Lack of excision of 4HAQO adducts from DNA by cell extracts that excise pyrimidine dimers

A substrate of DNA containing 4HAQO adducts, suitable for studies of excision repair, was prepared by reacting calf thymus DNA with [3H]monoacetyl-4HAQO. A crude HeLa cell extract was prepared by the method of Mortelmans et al (Proc. Natl. Acad. Sci. U.S.A. 73, 2757, 1976). The cell extract would specifically excise pyrimidine dimers from UV-irradiated DNA but would not release 4HAQO adducts in an acid soluble form. This result points to different initial steps in the excision repair process for these two forms of damage even though much of the repair mechanism is common to both.