Characterization of Ca++ and Mg++ Binding to Bovine Pancreatic Deoxyribonuclease A

Abstract Gel filtration studies of 45Ca++ binding to bovine pancreatic deoxyribonuclease A show that the enzyme can bind from 5 to 7 Ca++ ions. At pH 7.5 DNase binds 2 Ca++ ions strongly, with a Kd of 1.4 x 10-5 m, and 3 Ca++ ions weakly, with a Kd of 2 x 10-4 m. At pH 5.5 the protein binds only 1 Ca++ ion strongly, with a Kd of 5 x 10-4 m. DNase has two strong Mg++-binding sites, with a Kd of 2.3 x 10-4 m. Mg++ and Mn++ can compete with one of the two strong Ca++-binding sites present at pH 7.5. They do not complete for the single strong Ca++-binding site present at pH 5.5. Even a 103-fold molar excess of Mn++ or Mg++ does not affect Ca++ binding to the Ca++-specific site.