Reciprocal effects in human hemoglobin: direct measurement of the dimer-tetramer association constant at partial oxygen saturation.

An equilibrium gel permeation technique has been developed for determining as a function of oxygenation state the equilibrium constants for association of hemoglobin subunits. By using this method, the dimer-tetramer constant for human hemoglobin at a partial oxygenation state corresponding to 20% saturation for tetramers has been determined as 3.7 X 10(6) M-1 (dimers). Under the same conditions the corresponding constant for fully oxygenated hemoglobin is 4.1 X 10(5) M-1. These results are found to be in good agreement with the predicted behavior of the association reaction based upon oxygen binding curves measured as a function of protein concentration. Thus a high degree of consistency is found between the two independent experimental approaches to the reciprocal effects of this linkage system, lending support to the theory proposed earlier for these phenomena.