Mechanical instability of sickle haemoglobin Further evidence as to cause and clinical implications

Denaturation of oxyhemoglobulin S by mechanical shaking

The oxy form of sickle hemoglobin precipitates rapidly during mechanical shaking. The rate of precipitation depends on shaking conditions such as stroke amplitude, frequency, angle between the vial axis and the shaking motion, volume and viscosity of hemoglobin solution and temperature. The rate increases linearly with either stroke (S) or frequency (n) above a certain value (S = 5 mm or n = 10Hz). The rate constant was maximum when the angle 0 between the vial axis and the shaking direction was 30 degrees. The rate of precipitation of sickle oxyhemoglobin was faster at high temperatures and always about 10 times greater than that of normal hemoglobin. The activation energies of precipitation of hemoglobin S and A are 20.3 and 21.3 kcal/mol, respectively. The thermogram of hemoglobin was measured by a differential scanning calorimeter. The denaturation temperatures determined from the peak of denaturation curves were 83.5 degrees C for oxyhemoglobin S and 85 degrees C for oxyhemoglobin A. The precipitated hemoglobin after mechanical shaking did not show any denaturation peak indicating that the protein molecules are irreversibly denatured by shaking.

Interaction of serum albumin with normal and sickle hemoglobins

The stability of oxyhemoglobin S during mechanical shaking was enhanced by the addition of human serum albumin. The stabilizing effect was maximum when the concentration of serum albumin approached that of oxyhemoglobin, suggesting a molecular level interaction between them. The effects of serum albumin on oxyhemoglobin A were essentially similar to those on oxyhemoglobin S. Deoxy- and methemoglobins were also stabilized by serum albumin. The addition of human serum albumin to a solution containing sickle cell oxyhemoglobin slowly formed a compound which had an absorbance peak at 620 nm. After purification by Sephadex G-200 column chromatography, this compound was identified as methemalbumin. Comparison of the rates of formation of methemalbumin from hemoglobin with various ligand states and human serum albumin showed that the rate of formation from hemichrome was much faster than from met-, oxy- and deoxyhemoglobin. About 60% of the heme was transferred from hemichrome to albumin when the mixture was kept standing at room temperature for 5 min, in contrast to only 5% from methemoglobin. This result suggests that hemichrome, rather than methemoglobin, is the intermediate in the formation of methemalbumin from oxyhemoglobin and human serum albumin. This hypothesis is supported by the finding that the rate of formation of methemalbumin was faster at alkaline pH values than at acid pH values. Serum albumin from various animal sources showed different stabilizing effects. The formation of methemalbumin from these animal albumins was far less than that from human albumin.