that govern the steady-state kinetics of the coupled (or auxiliary) enzyme reaction (1)-( ). Mathematical statements regarding the validity of the above MM equations are derived. We show that as long as the reactant-stationary assumption (RSA) holds, the above equations describe the reaction dynamics during the QSS period. We con-clude with a discussion of our results, as well as a possible description of the experimental applicability of the interacting MM equations to estimate the kinetic parameters of the coupled (or auxiliary) enzyme reaction (1)-( ) via initial rate experiments for the indicator substrate and product. In addition, we show that the interacting MM equations given above can be derived when the relative speeds of the reactions are disparate.