Oxygen affinity and Bohr effect responses to 2,3-diphosphoglycerate in equine and human blood

The dependence of blood oxygen affinity and the Bohr effect on the concentration of 2,3-diphosphoglycerate (DPG) in erythrocytes was investigated in 24 trotter horses and 24 healthy men. The oxygen tension at half saturation and standard conditions (P₅₀st at pH 7·4 PCO₂40 mniHg and 37°C) and the carbon dioxide or fixed-acid-induced Bohr effect (dlogP₅₀/dpH) were determined. Samples of fresh blood and blood depleted of or enriched with DPG were studied. In the absence of measurable DPG, the equine and human blood had similar mean (SD) values of P₅₀st (16·6 0·6] and 16·2 0·7] mmHg, respectively). In both species these values increased with increasing DPG, but the response of equine blood was significantly lower, at least up to physiological values (P₅₀st =24·6 0·6] and 26·2 0·7]) mmHg; DPG=14(1·8] and 12·8 1·2] μmol gHb⁻¹, respectively, in fresh blood). concentrations above 20 to 25 μmol gHb⁻¹ of DPG the difference between the values of P₅₀st in the two species tended to de crease because the response in human blood reached a plateau. The interactions between the Bohr effect and the concentration of DPG showed that in the horses, as in the men, the level of DPG played an important role in governing the relative magnitude of carbon dioxide and fixed acid factors. The difference between them, which is associated with the oxylabile carbamino binding, was greatest in DPG-depleted blood, but whereas in the men the difference was suppressed by an above normal DPG concentration, in the horses it was still measurable.