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RBC Sickling:  Kinetics vs. Thermodynamics

The sickling process and its relationship to oxygen saturation can be described using either kinetic or  thermodynamic models. However, each offers its own perspective on SCD pathogenesis.

 Kinetic Model of HB S Polymerization: Delay Time to Nucleation

Eaton and Hofrichter have provided experimental evidence that under low O2 conditions a small aggregate of deoxyHb S molecules form a critical nucleus with an initial delay time[67-70]. This initial delay time, td,  that is required to form the critical nucleus is described by the following general equation:  td = k / [deoxyHb S]30  (Eq.1)[68]. Eq.1 is derived from laboratory findings; experiments demonstrate that td is inversely proportional to the initial deoxyHb S concentration, and temperature, but also particularly sensitive  to O2 tension, pH, and  the intracellular non-Hb S fraction[56].

Problems with the Kinetic Model

There is controversy over the 'kinetic' (delay time) explanation for Hb S polymerization based solely on oxygen saturation[73] . The dispute arises from in vivo observations of Hb S polymers inside non-sickled RBCs in arterial (oxygenated) blood, as well as in vitro studies showing persistence of Hb S polymers in RBCs even at high oxygen saturation[71-73]. According to the kinetic theory, Hb S polymers should dissolve under arterial conditions. These incongruent findings prompted other investigators to propose an alternative explanation for these phenomena [54].

Thermodynamic Model: Hb S Polymer Content

Noguchi and Schecter proposed a thermodynamic model[54] for Hb S polymerization that puts a greater emphasis on the solubility properties of deoxyHb S and explains the findings in arterial situations. Deoxygenated Hb S has a very low solubility in red cells, and might arguable play the central role in polymer formation[54].  Therefore, it would not be surprising that some Hb S-containing RBCs contain polymers even under arterial (high O2) conditions[71]. Thus, polymer formation might have a different relationship to O2 saturation than that predicted by the kinetic model[71]. Furthermore, the thermodynamic model considers the effect of high intracellular Hb S polymer content, but not necessarily morphological changes, in determining the SCD phenotype[72,73]. Indeed, in certain circumstances Hb S polymer content provides a better clinical index in SCD than cell morphology[63,72-73].

Kinetic and Thermodynamic Considerations in Polymerization

The two models bring to bear different points to consider in Hb S polymerization. The kinetic model puts a greater emphasis on the intracellular deoxyHb S concentration. Based on the kinetic model, the exponent in Eq.1 predicts that the level of deoxyHb S in a RBC inversely relates to the delay time for nucleation by the 30th power[68]. At the same time, the kinetic theory predicts that most cells will not sickle in the low O2 environment of the microvasculature, since the delay time is generally longer than the capillary transit time[70]. In contrast, the thermodynamic model essentially discounts the delay time and, by taking into account protein non-ideality[54], puts its emphasis on the low solubility of deoxyHb S. Based on thermodynamic considerations, the polymer fraction is proportional to the 3rd power of the Hb S concentration[54,63,73]. The thermodynamic paradigm stresses that other factors play a greater role in the disease’s severity, including concomitant sickle RBC viscosity[54] and rigidity (nondeformability) which affect blood rheology. Still, since both models offer insights into aspects of SCD pathophysiology, kinetic and biophysical theories about Hb S polymer and sickling remain as competing theories.

It is noteworthy, however, that neither the kinetic nor the thermodynamic paradigm fully explains how Hb S polymer formation leads to morphological sickling. The presence of Hb S fibers within the RBC causes other cellular effects, including abnormalities of the membrane and its underlying matrix[75]. Proof of the membrane's involvement comes from in vitro studies wherein ISC ghosts (lacking Hb S polymer) retain the sickle cell shape[76,77] and  in vivo observations of intact sickled cells that lack Hb S polymer[75].