Binding of organic ions by proteins. 2. Effects of 1-benzyl-3-indazoleoxyacetate at low concentration on the oxygen affinity of human hemoglobin.

The oxygen equilibrium of human hemoglobin has been studied in the presence of 1-benzyl-3-indazoleoxyacetate (BZ). The results show that: (a) The overall oxygen affinity of hemoglobin is a function of BZ concentration, but the cooperative character of the equilibrium curve appears insensitive to the drug up to the maximal concentration studied (5 x 10(-2) M); (b) The functional expression of the interaction between hemoglobin and BZ is not affected by the presence of protons, i.e., the change in oxygen affinity determined by BZ is the same at any pH value studied; (c) The aromatic region of BZ molecules is of primary importance for the functional change of hemoglobin; (d) The difference in moles of BZ bound per mole of tetrameric unliganded and oxygenated hemoglobin corresponds to 2; these functionally relevant binding sites on the protein are probably located at alpha 1, beta 1 and alpha 2 beta 2 interfaces; (e) Thermodynamically, entropy effects dominate the reaction between hemoglobin and BZ.