Effect of thermodynamic nonideality on the subcellular distribution of enzymes: adsorption of aldolase to muscle myofibrils.

An expression is derived whereby allowance may be made for the effects of thermodynamic nonideality on the biphasic interaction of a macromolecular solute with an immobilized reactant. This quantitative description, written in terms of activity coefficients expressed as virial coefficients on the basis of excluded volume, also takes into account the space-filling effect of an inert macromolecule present in the reaction mixture. Advantage is then taken of the theory to consider the effect of bovine serum albumin on the interaction of aldolase with bovine cardiac muscle myofibrils in I 0.158 imidazole-chloride buffer, pH 6.8. Partition equilibrium studies are used to establish that inclusion of a moderate concentration (14 mg/ml) of serum albumin in reaction mixtures leads to a 35-40% increase in the apparent binding constant written in terms of reactant molarities, and that the enhancement is attributable entirely to nonideality inasmuch as the same thermodynamic binding constant pertains. This investigation of thermodynamic nonideality arising from the space-filling effects of inert macromolecules on enzyme partition reinforces the possibility that some enzymes may be distributed between soluble and adsorbed states in the highly concentrated macromolecular environment of the cell cytoplasm.