Properties of sickle-cell haemoglobin

Abstract

The specific viscosity of sickle-cell hemoglobin was measured under different condi tions. Sickle-cell oxyhemoglobin shows the same type of viscosity behavior as normal adult oxyhemoglobin and hemoglobin. The viscosity rises steadily as the concentration is increased from 0.5 to 25 g/100 ml and falls at any one concentration as the temperature is increased from 0[degree] to 40[degree]. When the concentration of sickle-cell hemoglobin is raised above 12 g/100 ml the viscosity increases much more rapidly than that of normal adult hemoglobin, until at 16 g /100 ml a completely rigid, birefringent, liquid crystalline phase is formed. When the concentra tion of the solution is between these 2 critical points, spindle-shaped liquid crystalline masses or tactoids, separated by hemoglobin in a state of low order, are seen. When concentrated sickle-cell oxyhemo globin solutions are deoxygenated at low temperatures (0-15[degree]) the viscosity increment is small; it seems that finite polymers are formed. When the temperature is raised the usual liquid crystalline phase develops; this disappears when the solution is cooled. The increment in viscosity usually observed when concentrated sickle-cell oxyhemoglobin solutions are deoxygenated does not occur in the presence of molar urea or 0.5 [image] guanidinium chloride, but is unaffected by changes in pH from 6.0 to 8.0 or changes in the ionic strength of the solvent from 0.01 to 1. Hydrogen bonding may play an important part in aggregation of sickle-cell hemoglobin molecules, and formation of salt linkages only a minor part. Sickling of susceptible cells, and aggregation of sickle-cell hemoglobin molecules, do not take place in the presence of Ag+, Hg2+ and p-chloromercuribenzoate ions in concentrations of 4, 2 and 2 respectively per molecule of hemoglobin. These compounds form covalent bonds with exposed sulfhydryl groups in the vicinity of the complementary sites where sickle-cell hemoglobin molecules normally combine with one another (probably close to the heme groups), so that by steric hindrance the combination is prevented. Measurements of the viscosity of hemoglobin mixtures suggest that sickle-cell hemoglobin forms mixed aggregates with normal adult hemoglobin and hemoglobin C, but not with human fetal hemoglobin. Hemoglobin C can replace more molecules of sickle-cell hemoglobin in the mixed aggregates than can normal adult hemoglobin. This finding, and the formation of finite sickle-cell hemoglobin polymers at low temperatures, can be explained by postulating a helical rather than a linear process of aggregation of sickle-cell hemoglobin molecules when they are deoxygenated.