Possible role of metallothionein on the gastrointestinal absorption and distribution of cadmium.

Although the role of metallothionein (MT) has been suggested by many researchers, the exact biological or physiological roles of MT are still unclear. Up to the present, the role of this ubiquitous low molecular weight protein has variously been reported as 1) the regulation of the normal absorption and homeostasis of essential metals such as zinc (Zn), copper (Cu), 2) the chelation and detoxification of toxic metals such as cadmium (Cd) or mercury (Hg). The mechanisms of Cd uptake from gastrointestinal lumen and its consecutive transport from the intestinal mucosa into blood circulation and tissue are not well known. The chemical form of Cd is an important factor on its gastrointestinal absorption. It has been reported that Cd in foods such as meat, seafoods, and vegetable is existing mainly as Cd-MT or MT-like Cd binding proteins. Also it is well known that MT is induced in the intestinal tissue by oral Cd administration. Until today, the mechanism of gastrointestinal absorption has been mainly discussed about Cd ion. However, it needs the consideration including the chemical form of Cd such as Cd-MT in foods or Cd bound by MT induced in the intestinal tissue. Namely, it is important to assess the possible role of MT on gastrointestinal absorption and its consecutive tissue distribution of Cd.