Could you explain what the units for measuring the enzyme is. The ELISA I'm using measures in pg.mL
But many papers I've seen use U/L however they are measuring ACE activity not protein concentration.
Anything to clarify this would be useful.
If you are using some ACE ELISA kit you are indeed measuring ACE concentration, that's why the unit is pg.mL (pg.mL.-1, I guess you meant). The unit used in those papers you mentioned is U/L because they are probably estimating "how many" units (U) of ACE are necessary to convert the substrate into the product in 1L reaction. That's why they can claim ACE activity. The amount of enzyme not necessarily reflects in fully functional enzyme activity. Summarizing the answer for your question, if you are evaluating ACE levels, the unit should be pg.mL.-1 or equivalent units, while if you are measuring ACE activity the unit should be U/L or something equivalent.
The units you are using are for ACE level while U/L is for ACE activity