She said you do not want to dilute the enzymes more than you need to, as they are most happy when concentrated.+* The substrate concentration that is used in enzyme assays is chosen according to parameters such as the Km, the solubility of the substrate, whether high concentrations may inhibit, and the cost of the substrate. [http://www.life.illinois.edu/biochem/455/Lab%20exercises/B-gal/enzymology