I am also in need of clarification for  calculating the activity. Thanks for posting this query. 

During my experineces I used to calculate the CAT activity (according the method described by Clairborne, 1985) by means of this formula: Unit of CAT/mg-1 of proteins= (3.45* sample's diluiton factor )/minutes*ml of used sample, where 3.45 are the µmol of H2O2 which decompose in 3.0 ml of reaction mixture with an increasing in absorbance (240nm) from 0.45 to 0.40.

About SOD activity, I'm using the formula reported in the Sigma Aldrich SOD Kit assay, an indirect method for measure the SOD activity; so, to explain what I found,  I indicate the % of enzyme's inhibition directly in the graph.

If there is Catalase activity in your sample, absorbance will  always be decrease  with time duration (reaction period). If very high amount of endogenous peroxidase substrate is also present in an enzyme extract, then only there are chances of  increase in aborbance in visible range of the spectrum due to peroxidase activity.  Dr. Alessandro probably misspelled the term "increasing" in his answer.

I think first calculate the activity of an enzyme  (change in substrate or product concentration (µmol or mmol) per min or per hour) in a per unit volume of enzyme extract. Then measure and calculate the protein amount  in the same volume of enzyme extract. Finally, divide enzyme activity with the protein amount.  In this way you will get enzyme activity  (unit/mg or g of protein).  Therefore, it always required to define the enzyme unit (nmol, µmol or mmol of substrate per min or per h) depending on the enzyme efficiency and calculation choice. The activities of some enzymes are very fast and some take hours for their activity.

@ Rajesh Sir ..

thanks for answer.. but my real confusion is about the calculating the enzyme activity and specific activity .

since the total protein is always to be considered then how to calculate the enzyme activity.??

Enzyme activity expressed in per unit (mg) protein basis is called specific activity in general. If you have purified your protein of an enzyme before activity measurement then you could calculate specific activity more specifically. Enzyme activity could be expressed both on fresh (dry) matter basis and per mg protein basis. Enzyme activity expressed on protein weight basis is the specific activity of an enzyme.