Evidence for a difference in affinities of human hemoglobin beta A and beta S chains for alpha chains

A limiting amount of isolated human hemoglobin alpha chains was incubated with a mixture containing excess, but equal, amounts of beta A and beta S chains to form hemoglobin S = alpha 2 beta 2S and hemoglobin = alpha 2 beta 2A, presumably by the reaction 2 alpha + 2 beta leads to 2 alpha beta leads to alpha 2 beta 2. The initial concentration of each type of beta chain was varied from levels (greater than microM) at which the tetrameric form = beta 4 was predominant to levels (less than 1 microM) at which the tetramers had dissociated to monomers. The initial relative concentration of alpha chains remained constant at one-twentieth (0.05) that of total beta chains. About 50% as much hemoglobin S as hemoglobin A was formed in each reaction despite the 500-fold range in the beta chain concentrations among the assays. These results suggest that the difference in amounts of hemoglobins S and A formed was caused by a difference in affinities of individual beta S and beta A chain monomers for alpha chains and not by a difference in the concentrations of beta S and beta A monomers resulting from putative unequal rates of dissociation of beta 4S and beta 4S tetramers.