Primary structure and functional characteristics of a mammalian sodium-coupled high affinity dicarboxylate transporter

We have cloned a Na+-dependent, high affinity dicarboxylate transporter (Na DC3) from rat placenta. NaDC3 exhibits 48% identity in amino acid sequence with rat NaDC1, a Na+-dependent, low affinity dicarboxylate transporter. Na DC3-specific mRNA is detectable in kidney, brain, liver, and placenta. When expressed in mammalian cells, NaDC3 mediates Na+-dependent transport of su ccinate with a K-t of 2 mu M. The transport function of NaDC3 shows a sigmo idal relationship with regard to Na+ concentration, with a Hill coefficient of 2.7. NaDC3 accepts a number of dicarboxylates including dimethylsuccina te as substrates and excludes monocarboxylates. Li+ inhibits NaDC3 in the p resence of Na+. Transport of succinate by NaDC3 is markedly influenced by p H, the transport function gradually decreasing when pH is acidified from 8. 0 to 5.5. In contrast, the influence of pH on NaDC3-mediated transport of c itrate is biphasic in which a pH change from 8.0 to 6.5 stimulates the tran sport and any further acidification inhibits the transport. In addition, th e potency of citrate to compete with NaDC3-mediated transport of succinate increases 25-fold when pH is changed from 7.5 to 5.5. These data show that NaDC3 interacts preferentially with the divalent anionic species of citrate . This represents the first report on the cloning and functional characteri zation of a mammalian Na+-dependent, high affinity dicarboxylate transporte r.