*2.3.4 Metal ions*

Metal ions such as copper, iron and zinc are essential micronutrients required for the correct folding and activity of a broad range of enzymes. The contribution of the intestine to metal homeostasis has not been extensively investigated but midgut regions, the Cu cell and Fe regions, are the most proposed sites of metal ion absorption. The Cu cell region turns bright luminescent orange upon Cu ingestion due to the fixation of copper by metallothionein [101, 102], and appears to be an important site of accumulation of ingested radioactively labeled Cu [101, 103]. The Fe cell region in R4a stains by Prussian blue and also accumulates exogenously administered radioactive Fe [101, 104]. Many studies have confirmed the roles for the Cu/Fe regions by exploring the molecular machinery involved in the intestinal uptake, intracellular trafficking, and efflux of metal ions.
