Purification and Properties of a Mouse‐Cell DNA‐Repair Endonuclease, which Recognizes Lesions in DNA Induced by Ultraviolet Light, Depurination, γ‐Rays, and OsO4 Treatment

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Abstract
A DNA-repair endonuclease was purified 117-fold from mouse plasmacytoma cells (line MPC-11) by gel filtration, followed by ion-exchange and affinity chromatography. Its MW was determined by gel filtration to be 28,000 .+-. 2000. The enzyme recognizes apurinic and apyrimidinic sites induced by acid and .gamma.-rays in DNA and another type of lesion(s) which is introduced into DNA by both UV irradiation and OsO4. Quantitative measurements of the number of nicks the purified DNA-repair endonuclese makes in DNA treated with various amounts of OsO4 and UV light suggests that the endonuclease may act on 5,6-dihydroxydihydrothymine lesions. The endonuclease activity was sensitive to the ionic strength and was most active in the presence of 100 mM KCl, whereas the presence of divalent cations did not stimulate the activity.