DNA repair kinetics after low doses of X-rays. A comparison of results obtained by the unwinding and nucleoid sedimentation methods.

A re-examination of some of our previously published and our more recent data has led us to reconsider the interpretation of the kinetics of the repair of DNA single-strand breaks in mammalian cells. We investigated the detailed kinetics of this repair after low doses of X-rays in mouse Ehrlich ascites tumour cells grown in suspension cultures, using two different methods that measure strand breaks in DNA: the unwinding method and the 'nucleoid' sedimentation method. The results thus obtained, either under weak alkaline conditions (unwinding method) or under neutral conditions ('nucleoid' sedimentation method) both showed an initial fast repair component, with a half time, t1/2 of 5-6 min, followed by a component between 10 and 30 min after irradiation, in which the kinetic curves levelled off or turned upwards, indicating the possibility of the introduction of new breaks into the DNA. We propose that these additional DNA strand breaks may arise by incision of the DNA by endonucleases at base-damaged sites, and we therefore suggest that the previous interpretation of the kinetics of DNA single-strand break repair in terms of two or more first-order repair components may have been an approximation. The measured kinetics of strand break repair after X-irradiation probably represent a mixture of break ligation and incision, and therefore correspondingly deviate from first order, especially 10-30 min after exposure to X-rays.