from which we need to indirectly measure the activity of E 1 through means of an indicator reaction (S 1 denotes the substrate of the non-observable reaction, C 1 denotes the complex, and S 2 is the substrate generated in the non-observable reaction). In the coupled auxiliary enzyme reaction mechanism, the product of the non-observable reaction (S 2 ) is catalyzed by an auxiliary enzyme, E 2 , in the indicator reaction :