Comparative effects of CO2 on the affinity for O2 of fetal and adult erythrocytes.

1. Oxygen-linked carbamino formation in fetal erythrocytes was compared to that measured in adult erythrocytes. 2. Whole oxygen binding curves were recorded on washed intact erythrocytes either fresh or D-glycerate-2,3-P depleted with a continuous recording technique. Erythrocytes were resuspended in buffer media of different pH and PCO2 varying from 0-10.7 kPa (80 torr) at physiological ionic strength. Oxygen linked carbamates were estimated as deltalog PO2/delta log PCO2 at constant pH and constant saturation levels from 10-90% oxygen saturation. 3. The overall CO2 effect (deltalog P50/deltalog PCO2) was consistently lower in fetal erythrocytes than in the adult. The deltalog PO2/deltalog PCO2 ratio was markedly dependent on oxygen saturation in both types of erythrocytes and highest at the early part of the oxygen binding curve. This was more so in fetal erythrocytes. 4. Carbamino formation was lower in fetal erythrocytes than in adult erythrocytes at any pH value, indicating a higher apparent pK of the alpha amino groups involved in CO2 binding in fetal erythrocytes. This may be related to the different primary structures of the non alpha chains of HbFII and HbAI. 5. The large effect of low PCO2 on both fetal and adult erythrocytes was related to the higher affinity for CO2 of deoxyhemoglobin compared to oxyhemoglobin and a model for CO2 binding analogous to that described by de Bruin et al. [6] for anion binding is proposed. 6. It is concluded that the lower CO2 binding to fetal erythrocytes is in keeping with the lower allosteric effect of other major effectors of hemoglobin within the cells. This leads to a higher affinity for O2 of fetal erythrocytes well suited for O2 transport in utero.