Investigation of competitive binding of the essential trace elements zinc, iron, copper, and manganese by gastrointestinal mucins and the effect on their absorption in vitro

The molecular mechanism by which the essential trace elements (TEs) zinc (Zn), iron (Fe), copper (Cu), and manganese (Mn) interact during their uptake, particularly their competition for interaction with the gastrointestinal mucus layer, remains unclear. Thus, in this study we investigated the binding of TEs to gastrointestinal mucins individually and interdependently and analyzed the impact of mucus on their intestinal absorption. Cell-free binding and competition experiments with commercially available mucin and native MUC2 and MUC5AC from porcine gastrointestinal tract showed high TE-binding capacity (Fe>Zn>Cu>Mn) with an average affinity that was highest for Zn and lowest for Mn (Zn>Fe>Cu>Mn). Gastrointestinal mucins contain high-affinity TE-binding sites with physiologically relevant affinities, and TEs mutually affected their interaction with mucins in ratios reflecting the in vivo situation, leading to various cases of displacement or augmented binding. Solely Fe was unaffected by other TEs, apart from Mn that facilitated the Fe-mucin interaction. The relevance of TE-binding by the mucins was underlined by transport studies with two 3D in vitro intestinal models: Caco-2 mono- and Caco-2/HT-29-MTX co-cultures, showing that at least for Fe and Zn, mucus provides a buffering system and increases the availability of the TEs by delivering them to the underlying enterocytes. This study provides the first systematic analysis of the competition between Zn, Fe, Cu, and Mn for binding to gastrointestinal mucins, demonstrating the importance of the mucus layer for TE absorption and that competition for their intestinal uptake already starts at the mucus barrier, with translational relevance for nutrition and (malabsorption) diseases.