The presence of two mitochondrial DNA topoisomerases in human acute leukemia cells.

We have undertaken a study of DNA topoisomerases in mitochondria from human acute leukemia cells. Two activities have been detected in these organelles. One of the enzymes is presumably a type II topoisomerase, i.e., in ATP-dependent reactions it can catenate closed circular plasmid DNA, and decatenate closed circular kinetoplast DNA. A second topoisomerase is presumably a type I enzyme since, it can relax positive as well as negative supercoils in an ATP-independent reaction, it is unable to catenate plasmid DNA or decatenate kinetoplast DNA, and it is inhibited, rather than stimulated, by ATP.