JBC, Vol. 251, Issue 4, 1181-1187, Feb, 1976
Membrane-associated enzymes involved in nucleoside processing by plasma membrane vesicles isolated from L929 cells grown in defined medium
C. C. Li and J. Hochstadt
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.
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