Antibodies as Probes of Nitrobenzylthioinosine-Sensitive Nucleoside Transporters

Abstract

The passage of nucleosides across the membranes of mammalian cells is mediated by a variety of different transport systems [1]. In the kidney and small intestine active, sodium-linked uptake systems are present, at least four distinct transporters having been identified by virtue of their differing substrate specificities [2]. However, in most other mammalian cells nucleoside uptake and efflux occurs via the passive process of facilitated diffusion [1]. The passive transporters, like the active, can also be subdivided into classes, in this case by virtue of their sensitivity to inhibition by the 6-thiopurine analogue nitrobenzylthioinosine (NBMPR), to which the sodium-dependent transporters are insensitive. Passive transporters designated es (equilibrative-sensitive) bind and are inhibited by NBMPR with affinities in the 0.1–1 nM range, whereas transporters of the ei class (equilibrative-insensitive) are unaffected by micromolar concentrations of NBMPR [1,3,4].