Thermodynamics of all-or-none water channel closure in red cells

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Abstract
The relation of osmotic to diffusional water permeability of human red blood cells was compared after treating the cells with different concentrations of PCMBS (p-chloromercuribenzene sulfonate). After subtracting the PCMBS-insensitive permeability (presumably the water permeability of the lipid bilayer) from each, the ratio of osmotic to diffusional permeability remains invariant (≈11) as more and more water channels are inhibited by increasing concentrations of PCMBS. This result implies that the channels close in an all-or-none way and suggests a two-state model. Analysis of the dependence of osmotic water permeability on PCMBS concentration in terms of the model reveals a 1∶1 stoichiometry and a dissociation constant for the PCMBS/membrane receptor complex of about 0.019mm at 37°C. Temperature dependence studies show that the reaction is entropically driven (ΔHo≈25 kcal/mol, ΔSo≈100 cal/moldeg) and suggest the involvement of hydrophobic interactions.