Ullman B, Gudas L J, Clift S M, Martin D W
Proc Natl Acad Sci U S A. 1979 Mar;76(3):1074-8. doi: 10.1073/pnas.76.3.1074.
The inherited deficiency of purine-nucleoside phosphorylase (PNPase; purine-nucleoside:orthophosphate ribosyltransferase, EC 2.4.2.1) in humans is associated with a severe deficiency of the T lymphocytes of the immune system. Because of the unsatisfactory nature of previously described model systems, we have selected, cloned, and characterized a mutant mouse T cell lymphoma (S49) completely deficient in PNPase. Of the four substrates of PNPase, only deoxyguanosine at low concentrations is toxic to the PNPase-deficient (NSU-1) cells. In order to delineate the biochemical processes necessary for the sensitivity of the NSU-1 cells to deoxyguanosine, we have isolated a series of secondary mutants resistant to deoxyguanosine from the PNPase-deficient line. One of these mutants is defective in its ability to transport deoxyguanosine into the cell. A second type of mutant cannot phosphorylate the deoxyguanosine and is totally deficient in deoxycytidine kinase activity. A third type of mutant (NSU-1-dGuo-L) can both transport and phosphorylate deoxyguanosine and accumulates dGTP. However, unlike its parent, NSU-1-dGuo-L does not become depleted of dCTP and TTP when exposed to exogenous deoxyguanosine. This observation is accounted for by the fact that the reduction of CDP to dCDP by the ribonucleotide reductase (ribonucleoside-diphosphate reductase, 2'-deoxyribonucleoside-diphosphate:oxidized-thioredoxin 2'-oxidoreductase, EC 1.17.4.1) of NSU-1-dGuo-L cells is not normally sensitive to feedback inhibition by dGTP.Thus, in order to exert its toxicity deoxyguanosine must be transported into the cell, be phosphorylated by deoxycytidine kinase, and be accumulated as dGTP. By inhibiting ribonucleotide reductase, dGTP depletes the cell of dCTP and to some extent TTP, thus preventing the synthesis of DNA, a process necessary for any proliferation-dependent function of T cells.
人类嘌呤核苷磷酸化酶（PNPase；嘌呤核苷：正磷酸核糖基转移酶，EC 2.4.2.1）的遗传性缺陷与免疫系统中T淋巴细胞的严重缺陷有关。由于先前描述的模型系统存在不足之处，我们选择、克隆并鉴定了一种完全缺乏PNPase的突变小鼠T细胞淋巴瘤（S49）。在PNPase的四种底物中，只有低浓度的脱氧鸟苷对缺乏PNPase的（NSU - 1）细胞有毒性。为了阐明NSU - 1细胞对脱氧鸟苷敏感所需的生化过程，我们从缺乏PNPase的细胞系中分离出了一系列对脱氧鸟苷具有抗性的二级突变体。其中一个突变体在将脱氧鸟苷转运到细胞内的能力上存在缺陷。第二种类型的突变体无法将脱氧鸟苷磷酸化，并且完全缺乏脱氧胞苷激酶活性。第三种类型的突变体（NSU - 1 - dGuo - L）既能转运又能磷酸化脱氧鸟苷，并积累dGTP。然而，与它的亲本不同，NSU - 1 - dGuo - L在暴露于外源性脱氧鸟苷时不会耗尽dCTP和TTP。这一观察结果可以通过以下事实来解释：NSU - 1 - dGuo - L细胞的核糖核苷酸还原酶（核糖核苷二磷酸还原酶，2'-脱氧核糖核苷二磷酸：氧化型硫氧还蛋白2'-氧化还原酶，EC 1.17.4.1）将CDP还原为dCDP的过程通常对dGTP的反馈抑制不敏感。因此，为了发挥其毒性，脱氧鸟苷必须被转运到细胞内，被脱氧胞苷激酶磷酸化，并以dGTP的形式积累。通过抑制核糖核苷酸还原酶，dGTP会耗尽细胞内的dCTP，并在一定程度上耗尽TTP，从而阻止DNA的合成，而DNA合成是T细胞任何依赖增殖功能所必需的过程。
