A: DNA relaxation of supercoiled plasmid DNA. Lane 1, no extract; lane 2, 1.0 μg mitochondrial extract; lane 3, 0.5 μg mitochondrial extract; lane 4, 2.5 U purified topoisomerase 1; lane 5, linearized pBR322. B: 10 μM hydroxycamptothecin exacerbated DNA cleavage of LV mitochondrial extracts. Values are means ± SE and are normalized to no mitochondria (No Mito) control. *P < 0.05 compared with No Mito control; #P < 0.05 compared with mitochondria alone (+Mito). C: 0.5 mM doxorubicin exacerbated DNA cleavage of LV mitochondrial extracts. Values are means ± SE and are normalized to No Mito. D: diabetes increased myocardial mitochondrial topoisomerase DNA cleavage activity. Myocardial mitochondrial extracts were incubated with linear mtDNA for 30 min before being electrophoresed, and cleavage products were quantified by band intensity. Values are means ± SE and are normalized to Wistar DNA cleavage. *P < 0.05 compared with Wistar LV mitochondrial extract.