It is common practice to design an in vitro assay so that the indicator reaction is fast, making the non-observable reaction rate limiting. If the indicator reaction is able to "keep up" with the non-observable reaction, then the obvious slow timescale to employ in the analysis is t s1 . However, if the indicator reaction is slow, then the time to completion will be much greater than t s1 . We will initially assume (for the sake of simplicity) that the indicator reaction is extremely slow. The motivation here is to work out the extreme case when the indicator reaction is rate limiting, and then "work backwards" and analyze the indicator reaction as it picks up speed. If the indicator reaction is very slow, then it is easy to imagine a scenario in which the non-observable reaction simply produces s 2 , and the depletion of s 2 is negligible over the course of t s1 ; this will hold if k 4 (the catalytic constant of the indicator reaction) is very small, and if e 0 2 s 0 1 . Then, after the completion of the non-observable reaction, the mass action equations will (theoretically) reduce to: