Iron release from ferritin and its sensitivity to superoxide ions differs among vertebrates

The influence of the superoxide-generating system, xanthine oxidase, on the release of iron from various vertebrate ferritins was determined both in the presence and absence of superoxide dismutase. The initial rate of iron release in the presence of this system was higher for ferritins from human, trout and rat liver than for those from lamprey liver and horse spleen. The proportion of this iron release that was superoxide-dependent in the case of rat, human and trout ferritins was 92, 86 and 84% respectively, whereas no such superoxide-dependent iron release occurred from the ferritins of lamprey liver and horse spleen. On the other hand, the rate of superoxide-independent iron release was of comparable magnitude for all of the species examined. The rate of superoxide-dependent iron release was related neither to the iron: protein ratios nor to the subunit size of the ferritins. However, it is significant that the ferritins with a high rate of superoxide-dependent iron release came from tissues known to be susceptible to iron damage. It is thus proposed that the resistance of lamprey liver ferritin to the mobilization of iron by superoxide ions accounts in part for the tolerance of the lamprey liver to high iron loads.