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Abstract

The disulfonic acid APMB produces maximally about 85% inhibition of sulfate equilibrium exchange. Inhibition is half maximal at about 0.4 mM APMB. The reversibly binding APMB shares binding sites with the irreversibly binding inhibitors DNFB and H₂DIDS on the protein in band 3 and on the anion transport system. This was demonstrated by observing the inhibitory effect of increasing concentrations of APMB on the rate of dinitrophenylation of common binding sites of DNFB and H₂DIDS on band 3 and on the anion transport system. A mathematical description of the effect of APMB on dinitrophenylation of the anion transport system was consistent with the assumption that APMB reacts with 85% of the DNFB binding sites with a K_m value which is close to the K_I value observed in studies of the inhibitory effect of APMB.

An investigation of the effects of increasing sulfate concentration on sulfate transport showed, in accordance with previous work of Schnell (1975), that the sulfate flux passes through a maximum which suggests the existence of an inhibitory modifier site in addition to a transfer site. Inhibition by APMB decreases slightly over the sulfate concentration range 30 to 300 mM, with half maximal inhibition at APMB concentrations of 0.9 and 0.4 mM, respectively, at the upper and lower limits of this concentration range. The inferred low affinity of SO₄ for modifier and transfer sites and the variations of ionic strength associated with variations of sulfate concentration made it impossible to decide whether sulfate reduces inhibition by a specific reaction with one or both of the two sites, or by other interactions not involving specific binding sites.