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A novel nonapeptide DTDSEEEIR identified from Antarctic krill (Euphausia superba) iron-binding peptides was used in this study to analyze its iron-binding sites and structural changes after iron coordination. The enzymatic resistance and transport of DTDSEEEIR–iron during gastrointestinal digestion and absorption as well as the relationship between the DTDSEEEIR stability and the enhancement of iron absorption were further explored. Results revealed that iron ions spontaneously bound to the carboxyl, hydroxyl, and amino groups of the DTDSEEEIR peptide, which induced the folding of DTDSEEEIR to form a more orderly structure. The DTDSEEEIR peptide remained stable to a certain extent (79.60 ± 0.19%) after gastrointestinal digestion and the coordination of iron improved the digestive stability of the DTDSEEEIR peptide (93.89 ± 1.37%). Moreover, the stability of DTDSEEEIR across intestinal epithelium had a positive effect on iron absorption, which implied that DTDSEEEIR might carry iron ions through intestinal epithelial cells.

An antioxidant peptide derived from egg white, Asp-His-Thr-Lys-Glu (DHTKE), possesses specific amino acids related to zinc delivery. This study aimed to demonstrate the molecular basis of interactions between the egg white peptide (DHTKE) and zinc ions and investigate the effect of the DHTKE–Zn complex on zinc delivery through the gastrointestinal system. Approximately one DHTKE molecule can bind one zinc ion (n= 1.048 ± 0.085) through its carboxyl, amino, and imidazole nitrogen groups on Asp, His, and Glu. The formed DHTKE–Zn complex presented uniformly distributed globular particles with a particle size of 100–500 nm and underwent dissociation and re-chelation during gastrointestinal digestion. Moreover, the DHTKE peptide mostly remained stable, with a retention rate of 98.32% under gastrointestinal digestion, although one degradation product (DHTK) was identified by nanoscale liquid chromatography–electrospray ionization–tandem mass spectrometry in the gastrointestinal digests; the effectiveness of DHTKE–Zn digests on enhancing absorption of zinc was comparable to that of the initial complex.

In this study, the binding mechanism, morphological, and conformational analysis of the complex of a sea cucumber ovum derived octapeptide (EDLAALEK) with Ca2+as well as its calcium delivery behavior via the gastrointestinal (GI) tract were investigated. The Ca2+specifically bound to two carboxyl oxygen atoms of C-terminal Glu and Asp on the EDLAALEK peptide at a stoichiometric ratio of 1:1. Calcium coordination induced the self-assembly of the EDLAALEK peptide, resulting in the formation of a nanocomposite with a crystal structure. Furthermore, the formed nanocomposite went through dissociation and self-assembly during in vitro GI digestion, accompanied by the release and rechelation of Ca2+, which was related to changes in their secondary structure. Nevertheless, the GI digests of the EDLAALEK–calcium complex could significantly enhance Ca2+absorption across Caco-2 cell monolayers. The findings suggest that the sea cucumber ovum derived peptide has the potential as an efficient nanocarrier to transport calcium through the GI system.

Antarctic krill (Euphausia superba) might have specific peptides that chelate iron ions and are iron carriers. In this study, iron-binding peptides were isolated from Antarctic krill using enzymatic (trypsin) hydrolysis and immobilized metal affinity chromatography (IMAC); the binding mode and surface morphology of Antarctic krill peptides (AKP)-iron complexes as well as their iron delivery behavior in the gastrointestinal tract were investigated. Results showed that the AKP bound 79.3% of available iron from the enzymatic hydrolysis for 180 min. The contents of negatively-charged amino acids (Asp and Glu) on the AKP showed a positive correlation (r = 0.80) with iron-binding activities (the data includes Asn and Gln), while positively-charged amino acids showed a negative correlation (r = −0.83). Moreover, His, Ser and Thr on the AKP were also involved in the iron-binding activities. Iron-binding peptides from Antarctic krill hydrolysates, isolated using IMAC-Fe3+and identified using HPLC-MS/MS, showed a mass of 712–2451 Da, and were enriched in Asp (Asn), Glu (Gln), His, Ser or Thr. The iron ions mainly bound to carboxyl groups of AKP, thus forming nano-scale, spherical particles. The iron nanocarriers formed by AKP showed a significant iron transport activity after simulated gastrointestinal digestion, as compared to FeSO4(P < 0.05). The results suggested that the AKP-iron nanocomposite has the potential to be a nutraceutical supplement for improving iron absorption.