Oxygen Transport and 2, 3-Diphosphoglycerate (DPG)

¹ Assistant Professor of Medicine, Department of Medicine, State University of New York at Buffalo, New York

Reports from many laboratories have firmly established that control of oxygen affinity, and therefore, the position of the dissociation curve of whole blood, is effected through the concentration of DPG inside the red cell as well as by pH and temperature. An increase in P₅₀ (shift of the dissociation curve to the right) increases the amount of oxygen delivered by blood at normal arterial and venous oxygen tensions. This is a particularly effective mode of compensation in anemic states. In pulmonary disease, many patients have a right-shifted curve. However, with increasing arterial hypoxemia, the value of a right-shifted curve decreases and may even become a liability. Most likely this accounts for the observation of normal or left-shifted curves in some patients with lung disease. Changes in the position of the dissociation curve must be interpreted not only in terms of overall oxygen delivery, but also in terms of oxygen delivery to individual tissues.

DPG shifts the dissociation curve by direct preferential binding to deoxyhemoglobin, as well as by its effect on intracellular pH. Although strongly influenced by intracellular pH, the control of intracellular DPG concentration is still unclear. In acid-base disturbances, changes in DPG concentration tend to counteract alterations in the dissociation curve produced by the acid-base process, preserving the normalcy of the in viuo oxygen dissociation curve. The dissociation curve in the patient, rather than that obtained in vitro under standard conditions, is the determining factor in gas exchange. DPG is depleted in stored blood, but is rapidly regenerated following transfusion so that the resulting temporary left shift of the dissociation curve is probably innocuous except in the acutely-ill patient. While DPG facilitates oxygen transport, it may hinder carbon dioxide exchange due to its effect in decreasing the formation of hemoglobin-carbamate.