Substrate binding unequal

Figure 4-17 A simple representation of an equilibrium dialysis experiment where the volumes inside and outside the dialysis bag are unequal. The large circles inside the dialysis bag represent a nondiffusible protein, P, which may be an enzyme or a noncatalytic binding protein. The small dots represent a diffusible ligand, S, which may be a substrate, inhibitor, or activator, and so on. At equilibrium, the concentration of free ligand, [S]/, is the same on both sides of the membrane. The excess ligand inside the dialysis bag represents bound ligand, [S]t or [PS]. Note that if no protein were present inside the dialysis bagi the equilibrium [S] would be 1.2 (6/5 = 1.2 inside the bag. 12/10 =1.2 outside the bag, or 18/15= 1.2 overall). To minimize the amount of protein and ligand required, equilibrium dialysis is usually carried out with specially-made plastic chambers of equal volumes (e.g., 0.1 to 1.0 ml), separated by a semipermeable membrane (see Fig. 6-2).

Wang E-j, Casciano CN, Clement RP, Johnson WW. Fluorescent substrates of sister-P-glycoprotein (BSEP) evaluated as markers of active transport and inhibition evidence for contingent unequal binding sites. Pharm Res 2003 20 537-544. 

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