Martial, Sonia, Bernadette Olivès, Laurence Abrami, Cécile Couriaud, Pascal Bailly, Guofeng You, Matthias A. Hediger, Jean-Pierre Cartron, Pierre Ripoche, and Germain Rousselet. Functional differentiation of the human red blood cell and kidney urea transporters. Am. J. Physiol. 271 (Renal Fluid Electrolyte Physiol. 40): F1264-1268, 1996.-The recent cloning of two urea transporters will allow to better understand their role in the urinary concentrating mechanism. This physiological approach needs to be sustained by a knowledge of their functional characteristics. We compared the pharmacological properties of the human red blood cell and kidney urea transporters (HUT11 and HUT2) in the Xenopus oocyte expression system. Both proteins allow the rapid transfer of urea but not of water. Both are inhibited by phloretin, although with different half-maximal inhibitory concentrations (IC50 75 uM for HUT11 and 230 uM for HUT2). Whereas para-chloromercuribenzene sulfonate inhibits HUT11 with an ICsO of 150 uM, it does not inhibit HUT2, whatever the concentration used. We demonstrate that thiourea diffuses through HUT11 with a Michaelis constant (K,) of 40 mM, but not through HUT2. In contrast, it inhibits urea transport through both proteins. This identification of a substrate binding site independent from the transport activity is the first step in the understanding of the molecular events underlying urea transport. UT2; HUT11; phloretin; para-chloromercuribenzene sulfonate