Molecular and physicochemical characterization of hemoglobin from the high-altitude Taiwanese brown-toothed shrew (Episoriculus fumidus)

Red-toothed shrews (subfamily Soricinae) exhibit the highest mass-specific rates of O-2 consumption recorded among eutherian mammals, though surprisingly no data appears to be available on the functional characteristics of their hemoglobin (Hb). As a first step in addressing this shortcoming, we investigated the O-2 binding characteristics of Taiwanese brown-toothed shrew (Episoriculus fumidus) Hb and its temperature and pH dependence in the absence and presence of anionic red blood cell effectors. Although comparative data regarding the intrinsic O-2 affinity of other shrew species are currently unavailable, our data suggest that the sensitivity of this high-elevation endemic species' Hb to allosteric effector molecules is similar to that of the two lowland species of white-toothed (crocidurine) shrews examined to date. The efficient exploitation of blood O-2 reserves by E. fumidus appears to be achieved via synergistic modulation of O-2 affinity by Cl- and organic phosphates that moreover dramatically lowers the overall enthalpy of oxygenation of their Hb. Oxygen unloading is presumably further enhanced by a relatively high Bohr effect (Delta Log P (50)/Delta pH = -0.69) and marked reduction in the titratable histidine content (predicted low proton buffering value) of the component globin chains relative to human HbA. Notably, however, the limited data available suggest these latter attributes may be widespread among shrews and hence likely are not adaptations to chronic altitudinal hypoxia per se.