Uric acid organic anion transport

Certain molecules, such as p-aminohippuric acid (Fig. 3.18), a metabolite of p-aminobenzoic acid are actively transported from the bloodstream into the tubules by a specific anion transport system. Organic anions and cations appear to be transported by separate transport systems located on the proximal convoluted tubule. Active transport is an energy-requiring process and therefore may be inhibited by metabolic inhibitors, and there may be competitive inhibition between endogenous and foreign compounds. For example, the competitive inhibition of the active excretion of uric acid by compounds such as probenecid may precipitate gout. 

Sulfinpyrazone competitively inhibits the active transport of organic anions across the kidney tubule, both from the plasma to the tubular fluid and vice versa. The effect is dose-dependent for at low dose sulfinpyrazone prevents secretion of uric acid into tubular fluid, and at high dose, and more... 

At least partial clarification is provided by recent work fo Zins and Weiner (59) which demonstrated a secretory flux for taurocholate in the proximal tubule which can be inhibited by the administration of p-aminohip-purate, a secreted substance. This flux, which presumably traverses the well-known mechanism for organic anion secretion, is normally overshadowed by the larger active reabsorptive process which occurs in the same tubular segment. This is not a unique example of bidirectional transport in a single tubular segment uric acid is similarly handled (60). 

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