In order to caluclate the specific activity of the isolated
fractions, the protein concetration has to be determined. why is
the protein concetration necessary for accurate comparison of the
enzyme activity of the fractions?

Explanation:

This is evident that all enzymes are proteins but not all proteins are enzymes. The specific activity can be define as the number of enzyme units per milliliters that is divided by the available concentration of the proteins typically in mg/ml. Thus the value of the specific activity can be measured in  units/mg.

In others words this can be said that how much enzymes units can be found in the 1 mg of the total protein. So in the total concentration of the proteins the estimation of the enzyme units is possible. Thus protein concentration is necessary for calculating the number of the enzyme units.

i think that it is plants