Eaton's "The biophysics of sickle cell hydroxyurea therapy" 1995

From Biol557

• Hydroxyurea can help reduce symptoms of sickle cell patients.
• This is the first specific treatment for this class of genetic diseases.
• The pathology of sickle cell works like this:
  • abnormal hemoglobin S gets deoxygenated and (because it is abnormal) it polymerizes to form a viscous gel,
  • there is a large decrease in the deformability of RBCs with abnormal hemoglobin S,
  • occlusion of microcirculation vessels occurs because of stiffened cells,
  • insufficient oxygen supplication occurs,
  • tissue is damaged by lack of oxygen, often resulting in episodes of severe pain called sickle cell crisis.
• It has been shown that in patients with a higher hemoglobin F to hemoglobin S ratio have decreases symptoms.
• Hydroxyurea increases the hemoglobin F side of the ratio.
• We don't know how hydroxyurea increases hemoglobin F, but we do know how an increase in hemoglobin F decreases symptoms.
• The kinetics of polymerization are very important in this disease:
  • Polymerization is extremely sensitive to the concentration of (damaged )hemoglobin S; in the chemical reaction, the function of hemoglobin S is to the 30th power!
    • The more hemoglobin S is present, the faster it will polymerize.
  • It takes about 1 second for a RBC to get through the capillaries on it's circuit.
  • Recall that deoxygenation occurs just before or during transit across capillaries.
  • If polymerization occurs within the 1 second after deoxygenation, during which the RBC is in the capillaries, it will cause blockage and slow circulation.
  • If the kinetics of polymerization are delayed beyond the 10 to 20 seconds it takes a RBC to get back to the lungs for re-oxygenation, then polymerization will not occur.
• The authors show that the great dependence on concentration is due to a double nucleation event. That is, it is due to the fact that polymerization takes place by two different mechanisms.
  • The first mechanism is "homogenous" and slow and builds a linear polymer.
  • The occurrence of the first polymerization allows for the second mechanism to occur.
  • The second mechanism is called "heterogenous" polymerization and is fast.
  • Polymerization is dependent on the first event occurring and is therefore delayed.
  • Once started, polymerization grows rapidly because as the surface area grows, the surface area upon which polymerization can occur grows, too.
  • They have showed these polymerization events occurring through optical images taken upon low (homogenous, linear polymerization) and high (heterogenous, branching polymerization) concentrations of damaged hemoglobin S.
• The Glu->Val mutation in hemoglobin S creates a "sticky" hydrophobic patch on the molecular surface resulting in the polymerization of cells.
• It has been shown that hemoglobin units that contain a gamma subunit do not polymerize very well.
• Hemoglobin F has two gamma subunits.
• It has been shown that when hemoglobin S and hemoglobin F are mixed, a third species arises, made up of two alpha units, the beta unit from hemoglobin S and a gamma subunit. This species does not polymerize well.
• So, anything with a gamma unit acts with a dilution effect against the hemoglobin S units' polymerization.
• There are ways to calculate the expected dilution (and therefore decreased polymerization) effect.
  • Don't forget, however, that increased hemoglobin F will take up space in the cell and ... something about increasing the thermodynamics of polymerization.
• They calculated the critical concentration for polymerization (sickling) in an F cell: 0.3 grams / centimeter cubed.
• This means that any cells with a lower concentration of hemoglobin than this will never sickle. In vivo, cells can have higher concentrations because they cells don't have to never sickle they just can't sickle before they reach the lungs and get re-oxygenated.