The role of metallothionein in oxidative stress
A cell culture model was developed to examine the role of metallothionein in oxidative stress. Chinese hamster ovary cells, which contain little endogenous metallothionein, were transfected with a metallothionein-expressing gene. Metallothionein content of the resultant cell lines ranged from a physiological level to a very high level. The major antioxidant enzyme activities did not vary between the cell lines. Glutathione content, and the proportion of glutathione that was oxidised, was higher than that of the wild type in one of the cell lines, indicating that this cell line had a different phenotype from the wild type, which would affect its response to oxidative stress. Evidence gained from the metallothionein-enriched cells in which glutathione was unaltered from the wild type, indicated a protective effect of metallothionein against oxidative stress. In these cell lines high levels of metallothionein protected the cell against membrane-lysing effects of tert-butyl hydroperoxide (tBuOOH) and menadione sodium bisulphite (MSB). Physiological levels of metallothionein protected cells against tBuOOH. Metallothionein failed to protect DNA against tBuOOH or ferric chloride, but afforded protection to DNA against MSB or cupric chloride, agents with which it can form a conjugate or complex. This suggests that metallothionein was probably located in the cytoplasm, where it was able to scavenge free radicals and reactive oxygen species, thus protecting cytoplasmic components. It only affords protection to DNA when it can prevent the damaging agent from reaching the target site by conjugation or complexing. The cell line with altered phenotype from the wild type, exhibited an enhanced sensitivity to oxidative stress, despite high metallothionein content. Thus a protective role of metallothionein is dependent on the phenotype of the cell.