Reversible inhibition of cytochrome oxidase accumulation in human cells by ethidium bromide

Abstract

We investigated the ability of ethidium bromide (EB) to induce respiratory mutation in human cells in culture. Human cells grown in EB at non-toxic concentrations underwent a 75–85% reduction in cytochrome oxidase activity. This reduction was paralleled by disappearance of cytochrome a + a 3 as determined by difference absorption spectra; the cellular content of cytochromes b, c, and c 1 was unaffected by EB. When cells grown in EB were subcultured in EB-free medium, they regained cytochrome oxidase activity characteristic of cells not exposed to EB. The reversal of the EB effect was found in both mass cultures and in clones isolated following EB exposure. The reversibility of the effect of EB on cytochrome oxidase makes it unlikely that EB, in non-lethal concentrations, has the same mutagenic effect on the mitochondrial DNA of higher eukaryotes as on lower eukaryotes. Possible mechanisms by which EB causes reversible inhibition of cytochrome oxidase synthesis are considered.