Endoproteinase Glu-C has been reported to have specific activity of 500 units/mg casein as a substrate. Additionally, we use a different fluorescent substrate to generate our 38.3umol/min/mg activity, which is used as a reference to ensure that we have little lot to lot variation. The good news is that 0.5 units of the casein units equal 1 ug of NEB Endoproteinase GluC and because we sell the enzyme dry, it can be reconstituted in water to achieve a desired working concentration. We generally do our digestions using a ratio by weight of 20:1 substrate:GluC at 25-37°C for up to 18 hours. For Endoproteinase GluC digestions to be reproducible, concentration and incubation times should always be the same.