The present invention provides, among other things, methods and compositions for determining enzyme kinetic parameters (e.g., Vmax, Km, and specific activity, etc.) indicative of clinically relevant properties of glucocerebrosidase using a physiologically relevant substrate, in particular, a substrate that is representative of substrates that typically accumulate in patients suffering from Gaucher disease such as glucosylceramide. Thus, the present invention is particularly useful to measure a kinetic parameter relating to the activity of glucocerebrosidase in a drug substance, drug product, and stability sample for enzyme replacement therapy.