AN UNUSUAL MECHANISM FOR THE MAJOR HUMAN APURINIC APYRIMIDINIC (AP) ENDONUCLEASE INVOLVING 5'-CLEAVAGE OF DNA CONTAINING A BENZENE-DERIVED EXOCYCLIC ADDUCT IN THE ABSENCE OF AN AP SITE

The major human apurinic/apyrimidinic (AP) endonuclease (class II) is known to cleave DNA 5' adjacent to an AP site, which is probably the m ost common DNA damage produced hydrolytically or by glycosylase-mediat ed removal of modified bases, p-Benzoquinone (pBQ), one of the major b enzene metabolites, reacts with DNA to form bulky exocyclic adducts, H erein we report that the human AP endonuclease directly catalyzes inci sion in a defined oligonucleotide containing 3,N-4-benzetheno-2'-deoxy cytidine (pBQ-dC) without prior generation of an AP site, The enzyme i ncises the oligonucleotide 5' to the adduct and generates 3'-hydroxyl and 5'-phosphoryl termini but leaves the pBQ-dC on the 5' terminus of the cleavage fragment, The AP function of the enzyme is not involved i n this action, as no preexisting AP site is present nor is a DNA glyco sylase activity involved. Nicking of the pBQ-dC adduct also leads to t he same ''dangling base'' cleavage when two Escherichia coli enzymes, exonuclease III and endonuclease IV, are used, Our finding of this unu sual mode of action used by both human and bacterial AP endonucleases raises important questions regarding the requirements for substrate re cognition and catalytic active site(s) for this essential cellular rep air enzyme, We believe this to be the first instance of the presence o f a bulky carcinogen adduct leading to this unusual mode of action.