Cloning and functional characterization of a high-affinity Na+/dicarboxylate cotransporter from mouse brain

Neurons contain a high-affinity Na+/dicarboxylate cotransporter for absorpt ion of neurotransmitter precursor substrates, such as alpha -ketoglutarate and malate, which are subsequently metabolized to replenish pools of neurot ransmitters, including glutamate. We have isolated the cDNA coding for a hi gh-affinity Na+/dicarboxylate cotransporter from mouse brain, called mNaDC- 3. The mRNA coding for mNaDC-3 is found in brain and choroid plexus as well as in kidney and liver. The mNaDC-3 transporter has a broad substrate spec ificity for dicarboxylates, including succinate, alpha -ketoglutarate, fuma rate, malate, and dimethylsuccinate. The transport of citrate is relatively insensitive to pH, but the transport of succinate is inhibited by acidic p H. The Michaelis-Menten constant for succinate in mNaDC-3 is 140 muM in tra nsport assays and 16 mM at -250 mV in two-electrode voltage clamp assays. T ransport is dependent on sodium, although lithium can partially substitute for sodium. In conclusion, mNaDC-3 likely codes for the high-affinity Na+/d icarboxylate cotransporter in brain, and it has some unusual electrical pro perties compared with the other members of the family.