A steady-state kinetic method for the verification of the rapid-equilibrium assumption in allosteric enzymes

A method for testing the validity of the rapid-equilibrium assumption as it might apply to allosteric enzymes using exclusively steady-state kinetic data is presented. The method is based upon a recognition that the ratio of apparent dissociation constants for the allosteric ligand, obtained under conditions of limiting and saturating substrate concentration, must yield the thermodynamic value for the coupling parameter between the substrate and allosteric ligand even in the general steady-state case. If this value is found to be equal to the apparent coupling parameter determined from the ratio of limiting values of the Michaelis constant for substrate obtained in the absence and saturating presence of the allosteric ligand, then the substrate can be correctly viewed as effectively achieving a binding equilibrium with the enzyme in the steady-state. The utility and limitations of this method are demonstrated by examining the ADP activation of beef heart mitochondrial NAD-dependent isocitrate dehydrogenase.