K. Iseki et al., TRANSPORT MECHANISMS OF NUCLEOSIDES AND THE DERIVATIVE, 6-MERCAPTOPURINE RIBOSIDE ACROSS RAT INTESTINAL BRUSH-BORDER MEMBRANES, Biochimica et biophysica acta. Biomembranes, 1278(1), 1996, pp. 105-110
Na+-driven nucleoside transport processes across rat intestinal brush-
border membrane vesicles were investigated. 6-Mercaptopurine riboside
(6-MPR), an analogue of purine-nucleoside such as adenosine and inosin
e, was recognized by its purine- and pyrimidine-nucleosides transport
system, but their nucleo-bases did not entirely inhibit the 6-MPR tran
sport. The analysis according to the Hill equation of the curve for Na
+ activation of 6-MPR uptake was consistent with the notion of a Na+/6
-MPR coupling stoichiometry of 1:1. The expressed transport activities
of adenosine, uridine, and 6-MPR were Na+-dependent and saturable, an
d their affinity constants (K-m value) obtained by Eadie-Hofstee analy
sis were approx. 20, 15 and 100 mu M. Moreover, the uptake of radiolab
eled adenosine and uridine was trans-stimulated by 6-MPR inside vesicl
es in the absence of an inwardly directed Na+-gradient. On the other h
and, uridine did not exhibit any inhibitory effects on the uptake of a
denosine despite the fact that adenosine was a potent inhibitor for ur
idine uptake by intestinal brush-border membrane vesicles. These diffe
rences in the inhibition may be explained by the multiplicity of the n
ucleoside transport systems.