An endonuclease from calf liver specific for apurinic sites in DNA

An endonuclease specific for apurinic sites in double-stranded DNA has been partially purified from calf liver extracts. The enzyme has a pH optimum of 9.5, is only slightly stimulated by low concentrations of Mg2+, and has a molecular weight of 28 000. Inhibitors of the endonuclease include Ca2+, EDTA, p-HOHgBzO, NaCl, and tRNA. The enzyme introduces single- and double-stranded breaks in depurinated DNA. High concentrations of the enzyme preparation degrade untreated single-stranded DNA, but not ultraviolet (UV) irradiated DNA or DNA treated with methylmethanesulfonate or 7-bromomethyl-12-methylbenz[a]-anthracene. Enzymatic incisions produce 3'-hydroxyl and 5'-phosphate end groups. Some of the properties of the calf liver apurinic endonuclease differ from those of a similar endonuclease obtained from calf thymus by S. Ljungquist and T. Lindahl [(1974), J. Biol. Chem. 249, 1530] and in this laboratory. The data suggest that these are isozymes.