Effect of Inorganic Phosphate on Iron-Induced Lipid Peroxidation in Rat Liver Mitochondria

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The effect of inorganic orthophosphate (Pi) on iron-induced lipid peroxidation in non-respiring rat liver mitochondria was studied and compared with that of adenosine 5'-diphosphate (ADP) at pH 7.4. Under the experimental conditions used, lipid peroxidation was induced by Fe²⁺ in conjunction with Pi and/or ADP, but not by Fe²⁺ alone. The characteristics of Fe²⁺-induced lipid peroxidation supported by Pi and by ADP were different : ADP-supported peroxidation took place after a lag-time and proceeded until all the O₂ in the incubation medium was consumed, whereas the Pi-supported peroxidation exhibited a longer lag-time and a higher rate, but stopped when half the O₂ was consumed. These features were ascribed to the fact that Pi forms active iron-oxygen complexes more rapidly than ADP, but the lifetimes of these complexes are less than those of the complexes with ADP. Furthermore, the ability of Pi to form active iron-oxygen complexes was found to be less than that of ADP. Fe²⁺, in conjunction with Pi and/or ADP, was found to induce lipid peroxidation in respiring mitochondria, although to a lesser extent than in non-respiring mitochondria.