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The formation of ferritin from apoferritin. Kinetics and mechanism of iron uptake.

ABSTRACT: Ferritin has a high capacity as an iron store, incorporating some 4500 iron atoms as a microcrystalline ferric oxide hydrate. Starting from apoferritin, or ferritin of low iron content, Fe(2+) and an oxidizing agent, the uptake of iron can be recorded spectrophotometrically. Progress curves were obtained and the reconstituted ferritin was shown by several physical methods to be similar to natural ferritin. The progress curves of iron uptake by apoferritin are sigmoidal; those for ferritins of low iron content are hyperbolic. The rate of iron uptake is dependent on the amount of iron already present in the molecule. The distribution of iron contents among reconstituted ferritin molecules is inhomogeneous. These findings are interpreted in terms of a crystal growth model. The surface area of the crystallites forming inside the protein increases until the molecule is half full, and then declines. This surface controls the rate at which new material is deposited. The experimental results can best be accounted for by a two-stage mechanism, an initial slow ;nucleation' stage, which is apparently zero order with respect to [Fe(2+)], followed by a more rapid ;growth' stage. The rate of Fe(2+) oxidation is increased in the presence of apoferritin as compared with controls. Ferritin can therefore be regarded as an enzyme to which the product remains firmly attached. The protein appears to increase the rate of ;nucleation'. The apparent zero order of this stage suggests the presence of binding sites on the protein, which are saturated with respect to Fe(2+). These sites are presumed also to be oxidation sites. The oxidation and subsequent formation of the ferric oxide hydrate may proceed according to one of three alternative models.

SUBMITTER: Macara IG

PROVIDER: S-EPMC1178360 | biostudies-other | 1972 Jan

REPOSITORIES: biostudies-other

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Project description:Background:Iron deficiency is an enduring global health problem that requires new remedial approaches. Iron absorption from soybean-derived ferritin, an ?550-kDa iron storage protein, is comparable to bioavailable ferrous sulfate (FeSO4). However, the absorption of ferritin is reported to involve an endocytic mechanism, independent of divalent metal ion transporter 1 (DMT-1), the transporter for nonheme iron. Objective:Our overall aim was to examine the potential of purified ferritin from peas (Pisum sativum) as a food supplement by measuring its stability under gastric pH treatment and the mechanisms of iron uptake into Caco-2 cells. Methods:Caco-2 cells were treated with native or gastric pH-treated pea ferritin in combination with dietary modulators of nonheme iron uptake, small interfering RNA targeting DMT-1, or chemical inhibitors of endocytosis. Cellular ferritin formation, a surrogate measure of iron uptake, and internalization of pea ferritin with the use of specific antibodies were measured. The production of reactive oxygen species (ROS) in response to equimolar concentrations of native pea ferritin and FeSO4 was also compared. Results:Pea ferritin exposed to gastric pH treatment was degraded, and the released iron was transported into Caco-2 cells by DMT-1. Inhibitors of DMT-1 and nonheme iron absorption reduced iron uptake by 26-40%. Conversely, in the absence of gastric pH treatment, the iron uptake of native pea ferritin was unaffected by inhibitors of nonheme iron absorption, and the protein was observed to be internalized in Caco-2 cells. Chlorpromazine (clathrin-mediated endocytosis inhibitor) reduced the native pea ferritin content within cells by ?30%, which confirmed that the native pea ferritin was transported into cells via a clathrin-mediated endocytic pathway. In addition, 60% less ROS production resulted from native pea ferritin in comparison to FeSO4. Conclusion:With consideration that nonheme dietary inhibitors display no effect on iron uptake and the low oxidative potential relative to FeSO4, intact pea ferritin appears to be a promising iron supplement.

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The formation of ferritin from apoferritin. Kinetics and mechanism of iron uptake.

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