Enhanced excision repair activity in mammalian cells after ionizing radiation

Monkey CV-1 cells which had received 5 Gy 12 h before harvesting lysates from their cell cultures contained approximately three times as much DNA excision repair enzyme activity as unirradiated cells. The activity was determined in crude cell lysates by the release of intermediate mobility DNA fragments and fragments with 3'-phosphoryl ends from 5'-32P-end labelled irradiated 95 bp alpha DNA. Different 3'-termini endow the fragments with differing mobilities, signifying steps in the processing of radiation damaged DNA. Similar results were obtained when Krebs II mouse tumour cells growing in mice as ascites received 5 Gy 12 h before harvest. The enzyme activities from CV-1 cells and from Krebs II cells were partially purified as 60-70 kDa proteins on Superose 12 or Ultrogel AcA-54 columns. Divalent cations were not required for enzyme activity. A 23 nucleotide long defined duplex oligodeoxynucleotide substrate containing a single 8-oxodG residue was also very actively cleaved by the partially purified cell enzymes. 8-oxoguanine is a major product of ionizing radiation's action on DNA and was recognized by the enzymes described here. The mechanism by which radiation increased excision repair activity of cellular enzymes is not understood.