Kinetics and Mechanism of Complex Formation between Hemoglobin and Haptoglobin

The interaction between human hemoglobin and haptoglobin has been followed by measuring the quenching of haptoglobin fluorescence associated with complex formation. The effects of varying the concentration of the reactants suggest that hemoglobin tetramers do not react with haptoglobin, but that hemoglobin subunits are involved. Separated α chains of hemoglobin react with haptoglobin; although separated β chains do not do so, they react rapidly with a previously incubated mixture of haptoglobin and α chains. It is suggested that the haptoglobin site binds α chains specifically, and that the normal reaction between hemoglobin and haptoglobin proceeds either by consecutive binding of α and β monomers or by attachment of αβ dimers through the α chain. The rate of the reaction is markedly dependent on conditions: the observed second order constant may reach 4 x 106 m-1 sec-1, and the true value is probably 7 x 106 m-1 sec-1.