Datensatz

Zusammenfassung

The inhibition of inorganic anion transport by dipyridamole (2,6-bis(diethanolamino)-4,8-dipiperidinopyrimido[5,4-d] pyrimidine) takes place only in the presence of Cl^-, other halides, nitrate or bicarbonate. At any given dipyridamole concentration, the anion flux relative to the flux in the absence of dipyridamole follows the equation: J_rel = (1 + alpha 2[Cl^-])/(1 + alpha 4[Cl^-]) where alpha 2 and alpha 4 are independent of [Cl^-] but dependent on dipyridamole concentration. At high [Cl^-] the flux approaches alpha₂/alpha₄, which decreases with increasing dipyridamole concentration. Even when both [Cl^-] and dipyridamole concentration assume large values, a small residual flux remains. The equation can be deduced on the assumption that Cl^- binding allosterically increases the affinity for dipyridamole binding to band 3 and that the bound dipyridamole produces a non-competitive inhibition of sulfate transport. The mass-law constants for the binding of Cl^- and dipyridamole to their respective-binding sites are about 24 mM and 1.5 microM, respectively (pH 6.9, 26 degrees C). Dipyridamole binding leads to a displacement of 4,4'-dibenzoylstilbene-2,2'-disulfonate (DBDS) from the stilbenedisulfonate binding site of band 3. The effect can be predicted quantitatively on the assumption that the Cl^- -promoted dipyridamole binding leads to a competitive replacement of the stilbenedisulfonates. For the calculations, the same mass-law constants for binding of Cl^- and dipyridamole can be used that were derived from the kinetic studies on Cl^- -promoted anion transport inhibition. The newly described Cl^- binding site is highly selective with respect to Cl^- and other monovalent anion species. There is little competition with SO4^2-, indicating that Cl^- binding involves other than purely electrostative forces. The affinity of the binding site to Cl^- does not change over the pH range 6.0-7.5. Dipyridamole binds only in its deprotonated state. Binding of the deprotonated dipyridamole is pH-independent over the same range as Cl^- binding.