enzyme problems

[littlemok]

Hello, I was wondering if anyone could help advise me. I am trying to do enzyme inhibition kinetics but according to protocols I need to do it using the substrate to zero the spec with first and then look for a decrease at a certain wavelength. When this happens there is a decrease but the absorbance goes negative, consequently lineweaver burk plots face downwards. I do not know enough about this as this is the first time I have ever tried an enzyme assay, Can anyone suggest how I could change things? Many thanks to anyone who replies

[Bluenoise]

Could you tell us something about the reaction?