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Zinc and iron speciation in the cereal grain

  • Author(s): Schjoerring, Jan K.
  • Persson, Daniel P.
  • Hansen, Thomas H.
  • Laursen, Kristian H
  • Husted, Søren
  • et al.
Abstract

Phytic acid and proteins are believed to be important for the distribution and bio-availability of zinc and iron in the cereal grain, but little quantitative information is available on the relative importance of different organic ligands. We have used several chromatographic techniques including size exclusion chromatography (SEC) hyphenated to ICP-MS to analyse the molecular speciation of Fe and Zn in tissues of barley and rice seeds. The majority of Fe in barley embryos co-eluted with P as a species with the apparent mass of 12.3 kDa, whereas the majority of Zn co-eluted with S as a 3 kDa species, devoid of any co-eluting P. Subsequent ion pairing chromatography of the Fe/P peak showed that phytic acid (myo-inositol 1, 2, 3, 4, 5, 6-hexakisphosphate: IP6) was the main Fe binding ligand, with the stoichiometry Fe4(IP6)18. When incubating the embryo tissue with phytase, the enzyme responsible for degradation of phytic acid, the extraction efficiency of both Fe and P was doubled, whereas that of Zn and S was unaffected. Protein degradation on the other hand, using protease XIV, boosted the extraction of Zn and S, but not that of Fe and P. Similar observations were made for the endosperm of rice grains. It is concluded that Fe and Zn have a different molecular speciation in cereal grain tissues; Zn appears mainly bound to peptides, while Fe is mainly associated with phytic acid.

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