Human placental apurinic/apyrimidinic endonuclease. Mechanism of action.

The mechanism of action of the homogeneous preparation of human placental apurinic/apyrimidinic (AP) endonuclease, described in the previous paper (Shaper, N. L., Grafstrom, R. H., and Grossman, L. (1982) J. Biol. Chem. 257, 13455-13458), has been investigated in detail. This enzyme cleaves apyrimidinic DNA both 5' and 3' to the site of damage in a ratio of 60:40, respectively. Even though this enzyme can cleave on both sides of an internal AP site, it does not release deoxyribose 5-phosphate from terminal AP sites. However, a compound, tentatively identified as alpha, beta unsaturated deoxyribose 5-phosphate, is nonenzymatically released only from 5'-terminal AP sites, presumably by a beta-elimination mechanism.