The Apurinic/Apyrimidinic Endodeoxyribonuclease of Rat‐Liver Chromatin

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Abstract
The extract from rat liver chromatin contains 2 purinic/apyrimidinic (AP) endodeoxyribonucleases named 0.2 M and 0.3 M isozymes according to the phosphate concentration necessary to elute them from an hydroxyapatite column. The 0.3 M isozyme is the main and perhaps the only chromatin AP endodeoxyribonuclease in the living cell. This 0.3 M isozyme was purified by successive chromatographies on hydroxyapatite, phosphocellulose, heparin-Sepharose and alkylated-depurinated DNA-cellulose. It has a MW of .apprx. 39,000; its optimum pH is around 8.0; it needs Mg2+ or Mn2+ to be active and the optimum concentration for Mg2+ is between 5 mM and 10 mM. The 0.3 M isozyme has no action on intact DNA strands or on alkylated sites; it cuts the phosphodiester bridge which is the immediate neighbor of the AP site on its 5'' side leaving 3''-hydroxyl and 5''-phosphate ends. It has no associated exonuclease activity. To hydrolyze the phosphoester bond near the AP site, the enzyme makes a close contact with 3 base residues in the large groove of the DNA molecule.