Nicotianamine chelates both FeIII and FeII.: Implications for metal transport in plants

Nicotianamine (NA) occurs in all plants and chelates metal cations, including Fe-II, but reportedly not Fe-III. However, a comparison of the Fe-II and Zn-II affinity constants of NA and various Fe-III chelating aminocarboxylates suggested that NA should chelate Fe-III High-voltage electrophoresis of the FeNA complex formed in the presence of Fe-III showed that the complex had a net charge of 0, consistent with the hexadentate chelation of Fe-III. Measurement of the affinity constant for Fe-III yielded a value of 10(20.6), which is greater than that for the association of NA with Fe-II (10(12.8)). However, capillary electrophoresis showed that in the presence of Fe-II and Fe-III, NA preferentially chelates Fe-II, indicating that the Fe(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.