Iron(II) and Hydrogen Peroxide Detoxification by Human H-Chain Ferritin. An EPR Spin-Trapping Study

Abstract

Overexpression of human H-chain ferritin (HuHF) is known to impart a degree of protection to cells against oxidative stress and the associated damage to DNA and other cellular components. However, whether this protective activity resides in the protein's ability to inhibit Fenton chemistry as found for Dps proteins has never been established. Such inhibition does not occur with the related mitochondrial ferritin which displays much of the same iron chemistry as HuHF, including an Fe(II)/H(2)O(2) oxidation stoichiometry of approximately 2:1. In the present study, the ability of HuHF to attenuate hydroxyl radical production by the Fenton reaction (Fe(2+) + H(2)O(2) --> Fe(3+) + OH(-) + *OH) was examined by electron paramagnetic resonance (EPR) spin-trapping methods. The data demonstrate that the presence of wild-type HuHF during Fe(2+) oxidation by H(2)O(2) greatly decreases the amount of .OH radical produced from Fenton chemistry whereas the ferroxidase site mutant 222 (H62K + H65G) and human L-chain ferritin (HuLF) lack this activity. HuHF catalyzes the pairwise oxidation of Fe(2+) by the detoxification reaction [2Fe(2+) + H(2)O(2) + 2H(2)O --> 2Fe(O)OH(core) + 4H(+)] that occurs at the ferroxidase site of the protein, thereby preventing the production of hydroxyl radical. The small amount of *OH radical that is produced in the presence of ferritin (<or=1% of the iron oxidized) appears to arise from the reaction of H(2)O(2) with Fe(III) in the protein rather than from simple Fenton chemistry. The results are discussed in terms of recent experiments reporting both protective and degradative effects of ferritin iron on the integrity of nuclear DNA.