You can use Protazyme AK or OL at a range of pH values. Use acetate buffer in pH range 3.5–5.5; Maleate buffer at 5.5–6.5; Phosphate buffer at 6.2–7.5 and Tris buffer at 7.5–9.0.
In general, the substrates are less stable at high pH values for extended times, particularly at elevated temperatures. We have also found that with Aspergillus protease the pH optima with these substrates is 6.0–7.0, not approximately pH 5.0 as one would expect.
At low pH values, the substrate will hydrate less and thus the standard curve (substrate susceptibility) will be lower. You may also look at our new Azo-Casein. This is an excellent protease substrate, far superior to any other Azo-Casein commercially available.