Thermodynamic nonideality as a probe of macromolecular isomerizations: application to the acid expansion of bovine serum albumin.

The potential use of thermodynamic nonideality as an index of the coexistence of two isomeric protein states in equilibrium is explored in relation to the effect of inert space-filling macromolecular solutes on the gel chromatographic behavior of the system. Frontal gel chromatography, on Sephadex G-100, of bovine serum albumin at pH 3.2 in the presence and absence of a moderately high (15 g/liter) concentration of either Dextran T70 or Dextran T10 is then employed to establish that the progressive increase in the Stokes radius of albumin as the pH is lowered from 5 to 2 should not be viewed as a pH-dependent isomerization equilibrium between native and acid-expanded states. In addition, calculations based on parameters for the aspartate transcarbamoylase system point to the possible use of the present methodology to distinguish between preexistence and ligand induction of an isomerization equilibrium as the source of a sigmoidal effect in allosteric systems.