should still be interpreted to mean that the solution (39a)-(39b) to lies at the intersection of both the c 2 and s 2 -nullclines when t a s 2 t; however, since the intersection of the nullclines occurs at the origin, the biochemical interpretation is that the indicator reaction has completed. Another subtle difference between the phase-plane dynamics of the auxiliary enzyme assay and the zymogen activation assay is that both the s 2 and c 2 -nullclines move in the phase-plane of the zymogen activation reaction, whereas only the s 2 -nullcline moves in the phase-plane of the auxiliary enzyme reaction.