Submitted to: Food & Function
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 10, 2012
Publication Date: June 27, 2012
Citation: Zhiqiang, C., Tako, E.N., Yeung, A., Welch, R., Glahn, R.P. 2012. Evaluation of metallothionein formation as a proxy for zinc absorption in an in vitro digestion/caco-2 cell culture model. Food and Function. 3(7):732-736. Interpretive Summary: We used a cellular model to determine if metallotheionine (a cellular protein that is heavy metal toxicity sensitive) can be used as an indicator of cellular dietary zinc absorption. In addition, the effect of phytic acid on zinc uptake and cellular metallothionine concentration was studied. The results showed that as media zinc concentrations increases, cellular zinc and metallothionine uptake and formation increases. Phytic acid significantly lowered the cellular Zn and MT concentrations. However, when the cellular metallothionine concentrations in cells exposed to white or red beans and fish samples were not different from cell baseline. This study suggests that measurements of cellular zinc and metallothionine concentrations have some limitations. Phytic acid was observed to be a potent inhibitor of zinc uptake. Under the conditions of this cell model, it appears that this cellular model is not useful to evaluate zinc availability from foods.
Technical Abstract: Caco-2 cell metallothionein (MT) formation was studied to determine if MT could be used as a proxy for zinc (Zn) absorption in a cell culture model. MT intracellular concentration was determined by using a cadmium/hemoglobin affinity assay. Cellular Zn uptake was determined in acid digests (5% HNO3) using inductively-coupled argon-plasma emission spectroscopy. Phytic acid (PA) effect on cellular Zn and MT concentrations was also studied. Cells were treated with media containing 0, 2, 5, 10, 25, 50, 75 micro mol/L Zn (ZnCl2). Effect of varying Zn:PA molar ratios (1:0, 1:1, 1:5, 1:10, 1:20) on Zn uptake and MT formation was determined. Results showed a positive linear correlation between Zn-media concentrations and cellular Zn uptake and MT formation was observed. Zn and MT concentrations in cells treated with increasing levels of Zn (>25 micro mol/L Zn) were elevated. Zn and MT concentrations in cells incubated with Zn (when < 10 micro mol/L) were similar to untreated cells. PA significantly lowered the cellular Zn and MT concentrations. When Zn:PA molar ratios were > 1:5, cellular MT concentrations were not different from untreated cells. When a combined in vitro digestion/cell model was used, the cellular MT concentrations in white or red beans and fish samples were not different from cell baseline. This study suggests that measurements of cellular Zn and MT concentrations have some limitations (<10 micro mol/L Zn). PA was observed to be a potent inhibitor of Zn uptake. Under the conditions of this in vitro model, Caco-2 cell monolayers are not useful to evaluate Zn availability from foods.