Aconitase activity. (A) The level of aconitase activity in the total cellular homogenates and in the postmitochondrial fraction was analyzed and expressed as percentage of the activity of the control mock-transfected cells. Mean and SD of 7 experiments for the total and of 3 experiments for the cytosolic aconitase. *Significant difference (P < .05). (B, left) The total cell homogenates of untreated HeLa cells (Tot), the mitochondrial fraction (MF), and the postmitochondrial fraction (PMF) were separated on nondenaturing PAGE, and the activity of aconitase was revealed. The fast band corresponds to the cytosolic aconitase (c-aco), while the slower band, to the mitochondrial isoform (m-aco). (Right) HeLa cells, transfected with ABCB7 siRNA (siRNA), treated for 18 hours with 30 μM CCCP to abolish membrane potential, or for 18 hours with the iron chelator desferrioxamine 1 mM (DFO), as indicated, were analyzed for aconitase activity. Wells were loaded with 100 μg proteins, and the load was verified by protein stain of the gel.