Protein Digestion

Effect of activators
These give 5-15-fold activation with peptide substrate

SDS: no change observed in digestion rate (0-0.03 %)
Magnesium: no change in observed rate (0-500 mM)
Buffer ions: no change in observed rate (0-200 mM TAPS, pH 8.1)

Additional data
High appearant Km
no saturation point observed 
No change in rate when [E] is varied


What would account for this?