Changes in topoisomerase I activity after irradiation of lymphoid cells.

ABSTRACT

The activity of topoisomerase I in nuclear extracts increased about three-fold 5 min after gamma-irradiation (840-2500 rads) of human peripheral blood lymphocytes or cultured lymphoblastoid cells. The change may reflect modification of the enzyme by nuclear ADP-ribosyl transferase, which is known to be activated by DNA breaks.