Ultrastructure and microchemical composition of ethidium bromide-induced intramitochondrial complexes.

Ultrastructural changes in mammalian cells treated with ethidium bromide (EB) occur predominantly in the mitochondria. One hour after addition of 10 μg/ml of EB, an accumulation of electron-dense materials occurred in many mitochondria. After 4 h of treatment, mitochondrial complexes consisting of helically arranged fibers 30–260 Å in thickness were observed. ³H]TdR autoradiography demonstrated the presence of DNA in the complexes. EB-treated cells were also studied using energy dispersive X-ray analysis techniques. Mitochondria containing dense complexes were significantly different, microchemically, from mitochondria devoid of the structure, and contained analyzable amounts of bromine. These observations suggest that EB treatment of cells induces the formation of electron-dense mitochondrial complexes containing EB, mitochondrial DNA, and protein, and the complexes are associated with an inhibition of normal mitochondrial development.