The existence of a group translocation transport mechanism in animal cells: Uptake of the ribose moiety of inosine

✍ Scribed by Quinlan, Dennis C. ;Li, Chien-Chung ;Hochstadt, Joy

• Publisher: Wiley (John Wiley & Sons)
• Year: 1976
• Tongue: English
• Weight: 694 KB
• Volume: 4
• Category: Article
• ISSN: 0091-7419
• DOI: 10.1002/jss.400040402

✦ Synopsis

Abstract

After exposure to inosine, transport‐competent plasma membrane vesicles isolated from SV ‐40‐transformed Balb/c 3T3 cells accumulate intravesicular ribose 1‐PO~4~ 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‐PO~4~. In addition, hypoxanthine does not result in the efflux of transported uridine which is accumulated in these membrane vesicles as uridine. Exogenous ribose 1‐PO~4~ 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 nuceloside phosphorylase. The data are consistent with sites for inosine and hypoxanthine being on the outer membrane surface whereas the ribose 1‐PO~4~ site is only on the inner surface.