title: Nucleobase transport in opossum kidney epithelial cells and Xenopus laevis oocytes: the characterisation, structure-activity relationship of uracil analogues and oocyte expression studies of sodium-dependent and -independent hypoxanthine uptake.
abstract: The characteristics of hypoxanthine transport were examined in opossum kidney (OK) epithelial cells and Xenopus laevis oocytes. In both cell types hypoxanthine influx was mediated by two distinct transport systems: a high-affinity Na+-dependent system and a Na+-independent transporter. Na+-dependent hypoxanthine transport in OK cells was saturable (Km 0.78+/-0.29 microM) and was inhibited by guanine, uracil, thymine and 5-fluorouracil (Ki values 0.5-7 microM), whereas adenine had no effect. Substitutions at the 2- and 4-position had a marked effect on the ability of uracil to inhibit Na+/hypoxanthine influx by OK cells revealing that an oxo group at both the 2- and 4-positions of uracil is required for interacting with the transporter. The properties of Na+-dependent hypoxanthine influx in oocytes were similar to those observed in OK cells. In particular, xanthine and oxypurinol inhibited hypoxanthine influx, a characteristic not observed previously for the Na+/nucleobase carrier in pig LLC-PK1 renal cells. Na+-independent hypoxanthine influx in OK cells and oocytes was of a lower affinity (Km 90-180 microM). Adenine and guanine inhibited Na+-independent hypoxanthine flux in OK cells, but had no effect in oocytes. Injection of LLC-PK1 mRNA into oocytes resulted in a 1.5-fold stimulation of Na+/hypoxanthine flux over water-injected oocytes. These results reveal further heterogeneity in Na+/nucleobase cotransporters.
authors.affiliation: Research School of Biosciences, University of Kent, Canterbury, Kent CT2 7NJ, UK.
authors.forename: M
authors.initials: M
authors.lastname: Shayeghi
authors.forename: R
authors.initials: R
authors.lastname: Akerman
authors.forename: S M
authors.initials: SM
authors.lastname: Jarvis
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002352:Carrier Proteins / Q000096:biosynthesis* / Q000235:genetics; D004847:Epithelial Cells / Q000378:metabolism; D019271:Hypoxanthine / Q000378:metabolism*; D007668:Kidney / Q000378:metabolism*; D008845:Microinjections; D009865:Oocytes / Q000378:metabolism*; D009893:Opossums; D012333:RNA, Messenger / Q000096:biosynthesis / Q000494:pharmacology; D012965:Sodium Chloride / Q000494:pharmacology; D013379:Substrate Specificity; D014498:Uracil / Q000031:analogs & derivatives* / Q000494:pharmacology; D014982:Xenopus laevis
chemical_list: D002352:Carrier Proteins; D012333:RNA, Messenger; D019271:Hypoxanthine; D012965:Sodium Chloride; D014498:Uracil