MITOCHONDRIAL RESPIRATORY ENZYMES ARE A MAJOR TARGET OF IRON TOXICITYIN RAT HEART-CELLS

Our previous studies in iron-loaded rat heart cells showed that in vit ro iron loading results in peroxidative injury, manifested in a marked decrease in rate and amplitude of heart cell contractility and rhythm icity, which is correctable by treatment with deferoxamine (DF). In th e present studies we explored the role of mitochondrial damage in myoc ardial iron toxicity. Iron loading by 24-hour incubation with 0.36 mmo l/L ferric ammonium citrate resulted in a decrease in the activity of nicotinamide adenine dinucleotide (NADH)-cytochrome c oxidoreductase ( complex I+III) to 35.3% +/- 11.2% of the value in untreated controls; of succinate-cytochrome c oxidoreductase (complex II+III) to 57.4% +/- 3.1%; and of succinate dehydrogenase to 63.5% +/- 12.6% (p < 0.001 in all cases). The decrease in activity of other mitochondrial enzymes, including NADH-ferricyanide reductase, succinate ubiquinone oxidoreduc tase (complex II), cytochrome c oxidase (complex IV), and ubiquinol cy tochrome c oxidoreductase (complex III), was less impressive and range d from 71.5% +/- 15.8% to 91.5% +/- 14.6% of controls, That the observ ed loss of respiratory enzyme activity was a specific effect of Iron t oxicity was clearly demonstrated by the complete restoration of enzyme activities by in vitro iron chelation therapy. Sequential treatment w ith iron and doxorubicin caused a loss of complex I+III and complex II +III activity that was greater than that seen with either agent alone but was only partially correctable by DF treatment. Alterations in cel lular adenosine triphosphate measurements paralleled very closely the changes observed in respiratory complex activity These findings demons trate for the first time the impairment of cardiac mitochondrial respi ratory enzyme activity caused by iron loading at conditions formerly s hown to produce severe abnormalities in contractility and rhythmicity.