advice

[littlemok]

I am running an assay and wanting to do kinetics. From the literature I am using EDTA as an inhibitor and when I use it I get no pattern of inhibition. I am using 25,50,100 and 150uM concentrations and incubating 40ul of edta with 40ul of enzyme for 30 mins. the absorbance readings do not have a pattern from 25-100 they decrease but the 150 one increases? The isosbestic points do the same. Perhaps I should use more conc EDTA? I have even done the assay in a CaCl2 free buffer and when I add Calcium the absorbance drops even lower. I have no idea whats happening. Any ideas? Thank you in advance for any replies