The intracellular distribution of ATP, 2,3-bisphosphog1ycerate(P2-g1ycerate) and Mg2+ was calculated for the oxygenated and deoxygenated human erythrocyte for the normal range and that of pathophysiological variations based on the association constants of the relevant complexes. The data indicate that about 2001, of ATP is bound both in the oxygenated and deoxygenated cells, while 39 and 7301, of P,-glycerate is bound under these conditions. An increase of the free Mg2+ concentration from 0.7 to 1.1 mM is produced by complete deoxygenation of haemoglobin. The calculations would indicate that during deoxygenation of haemoglobin the hexokinase reacts to the increased concentration of the activator Mg2+ and the decline of the inhibitor P,-glycerate with an elevation of its activity which corresponds to experimental data on intact erythrocytes. The P,-glycerate formation rate in deoxygenated cells is stimulated about 2.5 times in comparison to oxygenated cells as estimated from the free concentration of P,-glycerate and the kinetic constants of bisphosphoglycerate mutase. An assessment of the possible influence of other anions including bicarbonate shows that the distribution of the species of ATP, P,-glycerate and Mg2+ are changed by less than 20°/,. Together with the data presented in the accompanying paper, the results given here indicate that the constants and estimates are approximately valid for intracellular conditions. I n the preceding paper [I] the interactions among ATP, 2,3-bisphosphoglycerate (P,-glycerate), Mg2+ and oxygenated and deoxygenated hemoglobin (HbO, and Hb, respectively) were reported. Association constants valid for intra-erythrocytic conditions were given. Glycolysis depends on the state of the metabolites within the cell; only their forms not bound to haemo-globin or other proteins are substrates and effectors in the enzymatic reactions. These forms enter in the mass action equations. Both ATP and P,-glycerate Abbreviations. P,-glycerate, 2,3-bisphosphoglycerate; Hb, deoxygenerated haemoglobin; HbO,, fully oxygenated haemoglobin; K,,, association constant; noHb system, mixture of metabolites without haemoglobin. Enzymes. play an important role in the regulation of glycolysis. A model of the regulation of the glycolytic chain in erythrocytes therefore requires the knowledge of the binding of these compounds. It is the aim of the present communication to describe quantitatively the intracellular distribution of the various species of ATP, P,-glycerate and Mg2+ for the oxygenated and deoxygenated cell based on the association constants of the relevant complexes. The range of variation for ATP, P,-glycerate and Mg2+ was chosen so as to cover both the normal range and that of pathophysiological variations. The data were compared with an assumed haemoglobin-free cell. CALCULATIONS
