Transport of Urate and Other Organic Anions by Anion Exchange in Human Renal Brush-Border Membrane Vesicles

Abstract

Earlier studies demonstrated that an apical anion exchanger which accepts urate, chloride, aliphatic monocarboxylates, and the aromatic anions, pyrazinoate (PZA) and nicotinate, plays a major role in urate reabsorption in human proximal tubules. In substrate competition studies for transport, we investigated the effect of PZA on 50 µM [¹⁴C]-urate uptake in exchange of lactate, human brush-border membrane vesicles (BBMV) being loaded with 5 mM lactate. 1 mM PZA cis-inhibited, whereas 0.1 mM PZA cis-stimulated [¹⁴C]-urate uptake in exchange for lactate. Such paradoxical effects suggest the cooperation of two anion exchangers, PZA having more affinity for a ‘lactate/anion exchanger’ than for a ‘urate/ anion exchanger’. Apparent K_m of [¹⁴C]-PZA uptake were measured in lactate- or urate-loaded BBMV, and were 0.33 ± 0.06 and 0.98 ± 0.44 mM, respectively. Unlike [¹⁴C]-PZA, [¹⁴C]-orotate and [¹⁴C]-nicotinate uptakes were stimulated only to a minor extent in BBMV loaded with lactate. In contrast, cis -inhibition studies in urate-loaded BBMV demonstrated that nicotinate and orotate had similar affinity as PZA for the ‘urate/anion exchanger’. Oxalate was shown to have affinity for the ‘lactate/anion exchanger’, but not for the ‘urate/anion exchanger’.

Human urate transporter 1 (hURAT1) mediates the transport of orotate

“…In addition, the profile of trans-stimulatory effects (strong stimulation by pyrazinoate and nicotinate, but weak by lactate) for hURAT1-mediated orotate uptake (Fig. 1d) is similar to urate uptake via hURAT1 as previously reported [7]. Since defects in the hURAT1 gene (SLC22A12) lead to idiopathic renal hypouricaemia (MIM number, 220150), hURAT1 is thought to participate in tubular urate reabsorption and to regulate blood urate levels in humans [17][18][19].…”

“…The findings that orotate trans-stimulates the uptake of urate via the urate/anion exchanger in brush-border membrane vesicles in humans [7] and that it inhibits cloned human renal urate transporter hURAT1-mediated urate uptake in Xenopus oocytes [8] prompted us to investigate whether orotate is a true transport substrate for hURAT1. The purpose of this study was to elucidate the molecular mechanism underlying the membrane transport of orotate using HEK293 cells stably expressing hURAT1 (HEKhURAT1).…”

Molecular and Cellular Physiology of Renal Organic Cation and Anion Transport

“…There is, however, little agreement on the mechanism for efflux of PAH and other "classical" OAs, with evidence in some species (dog and rat) consistent with carrier-mediated anion exchange, and in other species (pig and rabbit), consistent with electrogenic facilitated diffusion (342). In human and bovine luminal membranes, the presence of demonstrable OA/␣-KG exchange activity (357,380) further complicates efforts to develop an integrated view of how peritubular and luminal events work in concert to produce transepithelial OA secretion. Recent evidence has implicated the multidrug resistance-associated transport protein MRP2 in the active, ATP-dependent export of some OAs, including PAH (201).…”

Hyperuricemia and gout

“…1). Two anion exchangers have been described in human brush-border membranes, with high and low affinity [2,3], and may have been recently identified at the molecular level [4,5,6••]. Subsequently, proximal tubular secretion contributes to the main excreted uric acid before the so-called postsecretory reabsorption.…”