Membrane-associated enzymes involved in nucleoside processing by plasma membrane vesicles isolated from L929 cells...

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.