Molecular and functional analysis of SDCT2, a novel rat sodium-dependent dicarboxylate transporter

Kidney proximal tubule cells take up Krebs cycle intermediates for metaboli c purposes and for secretion of organic anions through dicarboxylate/organi c anion exchange. Alteration in reabsorption of citrate is closely related to renal stone formation. The presence of distinct types of sodium-coupled dicarboxylate transporters has been postulated on either side of the polari zed epithelial membrane in the kidney proximal tubule. Using a PCR-based ap proach, we isolated a novel member of the sodium-dependent dicarboxylate/su lfate transporter called SDCT2. SDCT2 is a 600-amino acid residue protein t hat has 47-48% amino acid identity to SDCT1 and NaDC-1, previously identifi ed in kidney and intestine. Northern analysis gave a single band of 3.3 kb for SDCT2 in kidney, liver, and brain. In situ hybridization revealed that SDCT2 is prominently expressed in kidney proximal tubule S3 segments and in perivenous hepatocytes, consistent with the sites of high-affinity dicarbo xylate transport identified based on vesicle studies. A signal was also det ected in the meningeal layers of the brain. SDCT2 expressed in Xenopus oocy tes mediated sodium-dependent transport of di- and tricarboxylates with sub strate preference for succinate rather than citrate, but excluding monocarb oxylates. SDCT2, unlike SDCT1, displayed a unique pH dependence for succina te transport (optimal pH 7.5-8.5) and showed a high affinity for dimethylsu ccinate, two features characteristic of basolateral transport. These data h elp to interpret the mechanisms of renal citrate transport, their alteratio n in pathophysiological conditions, and their role in the elimination of or ganic anions and therapeutic drugs.