Control of Oxygen Delivery from the Erythrocyte by Modification of Pyruvate Kinase

Abstract

This chapter discusses how modification of pyruvate kinase activity can control oxygen delivery from the erythrocyte. Hemoglobins of a variety of animals fall into two classes: low-0 2 -affinity hemoglobins insensitive to P 2 GA and high-O 2 -affinity, P 2 GA-sensitive hemoglobins. How the levels of 2,3-P 2 GA in the red cell are regulated is an important question that has not yet been fully answered. The discovery that the human red cell contains phosphoglycolate, a powerful activator of the phosphatase may hint at a previously unsuspected mechanism for control of 2, 3-P 2 GA concentration. The best established correlation with 2,3-P 2 GA concentration in red cells is the activity of the enzyme pyruvate kinase. Red cell pyruvate kinase activity can be affected by a variety of factors. The enzyme is an allosteric enzyme showing cooperative homotropic interactions with its substrate phosphoenolpyruvate (PEP) but not with adenosine diphosphate (ADP). The enzyme is activated by fructose bisphosphate (FBP) that appears to stabilize the PEP-binding conformation, decreasing the K m for PEP and changing the PEP saturation curve to a rectangular hyperbola. Activation is not specific to FBP because mannose bisphosphate and glucose bisphosphate show the same activating effects although at high affector concentrations. Erythrocyte pyruvate kinase, in the absence of FBP, is very sensitive to oxidation. Oxidation raises the K m for PEP but not for ADP and causes increased sigmoidicity of PEP saturation. No evidence has been obtained that oxidation of pyruvate kinase in vivo is a reversible control mechanism but it has been suggested that abnormal pyruvate kinase found in some genetic diseases may be an oxidized form of a normal enzyme rather than a mutant enzyme. The chapter presents the role of phosphorylation in the red cell as a determinant of the oxygen delivery capacity of that cell. Cyclic adenosine monophosphate (cAMP) added externally is clearly capable of stimulating phosphorylation of pyruvate kinase and decreasing its affinity for PEP. This necessarily causes an increase in the concentration of 2,3-P 2 GA and a decreased affinity of hemoglobin for oxygen.