Why is % yield in a purification table measured using activity instead of total protein? I'm trying to understand why % yield is measured with activity instead of total protein in mg. According to my biochemistry notes, activity is "1.0 units of enzyme activity = 1.0 μmol of substrate catalyzed to product under optimum conditions."

Then, yield is defined as "% of total starting protein/enzyme in final purified protein fraction."



With this definition of yield, it would make sense to say that you get 30% yield in the second step since you have 3,000 mg / 10,000 mg of the original protein amount. The definition of yield in my notes mentions nothing of activity.
Because total protein includes other proteins than the one desired.  

"% yield" is expressing how much of the enzyme desired was actually recovered.