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

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 (K-m 0.78 +/- 0.29 mu M) and was inhibited by guanine, uracil, thymine and 5-fluorouracil (K-i values 0.5-7 mu M), 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 (K-m 90-180 mu M). 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.