Fig. 3.

O₂ affinities of HbA and HbD isoforms from high- and low-altitude populations of house wrens. (A) P₅₀ values (mean ± SEM) for purified HbA variants of highland and lowland house wrens measured in 0.1 M Hepes buffer at pH 7.4 and 37 °C in the absence (stripped) and presence of allosteric effectors ([Cl⁻], 0.1 M; IHP/Hb tetramer ratio, 2.0; [heme], 0.3 mM). (B) P₅₀ values for HbD variants of highland and lowland wrens (experimental conditions as in A).