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.
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].
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].
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].
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].