ANALYSIS OF PHOSPHORYLATION LEVEL OF DNA TOPOISOMERASE-II-ALPHA AND TOPOISOMERASE-II-BETA IN HUMAN HELA-CELLS

We have studied the phosphorylation of DNA topoisomerases II in HeLa c ells focusing on the beta isoform of the enzyme which is very difficul t to analyze because of its instability. In proliferating cells, we ob served that both the a and beta isozymes are labeled after cell incuba tion with (32)p. The phosphorylation of beta enzyme occurs to a low ex tent, thus reflecting the expression of topoisomerases II during cell cycle. In cells treated with etoposide, the activity of topoisomerase II is inhibited and the level of phosphorylation decreases, suggesting a possible cooperation between this modification and drug response.