Inhibition of anion transport in the red blood cell by anionic amphiphilic compounds. II. Chemical properties of the flufenamate-binding site on the band 3 protein.

Flufenamate is a powerful inhibitor of anion exchange in red blood cells. It binds to the band 3 protein involved in the transport as discussed in the preceding paper (Cousin, J.-L. and Motais, R. (1982) Biochim. Biophys. Acta 687, 147-155). The present study is concerned with the chemical properties of the inhibitory binding site. Structure-activity studies were performed with two sets of compounds derivated from anthranilate (considered as the basic structure of flufenamate). The molar concentrations required to produce 50% inhibition (I50) varied over more than a 10(4) range. The inhibitory activity was quantitatively correlated with the hydrophobic character of the molecules and the electron-withdrawing capacity of the substituents. Comparison between the inhibitory potency of flufenamate analogs made a definition of the contribution of each part of the molecule in the binding to the receptor possible. The results suggest that anionic inhibitors bind to a site which presents a positively charged groups at the water-protein interface whereas the hydrophobic part of the molecule is inserted into an hydrophobic and electron-donor region of the protein. The specificity of amphiphilic compounds towards anion transport is discussed.