Li C C, Hochstadt J
J Biol Chem. 1976 Feb 25;251(4):1181-7.
adenosine, adenosine,Transport-competent plasma membrane vesicles isolated from a subline of L929 cells (L929se-) grown in serum-free, defined medium accumulate ribose-1-P when exposed to adensoine, inosine, guanosine, or uridine. This observation suggests the action of one or more nucleoside phosphorylases acting prior to, during, or subsequent to the transport event. Extravesicular ribose-1-P neither inhibits uptake nor exchanges with intravesicular ribose-1-P, indicating that the action of the phosphorylase is not prior to uptake. Preloading the vesicles with inosine prior to subjecting the vesicles to conditions under which further uptake could not take place (in the presence of caffeine) did not result in an alteration of the ribose-1-P to inosine ratio within the vesicles. This observation was interpreted as evidence that only exogenously derived, not intravesicular inosine, is the substrate for the nucleoside phosphorylase. This datum, when taken with the fact that hypoxanthine is never found to be a significant extent within the vesicles, suggests that the phosphorolytic cleavage of inosine occurs as a group translocation during the transport itself, so that hypoxanthine is released to the surrounding medium while the ribose-1-P accumulates intravesicularly. Thus, phosphorolysis would seem to occur during passage across the membrane.
腺苷、腺苷：从在无血清、限定培养基中生长的L929细胞亚系（L929se-）分离得到的具有转运活性的质膜囊泡，在暴露于腺苷、肌苷、鸟苷或尿苷时会积累核糖-1-磷酸。这一观察结果表明，一种或多种核苷磷酸化酶在转运事件之前、期间或之后发挥作用。囊泡外的核糖-1-磷酸既不抑制摄取，也不与囊泡内的核糖-1-磷酸交换，这表明磷酸化酶的作用不是在摄取之前。在使囊泡处于无法进一步摄取的条件下（存在咖啡因）之前，先用肌苷预加载囊泡，并未导致囊泡内核糖-1-磷酸与肌苷的比例发生改变。这一观察结果被解释为证据，表明只有外源性来源的肌苷，而非囊泡内的肌苷，是核苷磷酸化酶的底物。这一数据，结合在囊泡内从未发现次黄嘌呤大量存在这一事实，表明肌苷的磷酸解裂解在转运过程中作为一种基团转移发生，因此次黄嘌呤被释放到周围介质中，而核糖-1-磷酸在囊泡内积累。因此，磷酸解似乎发生在穿过膜的过程中。
