Nicotianamine Chelates Both FeIII and $\text{Fe}^{\text{II}}$ . Implications for Metal Transport in Plants

• Published in: Plant physiology (Bethesda) 1999-03, Vol.119 (3), p.1107-1114
• Main Authors: von Wirén, Nicolaus, Sukhbinder Klair, Suhkibar Bansal, Briat, Jean-Francois, Khodr, Hicham, Shioiri, Takayuki, Leigh, Roger A., Hider, Robert C.
• Format: Article
• Language: English
• Subjects: Chelates; Chelation; Citrates; Electrophoresis; Ligands; Metal ions; Phloem; Plants; Whole-Plant Processes; Xylem
• ISSN: 0032-0889; 1532-2548

Nicotianamine (NA) occurs in all plants and chelates metal cations, including $\text{Fe}^{\text{II}}$, but reportedly not FeIII. However, a comparison of the $\text{Fe}^{\text{II}}$ and $\text{Zn}^{\text{II}}$ affinity constants of NA and various FeIII-chelating aminocarboxylates suggested that NA should chelate FeIII. High-voltage electrophoresis of the FeNA complex formed in the presence of FeIII showed that the complex had a net charge of 0, consistent with the hexadentate chelation of FeIII. Measurement of the affinity constant for FeIII yielded a value of $10^{20.6}$, which is greater than that for the association of NA with $\text{Fe}^{\text{II}}$ ($10^{12.8}$). However, capillary electrophoresis showed that in the presence of $\text{Fe}^{\text{II}}$ and FeIII, NA preferentially chelates $\text{Fe}^{\text{II}}$, indicating that the $\text{Fe}^{\text{II}}$NA complex is kinetically stable under aerobic conditions. Furthermore, Fe complexes of NA are relatively poor Fenton reagents, as measured by their ability to mediate H2O2-dependent oxidation of deoxyribose. This suggests that NA will have an important role in scavenging Fe and protecting the cell from oxidative damage. The pH dependence of metal ion chelation by NA and a typical phytosiderophore, 2′-deoxymugineic acid, indicated that although both have the ability to chelate Fe, when both are present, 2′-deoxymugineic acid dominates the chelation process at acidic pH values, whereas NA dominates at alkaline pH values. The consequences for the role of NA in the long-distance transport of metals in the xylem and phloem are discussed.