Oxygen binding properties of blood and hemoglobin from the pigeon Columba livia

Oxygen binding curves were measured on pigeon red blood cells (RBCs) suspensions, at the temperatures of 25, 30, 37 and 41°C, from pH 7.0 to 8.2, and on pigeon hemoglobin solutions, in the absence and presence of organic phosphates. Data were analysed on the basis of the MWC model. P₅₀ for pigeon RBCs is 46.9 mmHg, at pH 7.4 and 41°C. This low O₂ affinity is due, in part, to the existence of a large pH gradient across the membrane of the RBCs. The overall Bohr effect ($\text{Δ}\text{log}\text{P}{}_{\mathrm{<mtext>m</mtext>}}\text{Δ}\text{pH}$) is –0.44 in the 7.0–8.2 extracellular pH range and increases to –0.70 if intracellular pH is considered (pHi range, 6.8–7.5). A significant pH dependence for K_T and K_R is observed. The maximal Hill coefficient (n_max) for pigeon RBCs never exceeds 3.35 in all the conditions of pH and temperature examined, n_max is slightly higher for RBCs than for hemoglobin dilute solutions. This might denote a diminished solubility of hemoglobin molecule in the physiological milieu, and is not observed for the oxygen equilibrium of human RBCs and hemoglobin solutions. Apart from this, the functional properties of pigeon RBCs are close to those of hemoglobin solutions, under comparable concentrations of protons, Cl^– and organic phosphates, and for the same temperature. The enthalpy change for overall oxygenation of pigeon RBCs is rather small (about – 7 kcal/mol).