Nuclear topoisomerase I and DNase activities in rat diethylnitrosamine-induced hepatoma, in regenerating and fetal liver.

DNase activity in the presence of Ca2+ + Mg2+, Mg2+ alone, Mn2+ alone, or EDTA, and topoisomerase I activity were measured in nuclear extracts of diethylnitrosamine (DEN)-induced hepatomas, regenerating, fetal, and normal rat livers. In hepatoma tissue, the Ca/Mg-dependent DNase activity was lower than in normal tissue and nearly the same as in fetal liver. In the poorly differentiated hepatomas, Mn-dependent DNase activity was higher than in both moderately and well differentiated ones and than in normal liver tissue. The activity of topoisomerase I in hepatomas and in regenerating liver was lower than in normal liver tissue.