23 Endonuclease G Isolation and Assays

Mitochondria in mammalian cells contain a nuclearly encoded, Mg⁺² (Mn⁺² )-dependent DNA endonuclease activity (reviewed in ref. 1 ), that has been termed Endonuclease G (or Endo G) (2 ). This enzyme activity, an approx 60-kDa homodimer of a 29-kDa polypeptide, accounts for nearly all of the potent DNA endonuclease activity identified in crude protein extracts of isolated mitochondria (3 ). The 1133-bp (base pairs) cDNA for the human Endonuclease G gene has been mapped to chromosome 9q34.1 (4 ).