The chloride shift may facilitate oxygen loading and unloading to/from the hemoglobin from the brown bear (Ursus arctos L.).

1. The oxygen binding properties of hemolyzed bear blood were studied in 0.1 M Tris and 0.1 M Hepes buffer with respect to the possible effects of temperature, pH, pCO2, 2,3-DPG, and chloride ions. 2. There was a significant Bohr shift with a Bohr factor (delta log P50/delta pH) of the magnitude of -0.5. The temperature sensitivity expressed by the apparent heat of oxygenation minus the heat of oxygen in solution was about -8.1 kcal/mol at pH 7.4. 3. Chloride ions decreased the oxygen affinity in the concentration range 50-200 mM, and there was a marked increase in the co-operativity of oxygenation up to a chloride concentration of about 200 mM. 4. There were no effects of pCO2 and 2,3-DPG in the presence of 200 mM Cl-, while in the absence of Cl-, 2,3-DPG had the same effect as 200 mM Cl- at 37 degrees C and pH 7.4. 5. Our results suggests at least two different binding sites for the chloride ion, one high affinity site which may also bind 2,3-DPG in the absence of chloride, and one or more low affinity sites, which only binds chloride. 6. The results further show, that a chloride shift of about 33 mM may account for as much as a 40% increase in O2 unloading, without taking into account the additional effect of the Bohr shift.