Regulation of metallothionein production in HeLa cells.

Metallothioneins are cysteine-rich, heavy-metal-binding proteins which have been assumed to participate in the detoxification of toxic metals. The mechanism of thionein (apoprotein of metallothionein) induction by cadmium was studied using cultured human cells. It was found that when thionein synthesis reaches a maximum (6-8 h after induction), it no longer responds to additional cadmium. Changes in cadmium uptake or induction of inhibitory proteins were not responsible. Together with our previous findings, a possible mechanism is proposed: loss of the secondary induction response might be due to increased intracellular levels of thionein, which has been overproduced by the initial induction.