Absorption of iron from ferritin is independent of heme iron and ferrous salts in women and rat intestinal segments

Abstract

Ferritin iron from food is readily bioavailable to humans and has the potential for treating iron deficiency. Whether ferritin iron absorption is mechanistically different from iron absorption from small iron complexes/salts remains controversial. Here, we studied iron absorption (RBC (59)Fe) from radiolabeled ferritin iron (0.5 mg) in healthy women with or without non-ferritin iron competitors, ferrous sulfate, or hemoglobin. A 9-fold excess of non-ferritin iron competitor had no significant effect on ferritin iron absorption. Larger amounts of iron (50 mg and a 99-fold excess of either competitor) inhibited iron absorption. To measure transport rates of iron that was absorbed inside ferritin, rat intestinal segments ex vivo were perfused with radiolabeled ferritin and compared to perfusion with ferric nitrilotriacetic (Fe-NTA), a well-studied form of chelated iron. Intestinal transport of iron absorbed inside exogenous ferritin was 14.8% of the rate measured for iron absorbed from chelated iron. In the steady state, endogenous enterocyte ferritin contained >90% of the iron absorbed from Fe-NTA or ferritin. We found that ferritin is a slow release source of iron, readily available to humans or animals, based on RBC iron incorporation. Ferritin iron is absorbed by a different mechanism than iron salts/chelates or heme iron. Recognition of a second, nonheme iron absorption process, ferritin endocytosis, emphasizes the need for more mechanistic studies on ferritin iron absorption and highlights the potential of ferritin present in foods such as legumes to contribute to solutions for global iron deficiency.

FIGURE 1

Ferritin iron absorption by healthy women administered ferrous sulfate at 0–1:99 molar ratios in GR and non-GR capsules. Individual data are in Supplemental Tables 2, 4, and 6. Values are mean ± SD, n = 13 or 14. **Different from 0, P < 0.01. GR, gastric resistant; non-GR, nongastric resistant.

FIGURE 2

Ferritin iron absorption by healthy women administered ascorbic acid in GR or non-GR capsules (A) or hemoglobin iron at 0–1:9 in non-GR capsules (B). Values are the mean ± SD, n = 15 (A) or 13 (B). Data for individuals are in Supplemental Tables 5 and 3, respectively. **Different from 0, P < 0.01. GR, gastric resistant; non-GR, nongastric resistant.

FIGURE 3

Accumulation and distribution of ⁵⁹Fe in cells, mainly enterocytes, from rat duodenum and jejunum perfused ex vivo with ⁵⁹Fe-ferritin or ⁵⁹Fe-NTA. The data are the mean ± SD, n = 2 or 3. *Different from corresponding Fe-NTA, P < 0.01. Note that the distribution of ⁵⁹Fe in the soluble (buffer-extractable) part of the cells and in proteins (TCA precipitable) was indistinguishable for perfused ⁵⁹ferritin and ⁵⁹Fe-NTA in duodenum and jejunum. Values were mean ± SD, n = 6, 95.5 ± 4.10 % (ferritin) and, n = 4, 92.8 ± 1.88% (Fe-NTA). Fe-NTA, ferric nitrilotriacetic acid; TCA, trichloroacetic acid.

FIGURE 4

Distribution of ⁵⁹Fe in the cytosolic proteins of cells, mainly enterocytes, from rat duodenum (A) and jejunum (B) perfused ex vivo with ⁵⁹Fe-ferritin or ⁵⁹Fe-NTA. (C) ⁵⁹Fe-HoSF, run at the same time. ⁵⁹Fe radioactivity was detected by autoradiography after native electrophoretic separation of soluble, cytoplasmic proteins (400 μg). (C) ⁵⁹Fe-HoSF, run at the same time. The cell samples described in Figure 3 were used. Higher order complexes of ferritin cages; 1: ferritin (24 subunits), 2: ferritin dimer (48 subunits); 3: ferritin trimer (96 subunits). Fe-NTA, ferric nitrilotriacetic acid; HoSF, horse spleen ferritin.