Hi there,

The most convenient method is to use specific and chromogenic/fluorogenic peptide with a unique cutting site as a substrate because activity may be monitored in a continuous assay. For info read the following paper:

https://www.ncbi.nlm.nih.gov/pubmed/24388866

Using BSA as a substrate would be much more tedious as you would have to identify cutting sites, separate products after the assay and quantify them. All the sites being not equivalent in terms of sensitivity to the protease the quantification of activity  would be complicated.

Thank you so much for your valuable suggestions.