The question is too general, because it doesn't show what was already performed on the samples. I can only suggest or ask if the protein was determined in these samples? If not, the Bradford assay should be performed on the same supernatants first. Then you need to divide the basic result (for instance the result given in U/mL) by mg of prot. per mL of the same supernatant.
You can simply follow Aebi 1984 protocol for the calculation of catalase activity where you have to divide unit activity of enzyme with the total protein content