The optimal pH for the enzyme is 8-8,5. Maybe adjusting your pH for optimal activity?

I found this tips too, apparently the enzyme is a zymogen that needs autocatalysis to activate itself. Also I have no source for this, but aureolysin appears to activate the enzyme, so it could be added for an amount of time before addition of sample and deactivated with EDTA to prevent inferference.

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Storage conditions: Supplied freeze-dried from Tris-HCl and sodium chloride buffer.

Reconstitution: Endoproteinase GluC should be reconstituted by the addition of 50-500 µl of high purity water. Finger flick the volume of water in the tube to fully resuspend the enzyme. Rapid autolysis is a function of enzyme concentration; any sample reconstituted in a small volume should be used immediately. To get the most use out the enzyme, resuspend the enzyme in 500 ul H2O and aliquot 50 ul each in 10 tubes. Freeze the tubes that are not being used immediately at -20°C for up to two weeks or less. Storage at - 80°C will prolong enzyme stability approximately 2-4 additional weeks.

Repeated freeze/thawing is not recommended.

GluC may also be used in 20-50mM phosphate or ammonium bicarbonate buffer. For optimal activity, supplement reactions with Glu-Glu dipeptide. Glutamic acid residues are highly favored in Tris, phosphate and ammonium bicarbonate buffers.

GluC is purified with a His-tag at its C-terminal of the protein. The predominant species has four histidine residues at the C-terminus with the remaining species showing a distribution from one to six His tags.

thanks a lot for your informative answer,Tomas