Fig. 6.

O₂ affinities of purified Hbs from representative pairs of high- and low-altitude mammals. O₂ equilibria were measured at pH 7.40, 37°C, in the presence and absence of allosteric effectors ([Cl⁻], 0.10 mol l⁻¹; [Hepes], 0.1 mol l⁻¹; DPG:tetrameric Hb ratio, 2.0: [heme], 0.2–0.3 mmol l⁻¹). For each taxon, P₅₀ values (±s.e.m.) 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 standardized treatments can provide insights into the functional mechanism responsible for observed differences in P_50(KCl+DPG). (A) Comparison between Hb variants of high- and low-altitude deer mice, Peromyscus maniculatus, from the Rocky Mountains and Great Plains, respectively [data from Natarajan et al., 2015a; for additional details, see Storz et al. (2009, 2010a); Jensen et al. (2016)]. (B) Comparison between Hbs of the high-altitude American pika (Ochotona princeps) and the low-altitude collared pika (O. collaris) (data from Tufts et al., 2015). (C) Comparison between Hbs of the high-altitude yellow-bellied marmot (Marmota flaviventris) and the low-altitude hoary marmot (M. caligata) (data from Revsbech et al., 2013). (D) Comparison between Hbs of the high-altitude snow leopard (Panthera uncia) and the low-altitude African lion (P. leo) (data from Janecka et al., 2015). For both cat species, P₅₀ is shown as the mean value for two co-expressed isoforms, HbA and HbB, which are present at roughly equimolar concentrations (Janecka et al., 2015).