Interaction of thiourea with band 3 in human red cell membranes

Although urea transport across the human red cell membrane has been studied extensively, there is disagreement as to whether urea and water permeate the red cell by the same channel. We have suggested that the red cell anion transport protein, band 3, is responsible for both water and urea transport. Thiourea inhibits urea transport and also modulates the normal inhibition of water transport produced by the sulfhydryl reagent, pCMBS. In view of these interactions, we have looked for independent evidence of interaction between thiourea and band 3. Since the fluorescent stilbene anion transport inhibitor, DBDS, increases its fluorescence by two orders of magnitude when bound to band 3 we have used this fluorescence enhancement to study thiourea/band 3 interactions. Our experiments have shown that there is a thiourea binding site on band 3 and we have determined the kinetic and equilibrium constants describing this interaction. Furthermore, pCMBS has been found to modulate the thiourea/band 3 interaction and we have determined the kinetic and equilibrium constants of the interaction in the presence of pCMBS. These experiments indicate that there is an operational complex which transmits conformational signals among the thiourea, pCMBS and DBDS sites. This finding is consistent with the view that a single protein or protein complex is responsible for all the red cell transport functions in which urea is involved.