beta-l-Dioxolane-cytidine (l-OddC, BCH-4556, Troxacitabine) is a novel unnatural stereochemical
nucleoside analog that is under phase II clinical study for
cancer treatment. This
nucleoside analog could be phosphorylated and subsequently incorporated into the 3' terminus of
DNA. The cytotoxicity of l-OddC was correlated with the amount of l-OddCMP in
DNA, which depends on the incorporation by
DNA polymerases and the removal by
exonucleases. Here we reported the purification and identification of the major
enzyme that could preferentially remove l-OddCMP compared with
dCMP from the 3' termini of
DNA in human cells. Surprisingly, this
enzyme was found to be apurinic/apyrimidinic
endonuclease (APE1) (), a well characterized
DNA base excision repair
protein. APE1 preferred to remove l- over d-configuration
nucleosides from 3' termini of
DNA. The efficiency of removal of these
deoxycytidine analogs were as follows: l-OddC > beta-l-2',3'-dideoxy-2', 3'-didehydro-5-fluorocytidine > beta-l-2',3'-dideoxycytidine > beta-l-2',3'-dideoxy-3'-thiocytidine > beta-d-2',3'-dideoxycytidine > beta-d-2',2'-difluorodeoxycytidine > beta-d-2'-deoxycytidine >/= beta-d-
arabinofuranosylcytosine. This report is the first demonstration that an
exonuclease can preferentially excise l-configuration
nucleoside analogs. This discovery suggests that APE1 could be critical for the activity of l-OddC or other l-
nucleoside analogs and may play additional important roles in cells that were not previously known.