The utilization of iron and its complexes by mammalian mitochondria

Sonicated mitochondria catalyse the reduction of ferric salts, and the subsequent incorporation of Fe(2+) into haem, when provided with a reducing substrate such as succinate or NADH. The rate of haem synthesis was low under aerobic conditions and, after a short lag period, accelerated once anaerobic conditions were achieved; it was insensitive to antimycin A. The lag period was decreased by preincubating the mitochondria with NADH and Fe(3+). Newly formed Fe(2+) was autoxidized rapidly and the consequent O(2) uptake was measured with an oxygen electrode to determine the rate of enzymic formation of Fe(2+) from FeCl(3); this reaction was rapid in sonicated mitochondria provided with NADH or succinate and was insensitive to antimycin A. The reaction was very slow in intact mitochondria, suggesting a permeability barrier to Fe(3+) ions. This system was used to test the permeability of the mitochondrial membrane to various iron complexes of biological importance. Of the compounds tested only ferrioxamine G appeared to penetrate readily and the iron of this complex was reduced when intact mitochondria were supplied with succinate or NADH-linked substrates. The reduction was insensitive to rotenone or antimycin A. Both ferrioxamine G and ferrioxamine B were, however, reduced by particles. The membrane fraction of sonicated mitochondria was necessary for the reduction. The rate of ferrioxamine B reduction by sonicated mitochondria was measured by a dual-wavelength spectrophotometric assay and was found to be stimulated in conditions where the Fe(2+) produced was utilized for haem synthesis. The addition of FeCl(3) to anaerobic particles caused an oxidation of cytochrome b when this region of the respiratory chain was isolated by treatment with rotenone and antimycin A. These results suggest that the reduction of ferric iron and its complexes occurs inside the inner mitochondrial membrane in proximity to ferrochelatase. Possible sites for this reduction are the flavoproteins, succinate and NADH dehydrogenase.