As a final remark, we note that in typical in vitro assays, the auxiliary enzyme concentration e 0 2 is usually large. In this scenario, the indicator reaction can be modeled using pseudo-first order (PFO) kinetics. We will not explore this idea in detail since the intent of this work was to investigate the RSA kinetics of the in vitro enzyme assays. However, we remark that the subject of an indicator reaction that follows PFO kinetics, combined with a non-observable reaction that obeys MM kinetics, is no doubt an interesting formulation that is deserving of further investigation. Furthermore, numerical simulations seem to confirm the idea that it is possible ṗ ≈ k 2 c 1 as e 0 2 gets large, even in cases when the catalytic constants are of the same order (see Fig. ). A similar analysis of the PFO model can be carried out based on the unique geometry of the phase-plane (i.e., intersecting nullclines). Of course, both the utility and validity of the of both the PFO model as well as the QSS model are best addressed as part of a more thorough analysis of the inverse problem. We will examine the inverse problem for coupled (or auxiliary) enzyme reactions in future work. Parameter values for numerical solutions are: s 0 1 = 100, s 0 2 = 0, e 0 1 = 1, e 0 2 = 1000, k 1 = 1, k 2 = 10, k -1 = 10, k 3 = 1, k 4 = 10 and k -3 = 1 (units have been omitted). Time has been mapped to the t∞ scale: t∞(t) = 1 -1/ ln(t + e).