In addition, we have shown that the lag time does not necessarily coincide with the onset of the validity of the QSSA. As we have shown, the QSSA can be imposed long before the lag time if appropriate conditions hold. This is a novel and more rigorous study of the lag time of coupled (or auxiliary) enzyme catalyzed reactions which, as reported in previous studies, does not rely on linear approximations to estimate the complex nonlinear dynamics . However, the practical utility of the interacting MM equations approximations in the context of the inverse problem has not been established. Our analysis suggests that previous approaches relying on lag time studies can be ineffective to estimate non-observable reaction parameters for the coupled (or auxiliary) enzyme reaction (1)-( ). The development of standard-based approaches to measure kinetic parameters is a complex inverse problem as the conditions for the validity of mathematical approximations to described chemical reactions do not guarantee an accurate estimation of the kinetic parameters .