with s 0 1 denoting the initial non-observable S 1 concentration. Thus, the indicator reaction is described by a non-autonomous set of equations with e A 2 (t) as its forcing term. As with the coupled auxiliary enzyme reaction, the basic analysis of the zymogen activation assay can be carried out with three timescales: t c 1 , t s 1 and t a s 2 . The timescales t c 1 and t s 1 are identical to those defined earlier in the coupled auxiliary enzyme reaction. The additional timescale,