The Renal Na+-Dependent Dicarboxylate Transporter, NaDC-3, Translocates Dimethyl- and Disulfhydryl-Compounds and Contributes to Renal Heavy Metal Detoxification

Abstract

Abstract. The active transport of Krebs cycle intermediates, such as succinate, ␣-ketoglutarate, and citrate, is mediated by sodium-coupled transporters found in the luminal (NaDC-1) and basolateral plasma membranes (NaDC-3) of proximal tubule cells. This study used the two-electrode voltage clamp technique to examine steady-state currents associated with the influx of three sodium ions and one divalent dicarboxylate into oocytes expressing the sodium-dicarboxylate transporter from winter flounder kidney, fNaDC-3. The substrate concentration, where half-maximal current was observed (K 0.5 ), was 30 M for succinate. Besides 2,2-dimethylsuccinate, fNaDC-3 also accepted 2,3-dimethylsuccinate and the oral lead-chelating agent, meso-2,3-dimercaptosuccinate (DMSA or Succimer).Whereas the K 0.5 for succinate and 2,2-dimethylsuccinate was independent of membrane voltage within Ϫ90 and Ϫ10 mV, K 0.5 for 2,3-dimethylsuccinate and 2,3-dimercaptosuccinate increased with decreasing voltage, indicating a critical role of the position of the methyl-or sulfhydryl-group in voltage-sensitive affinity. In addition to meso-2,3-dimercaptosuccinate, fNaDC-3 translocated dimercaptopropane-1-sulfonate (DMPS or Dimaval), an oral chelator for the treatment of mercury intoxication. The chelates formed by HgCl 2 and DMSA or DMPS and by Pb(NO 3 ) 2 and DMSA, however, were not translocated by fNaDC-3. The data suggest that NaDC-3 is an essential component in the delivery of uncomplexed antidotes for renal heavy metal detoxification.Di-and tricarboxylates, such as succinate, ␣-ketoglutarate, and citrate, are actively taken up by renal proximal tubule cells both from the peritubular capillaries and the glomerular filtrate. These compounds serve as metabolic fuels and as substrates for gluconeogenesis (1,2,3). In addition, intracellular ␣-ketoglutarate drives the uptake of organic anions by an organic anion/ dicarboxylate exchanger, OAT1, located in the basolateral membrane (4,5). This exchange process constitutes the first step in the proximal tubular excretion of a large number of organic anions, including widely used drugs such as ␤-lactam antibiotics, antiviral drugs, diuretics, and nonsteroidal antiinflammatory drugs (6 -9).The transport of di-and tricarboxylates across the luminal and the basolateral cell membrane of proximal tubule cells is mediated by two distinct Na ϩ -dependent dicarboxylate transporters, NaDC-1 and NaDC-3 [reviewed in references 10 and 11]. Both translocate three sodium ions and a di-or tricarboxylate in its divalent form. NaDC-1 has been cloned from rabbit, rat, mouse, and human. Except for rat NaDC-1 (or SDCT1), these transporters exhibit a comparatively low affinity for succinate (K m above 0.2 mM). By immunohistochemical studies, rabbit and rat NaDC-1 were located to the luminal membrane of proximal tubule cells. NaDC-3 (or SDCT2) cloned from human (12), rat (13,14), mouse (15), and flounder (16) showed a comparatively high affinity for succinate in radiotracer uptake experiments (K m : 20 Ϯ 1 M for human, 15 Ϯ 1...

Organic anion transporters OAT1 and OAT4 mediate the high affinity transport of glutarate derivatives accumulating in patients with glutaric acidurias

“…The two dicarboxylates with the lowest affinities (3OHGA, GC) exhibited a potentialdependent K m , whereas unsubstituted and C2-substituted dicarboxylates showed K m values independent of membrane potential between −90 and −30 mV. The heavy metal chelator, 2,3-dimercaptosuccinate, also had a K m increasing with depolarization [22], suggesting that hNaDC3 has to perform a voltage-dependent step in interacting with compounds carrying a substitution at C3 or a double bond between C2 und C3. The maximal currents, I max , decreased in the order D-2OHGA>3OHGA=L-2OHGA>GA= adipate> GC>αKG.…”

Determinants of Substrate and Cation Transport in the Human Na+/Dicarboxylate Cotransporter NaDC3

“…In addition to providing citric acid cycle metabolites for energy metabolism, NaDC3 also participates in the secretion of drugs and xenobiotics by providing glutarate and ␣-ketoglutarate to the organic anion exchangers OAT1 and OAT3, which are important for drug pharmacokinetics (9). The range of NaDC3 substrates also includes the clinically used heavy metal poisoning antidotes, such as succimer (10), and the antioxidant glutathione (11). NaDC3 is also important for the transport of N-acetyl aspartate in neurons and astrocytes, thought to contribute to osmoregulation in the brain (12).…”

Renal Uptake of 99mTc-Dimercaptosuccinic Acid Is Dependent on Normal Proximal Tubule Receptor–Mediated Endocytosis

“…However, the significance of both routes is debated (3)(4)(5)(6)(7)(8)(9)(10)(11)(12). The sodium-dependent dicarboxylate transporter NaDC-3 has been implicated in the basolateral uptake of 99m Tc-DMSA from peritubular capillaries into proximal tubule cells (13). The mechanism by which 99m Tc-DMSA may be reabsorbed from the glomerular ultrafiltrate is unknown; however, studies have indicated that this route may contribute substantially to the renal uptake of the tracer (4,5,14).…”