Although not otherwise discussed in this guide, one should also be aware that enzymes can denature over time, especially if they are very dilute, and the products of some reactions can inhibit the enzyme. Indeed there are other possible reasons for non-linear behaviour but it is usually relatively easy to find the linear range by trial and error using serial dilutions of the enzyme as described above.
Assay time and temperature
These parameters can also influence the linear range as they affect the rate of substrate conversion. For example, an assay may be linear after 15 min, but at 60 minutes too much substrate may have been consumed. In this situation you would have to reduce the amount of enzyme in order to run your assay for 60 minutes. The same considerations apply to assay temperature as it can also affect the rate of reaction.
Most people adopt an assay time of somewhere between 15 min and 60 min. Very short times (e.g. 2 min) are to be avoided because a slight delay in stopping the reaction will lead to a significant error in calculations of activity. It is important to ensure that reagents stored in a fridge or freezer have equilibrated to the correct temperature before use, and this is especially important if you have a relatively short assay time.
Assay volume/sensitivity
From a practical perspective the assay volume is determined/limited by the consumable item that you use for your assays (e.g. cuvettes, tubes, or microplates). The signal for most enzyme assays is proportional to the assay volume and attempts to miniaturise the