Urate uptake in membrane vesicles of rat renal cortex: effect of copper

Abstract

[2-14C]Urate uptake was studied in brush border and basolateral membrane vesicles of rat renal cortex. In the absence of copper, urate equilibrated without metabolism of transported urate. Exposure of the vesicles to copper significantly stimulated uptake, and in these vesicles uptake was also stimulated by NaCl or KCl gradients. Allantoin accumulated in these vesicles due to oxidation of transported urate. This oxidation is ascribed to a copper-stimulated, membrane-associated uricase since purified uricase and the membranes had similar Km values, both were inhibited by oxonic acid, and extramembranal uricase was not detected. Oxonic acid and pyrazinoic acid inhibited both uptake and enzyme activity. These findings suggest that urate uptake is carrier mediated and that uricase may play some role in transport. In addition, it appears that a significant loss of copper occurs during isolation of membrane vesicles that profoundly affects the characteristics of urate uptake. Those properties of the membrane that influence urate uptake, however, can be restored by exposure of the membranes to copper.

The Uricosuric Effect in Rats of E5050, a New Derivative of Ethanolamine, Involves Inhibition of the Tubular Postsecretory Reabsorption of Urate

“…The usefulness of this model has been supported by a micropuncture study (13) in which oxonate was shown not to influence CURATE/CINULIN in rats. However, it was subsequently reported that oxonate inhibited urate uptake concentration-dependently in the brush border and basolateral membrane vesicles of rats (14) and further reported that CURATE/CINULIN was decreased by oxonate in the isolated rat kidney (15). Our preliminary study using rats loaded i.v.…”

Functional Analysis and Molecular Modeling of a Cloned Urate Transporter/Channel

“…In the multiple mammalian species studied, urate excretion represents the net effect of its free filtration at the glomerulus followed by both tubular reabsorption and secretion that occur primarily within the convoluted and straight portions of the proximal tubule [3]. Mechanistically, two modalities of transport have been reported in renal cortical plasma membranes, an electroneutral urate/ anion exchanger [10,24,25,[32][33][34]57] and an electrogenic urate transporter [1,2,36,57].…”

“…It is of note that a polyclonal antibody to affinity purified porcine uricase was used to identify the urate transporter/channel cDNA clone in a rat renal cDNA library [40]. This antibody was selected as a tool to clone this transporter based on a number of characteristics of the electrogenic urate transporter that resides in rat and rabbit renal cortical cell membranes [1,2,36]. First, the renal electrogenic urate transporter displays some similarities to the enzyme uricase [1,2,36,37].…”

“…This antibody was selected as a tool to clone this transporter based on a number of characteristics of the electrogenic urate transporter that resides in rat and rabbit renal cortical cell membranes [1,2,36]. First, the renal electrogenic urate transporter displays some similarities to the enzyme uricase [1,2,36,37]. Second, hepatic uricase functions as a urate transporter when incorporated in proteoliposomes [54] and a highly selective urate channel when inserted in lipid bilayers [39].…”

“…Fourth, this antibody specifically inhibits electrogenic urate transport in rat renal cortical membrane vesicles and finally, in immunocytochemical studies, anti-uricase is reactive in proximal tubules, the nephron site of urate transport [37]. Since the urate transporter/channel cDNA was not only identified with the polyclonal antibody to uricase, but the recombinant UAT protein was immunoreactive to anti-uricase and the antibody blocked urate channel activity in the lipid bilayer system [40], we proposed that this cDNA may be the molecular representation of the renal proximal tubule electrogenic urate transporter [1,2,36]. Based on the wide tissue distribution of the mRNA for this protein, we also postulated that this channel may serve an important "housekeeping" function to permit efflux of urate subsequent to its intracellular production [40].…”