Schneider E G, Whitten D J
Exp Cell Res. 1987 Jan;168(1):1-14. doi: 10.1016/0014-4827(87)90411-3.
Uptake and metabolism of thymidine and adenosine have been studied in embryos of the sea urchin Strongylocentrotus purpuratus. Uptake of these nucleosides is found to be mutually competitive, with the Km for uptake of thymidine similar to its Ki for inhibition of adenosine uptake and vice versa. The metabolic studies show that adenosine is rapidly and completely phosphorylated upon entry, even at high exogenous concentrations which saturate the uptake mechanism. In contrast, at concentrations which saturate nucleoside uptake, thymidine becomes appreciably catabolized (up to 60%) to thymine and beta-amino-isobutyric acid in addition to its phosphorylation to thymine nucleotides. Negligible amounts of endogenous thymidine appear to remain unmetabolized following uptake in these embryos. The data provide strong in vivo evidence for separate metabolic pathways for thymidine and adenosine which have not previously been described in this organism. The observation of mutual competition during uptake, together with different routes of metabolism for these nucleosides, would suggest that the rate-limiting step in the uptake process is transport rather than metabolism. The specificity of this transport system for its nucleoside substrate has been examined in some detail in the present report. All naturally occurring nucleosides but only a limited number of nucleoside analogs are recognized by this membrane carrier. Neither purine nor pyrimidine bases are substrates for this transport system. Previous work by this laboratory has demonstrated the strict Na+-dependence of this carrier, its high affinity for nucleoside substrate, and its activation at fertilization. These observations and the substrate specificity studies of the present work together describe a unique transport system for nucleosides in sea urchin embryos which is quite different from those previously described in mammalian cells.
在紫海胆胚胎中对胸苷和腺苷的摄取及代谢进行了研究。发现这些核苷的摄取相互竞争，胸苷摄取的Km与其抑制腺苷摄取的Ki相似，反之亦然。代谢研究表明，腺苷进入后会迅速且完全磷酸化，即使在使摄取机制饱和的高外源浓度下也是如此。相比之下，在使核苷摄取饱和的浓度下，胸苷除了磷酸化生成胸腺嘧啶核苷酸外，还会明显分解代谢（高达60%）生成胸腺嘧啶和β-氨基异丁酸。在这些胚胎中摄取后，似乎只有极少量的内源性胸苷未被代谢。这些数据为胸苷和腺苷的单独代谢途径提供了有力的体内证据，而此前在该生物体中尚未有过描述。摄取过程中相互竞争的观察结果，以及这些核苷不同的代谢途径，表明摄取过程中的限速步骤是转运而非代谢。在本报告中已对该转运系统对其核苷底物的特异性进行了详细研究。该膜载体识别所有天然存在的核苷，但只识别有限数量的核苷类似物。嘌呤和嘧啶碱基都不是该转运系统的底物。本实验室之前的工作已证明该载体严格依赖Na +，对核苷底物具有高亲和力，且在受精时被激活。这些观察结果以及本工作的底物特异性研究共同描述了紫海胆胚胎中一种独特的核苷转运系统，它与之前在哺乳动物细胞中描述的转运系统有很大不同。
