Nuclear DNA endonuclease activities on partially apurinic/apyrimidinic DNA in normal human and xeroderma pigmentosum lymphoblastoid and mouse melanoma cells

DNA endonuclease activities from nuclear proteins of normal human and xeroderma pigmentosum (XP), complementation group A, lymphoblastoid and Cloudman mouse melanoma cells were examined against partially apurinic/apyrimidinic (AP) DNA. Non-histone chromatin-associated and nucleoplasmic proteins, obtained from isolated nuclei, were subfractionated by isoelectric focusing and assayed for DNA endonuclease activity against linear, calf thymus DNA. All of the nine chromatin-associated and three of the nucleoplasmic fractions, which lacked DNA exonuclease activity, were tested for DNA endonuclease activity against both native and partially AP, circular, duplex, supercoiled PM2 DNA. In all three cell lines, four chromatin-associated, but none of the nucleoplasmic fractions, showed increased activity against DNA rendered AP by either heat/acid treatment or by alkylation with methyl methanesulfonate (MMS) followed by heat. One chromatin-associated activity, with pI 9.8, which was not active on native DNA, showed the greatest activity on AP DNA. AP activity was moderately decreased in XP cells and slightly decreased in mouse melanoma cells, as compared with normal cells, in the fraction at pI 9.8. Little or no increased activity was observed in any of the endonucleases from any of the cell lines on MMS alkylated DNA.