We have recently identified sodium-dependent nucleobase transporter 1 (SNBT1, Slc23a4), 1) from the rat small intestine as a first nucleobase transporter in mammals. Rat SNBT1 (rSNBT1) was found to mediate sodium-dependent transport of several nucleobases such as uracil, thymine, guanine, hypoxanthine and xanthine, but not adenine and cytosine, and was suggested to be responsible for the intestinal nucleobase absorption. The substrate specificity of rSNBT1 suggests a role of a oxo group at the 4th position (C-4) of pyrimidine ring (or corresponding position of purine ring) in substrate recognition. To further clarify the role of this transporter in the intestinal disposition of nucleobases and relevant compounds, we examined the transport of urate, which is an important metabolite of purine nucleobases and has an oxo group at the required position. The uptake of [14C]urate was evaluated at 37°C and pH 7.4 for a 2-min period, using Madin–Darby canine kidney (MDCK) II cells stably expressing rSNBT1. The uptake of [14C]urate was also evaluated by using everted intestinal tissue sacs prepared from the small intestine of male Wistar rats, with the uptake condtion of 37°C and pH 7.4 for a 2-min period. Urate was found to be efficiently transported by rSNBT1 in a sodium-dependent manner. rSNBT1-mediated urate transport was saturable with the Michaelis constant (Km) of 433 µM and, at a low concentration of 5 µM, inhibited by uracil, hypoxanthine, thymine and guanine with the IC50 values of 28.4, 10.1, 2.7 and 8.8 µM, respectively. Particularly, the IC50 value for uracil was comparable with the Km (21.2 µM) of uracil transport by rSNBT1, suggesting that urate shares the substrate recognition site with uracil, and rSNBT1 could mediate absorption of urate as well as uracil in the small intestine. However, the urate uptake was not detectable in the evered sacs of the rat small intestine, although the marked uptake of nucleobases such as hypoxanthine, uracil and thymine could be observed. Thus, rSNBT1 was found to transport urate by a sodium-dependent mechanism shared by nucleobases, although the uptake of urate was not observed physiologically in the rat small intestine. These results suggest that urate absorption is regulated by futile cycling via SNBT1 and other transporters such as Abcg2 and/or Glut9, both of which have been known to mediate the efflux of urate. Further studies will be needed to clarify the role of SNBT1 in urate disposition in the small intestine.
1) Yamamoto S, Inoue K, Murata T, Kamigaso S, Yasujima T, Maeda J, Yoshida Y, Ohta K, and Yuasa H: Identification and functional characterization of the first nucleobase transporter in mammals: implication in the species difference in the intestinal absorption mechanism of nucleobases and their analogs between higher primates and other mammals. J Biol Chem, 285:6522 – 6531, 2010.