| dc.description.abstract | Iron deficiency, a condition currently affecting ;3 billion people, persists in the 21st century despite half a millennium of
medical treatment. Soybean ferritin (SBFn), a large, stable protein nanocage around a mineral with hundreds of iron and
oxygen atoms, is a source of nutritional iron with an unknown mechanism for intestinal absorption. Iron absorption from
SBFn is insensitive to phytate, suggesting an absorption mechanism different from for the ferrous transport. Here, we
investigated the mechanism of iron absorption from mineralized SBFn using Caco-2 cells (polarized in bicameral inserts) as
an intestinal cell mode and analyzed binding, internalization and degradation with labeled SBFn (131I or fluorescent labels),
confocal microscopy, and immunoanalyses to show: 1) saturable binding to the apical cell surface; dissociation constant of
7.7560.88 nmol/L; 2) internalization of SBFn that was dependent on temperature, concentration, and time; 3) entrance of
SBFn iron into the labile iron pool (calcein quenching); 4) degradation of the SBFn protein cage; and 5) assembly peptide 2
(AP2)-/clathrin-dependent endocytosis (sensitivity of SBFn uptake to hyperosmolarity, acidity, and RNA interference to the
m2 subunit of AP2), and resistance to filipin, a caveolar endocytosis inhibitor. The results support a model of SBFn
endocytosis through the apical cell membrane, followed by protein cage degradation, mineral reduction/dissolution, and iron
entry to the cytosolic iron pool. The large number of iron atoms in SBFn makes iron transport across the cell membrane a
much more efficient event for SBFn than for single iron atoms as heme or ferrous ions. | |
