The existance of a group translocation transport mechanism in animal cells: uptake of the ribose moiety of inosine.
After exposure to inosine, transport-competent plasma membrane vesicles isolated from SV-40-transformed Bal/c 3T3 cells accumulate intravesicular ribose 1-PO4 at a concentration 200-fold greater than the extravesicular concentration. An analysis of the purine nucleoside phosphorylase activity distribution in various subcellular fractions, relative to other enzyme activities, indicated the presence of plasma membrane-associated purine nucleoside phosphorylase activity. The plasma membrane vesicles appear relatively impermeable to hypoxanthine. However, hypoxanthine, which is a competitive inhibitor of the transport reaction, is the only compound tested capable of mediating efflux of already accumulated ribose 1-PO4. In addition, hypoxanthine does not result in the efflux of transported uridine which is accumulated in these membrane vesicles as uridine. Exogenous ribose 1-PO4 neither results in counterflow nor does it inhibit the original uptake reaction. The following transport reaction is proposed: uptake occurs by group translocation, mediated by membrane-localized purine nucleoside phosphorylase. The data are consistent with sites for inosine and hypoxanthine being on the outer membrane surface whereas the ribose 1-PO4 site is only on the inner surface.
D000225:Adenine / Q000494:pharmacology; D001693:Biological Transport, Active; D002460:Cell Line; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002471:Cell Transformation, Neoplastic; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D007042:Hypoxanthines / Q000494:pharmacology; D007288:Inosine / Q000378:metabolism*; D008954:Models, Biological; D011683:Purine-Nucleoside Phosphorylase / Q000378:metabolism; D012266:Ribose / Q000378:metabolism*; D012267:Ribosemonophosphates / Q000494:pharmacology; D013539:Simian virus 40 / Q000378:metabolism
D007042:Hypoxanthines; D012267:Ribosemonophosphates; D007288:Inosine; D012266:Ribose; D011683:Purine-Nucleoside Phosphorylase; D000225:Adenine