Endonuclease specific for apurinic/apyrimidinic sites from human liver

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Abstract
The endonuclease (AP endonuclease) specific for apurinic/apyrimidinic sites (AP sites) was purified and characterized from normal human autopsy liver. Three chromatographically distinct species of AP endonuclease were resolved by phosphocellulose chromatography. The species had MW ranging from 24,000-31,000. Species 3 was significantly more thermostable than was species 1 or species 2. Species 2 had an apparent Km of 400 nM AP sites whereas species 3 had an apparent Km of 2000 nM AP sites. All of the species required Mg2+ for activity. The concentration of Mg2+ which resulted in half maximal velocity was 5 mM for species 3 but was only 1.2 mM for species 1. The observed differences in physical and catalytic properties of the 3 species suggest they are isozymes. Incision of damaged DNA by the 3 putative isozymes was restricted to AP sites and resulted in the production of 3''hydroxyl and 5''phosphorylated termini. [The investigation of DNA repair in human cells could provide information which will ultimately help lead to the prevention of cancer in man.].