Reversible inhibition of anion exchange in human erythrocytes by an inorganic disulfonate, tetrathionate

Abstract

Tetrathionate (S4O6--) markedly inhibits anion exchange across the human erythrocyte membrane. This phenomenon has been studied in order to obtain further insight into the mechanism of action of reversible inhibitors, in particular disulfonate inhibitors, of anion exchange. Anion fluxes were measured by tracer techniques at equilibrium. The following results were obtained: Tetrathionate, although an inorganic compound, inhibits the self-exchange of sulfate and of divalent organic anions (oxalate, malonate) noncompetitively at Ki values (less than or equal to 0.5 mM) as yet only observed for amphiphilic inhibitors. The inhibitor is effective only from the outside of the cell. The inhibition is temperature-dependent, Ki increasing by a factor of 5 between 5 and 35 degrees C, and instantaneously and fully reversible. The presence of small monovalent anions (fluoride, bromide, chloride, nitrate, acetate) counteracts inhibition by tetrathionate to a varying and concentration-dependent extent, divalent anions have only a minor effect at high concentrations. Chloride exchange is also inhibited, while glycolate and lactate fluxes are much less sensitive or almost insensitive, in agreement with their alleged transfer by a different transport system. Tetrathionate is unique in its inhibitory action, its structural congeners, peroxodisulfate (S2O8--) and ethanedisulfonate (C2H4S2O6--) are much less effective. The results can be interpreted by assuming that tetrathionate inhibits the movement of anions via the inorganic anion exchange system by binding--in a 1 : 1 stoichiometry--to inhibitory "modifier sites", for which it competes with other anions. These sites are located only on the exofacial surface of the membrane. The high affinity of tetrathionate is probably due to a local excess of pi electrons in the region of its central disulfide bond. These may stabilize the binding by their ability to form electron donor-acceptor complexes with membrane sites, thus compensating for the absence of a hydrophobic binding domain in tetrathionate.