Sensitivity of mitochondrial transcription to different free radical species

Proper mitochondrial function requires the continual maintenance of the integrity of the mitochondrial gene expression system. The sensitivity of both mitochondrial lipids and mitochondrial respiration to free radicals has been well recognized, but the effect of free radicals and lipid peroxidation on mitochondrial transcription has not yet been examined. Using an in vitro mitochondrial transcription assay, we tested the ability of five prooxidants to affect mitochondrial transcription. Results show that mitochondrial transcription is extremely sensitive to inhibition by peroxyl radicals generated by either 2,2'-azobis-(2-amidinopropane) (AAPH) or 2,2'-azobis-(2,4-dimethylvaleronitrile) (AMVN), and that this inhibition occurs prior to detectable evidence of lipid peroxidation as measured by thiobarbituric acid (TBA)-reactive substances, 4-hydroxynonenal accumulation, and oxygen consumption. Furthermore, although mitochondrial transcription was sensitive to 4-hydroxynonenal, it was resistant to malondialdehyde as well as high levels of lipid peroxidation induced by ADP/Fe/NADPH. Together, these results suggest that the repression of mitochondrial transcription is differentially sensitive to specific modes of free radical reaction.