RECOGNITION OF APURINIC DNA BY APURINIC APYRIMIDINIC ENDONUCLEASE FROM HUMAN PLACENTA/

A study was made of the dependence of the affinity of oligonucleotides d(pC)(n), d(pT)(n), and d(pA)(n), and some duplexes to apurinic/apyri midinic endonuclease on their length. Simple algorithms were developed for estimating the affinity of single-stranded oligonucleotides and t heir duplexes to this enzyme. Nucleoside monophosphates (dNMPs) were s hown to be the minimal ligands for AP endonuclease (K-i = 420-500 mu M ). The enzyme interacts with 10 units of a DNA ligand regardless of it s length. Raising the oligonucleotide length by one unit (at n 10) inc reases its affinity (factor f) 1.5-1.6-fold. The affinity increases ac cording to the progression K-i[d(pN)(n)] = K-i[dNMP]f(1-n). The affin ity of d(pA)(n) d(pT)(n) changes in a similar way, f = 1.95. Introduct ion of an apurinic site into single-stranded oligonucleotides and comp lementary duplexes increases their affinity only 3-10-fold, suggesting that recognition of apurinic sites by the enzyme does not play a key role in its interaction with modified DNA. Recognition of the substrat e takes place basically through weak additive interactions of the enzy me with nine intact pairs.