Abstract
Nicotianamine (NA), a plant-derived metal chelator abundant in vegetables, is a bioavailable iron-chelating compound in mammals. We recently reported that NA-iron (NA-Fe) complexes are absorbed in the small intestine via a proton-conjugated amino acid transporter (PAT1). However, the subsequent process remains unknown. In this study, using radioactive iron tracers and NA export quantification in Xenopus laevis oocytes, we identified the heterodimeric amino acid transporter as being responsible for transporting NA-Fe(II) complexes into the vascular system; this complex is composed of L-type amino acid transporter 2 (LAT2) and 4F2 heavy chain (4F2hc). Oocytes expressing LAT2/4F2hc exhibited significantly higher efflux of NA-59Fe(II) than free Fe via ferroportin 1, indicating an NA-Fe(II)-specific excretion mechanism. A LAT2 mutant (N134S), which is known to enhance amino acid efflux, also increased radioactive iron-NA excretion. Molecular modeling revealed that the substrate-binding cavity of LAT2 could accommodate NA-Fe(II), suggesting that NA-Fe(II) could be transported through the same cavity entrance as amino acids. The expression profiles of PAT1 and LAT2/4F2hc in the small intestine were higher in the proximal jejunum than in the duodenum, which was consistent with the NA and iron content profiles in the small intestine of mice treated with NA-Fe(II). These findings suggest the presence of different iron absorption mechanisms. Iron is directly absorbed in the duodenum via the divalent metal transporter 1/ferroportin 1 system and as an iron-chelator complex in the proximal jejunum via the PAT1/LAT2(4F2hc) amino acid transporter system.