The presence of DNA topoisomerase II alpha was investigated in interphase and metaphase mouse erythroleukemia (MEL) Friend-S cells, and in extracted with 25 mM lithium diiodosalicylate buffer (Lis) nuclei using indirect immunofluorescence. The results showed that DNA topoisomerase II alpha is localised in the nuclei. In the metaphase cells, we found high concentrations of this enzyme in the mitotic chromosomes. Our results support the idea of the accumulation of DNA topoisomerase II alpha at the end of the cell cycle. The extractions of nuclei with 25 mM Lis led to the complete depletion of DNA topoisomerase II alpha from the residual nuclear matrix. Using a high dilution of the first antibody, we established that the high level of heterochromatin compactisation in the interphase nuclei is caused by the high concentration of DNA topoisomerase II alpha
