Fig. 3.

Highland deer mice (Peromyscus maniculatus) from the Rocky Mountains have a higher Hb-O₂ affinity than lowland mice from the Great Plains, as indicated by lower values of P₅₀ (the PO₂ at which Hb is 50% saturated). O₂ equilibria were measured at pH 7.40, 37°C, in the presence and absence of allosteric cofactors that modulate Hb-O₂ binding ([Cl⁻], 0.10 mol/l; [HEPES], 0.1 mol/l; DPG:tetrameric Hb ratio, 2.0: [heme], 0.2–0.3 mmol/l). P₅₀ values (± SE) are reported for stripped Hbs in the absence of added anions, in the presence of Cl⁻ alone (added as KCl), in the presence of DPG alone, and in the presence of both anions combined. This latter “KCl+DPG” treatment is most relevant to in vivo conditions in mammalian red blood cells, but measurements of O₂ affinity under each of the four treatments provide insights into the functional mechanism responsible for observed differences in P_50(KCl+DPG). Data from Natarajan et al. (2015a) (for additional details, see Storz et al. 2009, 2010a and Jensen et al. 2016).