Abstract
Hemoglobin tetramers which cannot split into alphabeta dimers, because they are covalently cross-linked between the beta chains across the polyphosphate binding site, form complexes with haptoglobin. The reaction is biphasic as measured by fluorescence quenching and peroxidase activity. A complex in which one of the alpha beta dimers of the cross-linked hemoglobin is bound to one of the sites in the divalent haptoglobin molecule, is formed reversibly during the initial fast phase. In the subsequent slower step, this product then either polymerizes, adds another cross-linked hemoglobin molecule or, in the presence of excess haptoglobin, combines with a second haptoglobin molecule. This latter complex, in which two haptoglobin molecules are bridged by a cross-linked hemoglobin tetramer, can still combine with normal alpha beta dimers at the vacant haptoglobin combining sites. In spite of the very low oxygen affinity of the cross-linked hemoglobin, combination with haptoglobin shifts if oxygen affinity to the very high value of the normal hemoglobin-haptoglobin complex.
Highlights
Hemoglobin tetramers which cannot split into a$ dimers, because they are covalently cross-linked between the p chains across the polyphosphate binding site, form complexes with haptoglobin
Aliquots (0.1 ml) of HbXL solution containing 0.30 nmol were mixed with the indicated amount of haptoglobin and incubated at 25” in 0.01 M phosphate buffer, pH 7.0, for 17 hours, followed by measurement of the peroxidase activity
Has combined with one of the sites on the Hp, and that it is formed in a rapid reversible reaction. This intermediate is consistent with the stoichiometry of the overall reaction as observed by the titration of HbXL with haptoglobin using peroxidase activity as the end point (Fig. 6)
Summary
Hemoglobin tetramers which cannot split into a$ dimers, because they are covalently cross-linked between the p chains across the polyphosphate binding site, form complexes with haptoglobin. A complex in which one of the ctp dimers of the cross-linked hemoglobin is bound to one of the sites in the divalent haptoglobin molecule, is formed reversibly during the initial fast phase. In the subsequent slower step, this product either polymerizes, adds another cross-linked hemoglobin molecule or, in the presence of excess haptoglobin, combines with a second haptoglobin molecule. This latter complex, in which two haptoglobin The concept that only the dimer can react with haptoglobin is further supported by the increase in the rate constants with dilutionjl2)
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