Purification and characterization of an apurinic/apyrimidinic endonuclease from HeLa cells

Abstract

An endodeoxyribonuclease from HeLa cells acting on apurinic/apyrimidinic (AP) sites has been purified to apparent homogeneity as judged by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The presence of Triton X-100 was necessary throughout the purification for stabilization and stimulation of activity. The endonuclease has an apparent native molecular weight of 32,000 determined by molecular sieving and an apparent subunit molecular weight of 41,000 as judged by its electrophoretic mobility in SDS-polyacrylamide gels. The activity has an absolute requirement for Mg2+ or Mn2+ and a broad pH optimum between 6.7 and 9.0 with maximal activity near pH 7.5. The enzyme has no detectable exonuclease activity, nor any endonuclease activity on untreated duplex or single-stranded DNA. It is inhibited by adenine, hypoxanthine, adenosine, AMP, ADP-ribose, and NAD+, but it is unaffected by caffeine, the pyrimidine bases, ADP, ATP, or NADH. The use of a variety of damaged DNA substrates provided no indication that the enzyme acts on other than AP sites. The enzyme appears to cleave AP DNA so as to leave deoxyribose-5-phosphate at the 5' terminus and a 3'-OH at the 3' terminus; it also removes deoxyribose-5-phosphate from AP DNA which has deoxyribose at the 3' terminus. Specific antibody has been produced in rabbits which interacts only with a 41,000-dalton protein present in the purified enzyme (presumably the enzyme itself), as well as with partially purified AP endonuclease fractions from human placenta and fibroblasts.