Enzyme activities related to 2,3-P₂-glycerate metabolism in embryonic and fetal red cells

The intraerythrocytic concentration of 2,3-P₂-glycerate decreases conspicuously during the intrauterine development of rabbits. In order to obtain informations on possible causes for these changes, we have measured in embryonic and fetal red cells the activities of those glycolytic enzymes which are thought to be most important in the regulation of the steady-state level of 2,3-P₂-glycerate, namely hexokinase, phosphofructokinase, bisphosphoglyceromutase and pyruvate kinase. Hexokinase and bisphosphoglyceromutase activities are significantly higher in embryonic red cells, when compared to fetal or adult erythrocytes, whilst the activity of phosphofructokinase is not significantly different at either developmental stage. Most important are the changes found in the activity of pyruvate kinase which is low in embryonic and early fetal erythrocytes and increases drastically thereafter. The activity of this enzyme is inversely related to the concentration of 2,3-P₂-glycerate, which is highest in embryonic red cells and very low in the late fetal period. These results support the conclusion that the pyruvate kinase activity is of great importance for the adjustment of the concentration of 2,3-P₂-glycerate and therefore of the blood oxygen affinity during rabbit ontogeny.