结构（8 - 11, 22, 26, 32 - 34, 35, 36）。
