Alford B L, Barnes E M
J Biol Chem. 1976 Aug 25;251(16):4823-7.
The uptake of hypoxanthine by Chinese hamster lung fibroblasts grown in tissue culture was studied in wild type clones and 8-azaguanine-resistant mutant clones devoid of hypoxanthine-guanine phosphoribosyltransferase. Wild type fibroblasts rapidly accumulate [3H]hypoxanthine from the medium and over 80% of the intracellular radioactivity is found in acid-soluble nucleotides. The phosphoribosyltransferase-deficient clones accumulate much lower levels of hypoxanthine and over 85% of the intracellular 3H label is associated with chemically unaltered hypoxanthine. The internal level of hypoxanthine in the mutant clones rapidly approaches but does not exceed that present in the medium. Wild type and phosphoribosyltransferase-deficient cells take up hypoxanthine at almost identical initial rates at external hypoxanthine levels from 2 to 300 muM. Analysis of these data reveals two transport systems that obey the Michaelis-Menten relationship. These differ markedly in affinity, yielding average Km values of 20 and 600 muM for both cell types. Hypoxanthine transport by both low and high affinity transport systems is blocked by p-chloromercuriphenylsulfonate and N-ethylmaleimide. Counter-transport of hypoxanthine was demonstrated in phosphoribosyltransferase-deficient fibroblasts. It is concluded that hypoxanthine is transported into Chinese hamster cells by means of carrier-mediated processes (facilitated diffusion) that operate independently of phosphoribosylation.
在组织培养中生长的中国仓鼠肺成纤维细胞对次黄嘌呤的摄取情况，在野生型克隆和缺乏次黄嘌呤 - 鸟嘌呤磷酸核糖基转移酶的8 - 氮杂鸟嘌呤抗性突变克隆中进行了研究。野生型成纤维细胞能迅速从培养基中积累[3H]次黄嘌呤，并且超过80%的细胞内放射性存在于酸溶性核苷酸中。缺乏磷酸核糖基转移酶的克隆积累的次黄嘌呤水平要低得多，并且超过85%的细胞内3H标记与化学性质未改变的次黄嘌呤相关。突变克隆中次黄嘌呤的内部水平迅速接近但不超过培养基中的水平。在外部次黄嘌呤水平为2至300μM时，野生型和缺乏磷酸核糖基转移酶的细胞摄取次黄嘌呤的初始速率几乎相同。对这些数据的分析揭示了两种遵循米氏关系的转运系统。这两种系统在亲和力上有显著差异，两种细胞类型的平均Km值分别为20和600μM。低亲和力和高亲和力转运系统对次黄嘌呤的转运都被对氯汞苯磺酸盐和N - 乙基马来酰亚胺阻断。在缺乏磷酸核糖基转移酶的成纤维细胞中证明了次黄嘌呤的反向转运。得出的结论是，次黄嘌呤通过载体介导的过程（易化扩散）进入中国仓鼠细胞，该过程独立于磷酸核糖基化作用。
