Kinetics and Mechanism of Deoxyhemoglobin S Gelation: A New Approach to Understanding Sickle Cell Disease

Abstract

We report the results of a kinetic investigation on the gelation of purified deoxyhemoglobin S. Gelation was induced by raising the temperature and was monitored by measuring both the heat absorbed, with a microcalorimeter, and the appearance of linear birefringence, with a microspectrophotometer. The kinetics are unusual. Prior to the onset of gelation there is a delay period, followed by a sigmoidal progress curve. The delay time is formally dependent on approximately the 30th power of the deoxyhemoglobin S concentration; a decrease in concentration from 23 to 22 g/dl increases the delay time by a factor of four. It is also extremely temperature dependent; a 1C temperature rise in the range 20-30'C almost halves the delay time. From these results we conclude that the initial rate is controlled by the nucleation of individual fibers. We present a kinetic model that accounts for the concentration, temperature, and time dependence of the initial phase of the gelation reaction. Extrapolation of our data to physiological conditions predicts that changes in intracellular hemoglobin concentration and oxygen saturation, realizable in vivo, produce enormous changes in the delay time. The range of delay times spans both the mean capillary transit and total circulation times. This result points to the delay time as an extremely important variable in determining the course of sickle cell disease, and suggests a new approach to therapy.Hemoglobin S in concentrated solutions aggregates to form a highly viscous material, referred to as a gel. It is the formation of this gel that rigidifies and distorts deoxygenated erythrocytes of patients with sickle cell anemia. Considerable new insight into the structure and equilibrium behavior of this system has been gained through recent electron microscope (1), x-ray diffraction (1, 2), optical (3), solubility (4), sedimentation (5-7), and theoretical (8, 9) studies. The gel may be tentatively described as consisting of two phases in reversible equilibrium: a liquid phase containing mainly monomeric (molecular weight of 64,000) hemoglobin and a solid phase containing polymeric hemoglobin in the form of bundles of long straight fibers (compare ref. 8). The-solid phase, which exhibits optical birefringence and other properties of a liquid crystal, is favored by low oxygen concentrations and high temperatures.Much less is known about gelation kinetics. The rate of gelation of deoxyhemoglobin S at a fixed temperature T may be measured in two ways:(1) oxyHbS (T)In the first experiment gelation is induced by removing oxygen from an oxyhemoglobin S solution. In the second, gelation is induced by raising the temperature of an already deoxygenated hemoglobin S solution from 0C, where it is a nonbirefringent liquid, to the higher temperature T. Although the first experiment may be considered more relevant to the in vivo sickling process, the two experiments should give identical results if the deoxygenation and temperature change are both fast when compared to the rate of gelation. Neit...

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