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

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 (approximately equal to 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 0.019 mM at 37 degrees C. Temperature dependence studies show that the reaction is entropically driven (delta H degrees approximately equal to 25 kcal/mol, delta S degrees approximately equal to 100 cal/mol-deg) and suggest the involvement of hydrophobic interactions.