Biological Functions of Zinc.

Zinc is an essential trace element for the body. In 1933, Todd et al. first reported that zinc was a necessary nutrient for rats. Later, Tucker and Salmon reported that zinc deficiency caused skin damage in pigs, and O'Dell recognized zinc’s essential role in the growth of chickens. Over the past decades, our understanding of the biological functions of zinc has advanced rapidly.

1. Zinc Absorption

Zinc is primarily absorbed in the duodenum and jejunum, with minimal absorption occurring in the ileum. However, the exact contribution of each part of the intestine to zinc absorption is still uncertain (Solomons and Cousins, 1984). Zinc absorption involves two main pathways: carrier-mediated absorption and non-mediated diffusion absorption. Both pathways are influenced by the concentration of zinc in the intestinal lumen (Cousins, 1989). Most zinc is absorbed via the carrier-mediated pathway, especially when the zinc concentration in the lumen is low. When the intestinal zinc concentration is high, such as in high-zinc diets, zinc can also be absorbed through the non-mediated mechanism. As food is further digested in the small intestine, zinc is presented to the intestinal epithelial cells in the form of smaller zinc-binding ligands, rather than as free zinc. In this case, solubility appears to be a key factor influencing zinc uptake by the intestinal epithelial cells. Additionally, vesicular transport is considered another way for zinc to be transferred across cells.

2. Zinc Metabolism

Zinc-binding proteins, particularly metallothionein (MT), are crucial regulators of zinc metabolism. Dietary zinc induces the synthesis of metallothionein through the metal response element (MRE) and the metal-binding transcription factor 1 (MTF-1) mechanism. This dietary regulation of metallothionein expression seems to form an automatic regulatory system, where the increase in metallothionein synthesis correlates with the rise in intracellular zinc binding. Metallothionein likely acts as a metabolic buffer for zinc in the body, regulating the levels of free zinc ions and coordinating the intracellular zinc pool that responds to both hormones and dietary inputs. Kinetic studies show that a decrease in plasma zinc levels corresponds with the synthesis of metallothionein and the zinc stored in the metallothionein-bound state. The transport carriers regulating zinc entry and exit from cells may allow cells to adapt further to varying levels of zinc intake, even without metallothionein involvement.

The small intestine plays a key role in the regulation of zinc metabolism by controlling the absorption of dietary exogenous zinc and the retention of endogenous zinc. This regulatory process responds to changes in the body’s zinc nutritional status, and is also influenced by dietary factors that can affect exogenous zinc absorption. Conditions such as diarrhea and malabsorption can also impact the regulatory process.

Conclusion:

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