Substitution of Zinc Bound to Metallothionein for Cadmium in Vitro and in Vivo

Rats were injected subcutaneously with cadmium (3 mg/kg) ; 5 ml of the supernatant of the liver homogenate from these animals was incubated with cadmium, and the reaction mixture was gel filtered over Sephadex G-75 column. Cadmium in vitro was mainly bound to metallothionein fraction and the amount of zinc, approximately equal to the increased amount of cadmium in metallothionein fraction, decreased. Rats pretreated subcutaneously with cadmium (3 mg/kg) were injected 2 days later intravenously with labeled cadmium (0-3 mg/kg). Almost all of the radioactivity was already bound to metallothionein fraction after 30 min. The ratio of cadmium to zinc both bound to two different types of metallothionein fractionated by gel filtration and ion exchange chromatography increased in proportion to the amount of cadmium additionally injected. These findings indicate that the zinc bound to metallothionein is substituted with cadmium both in vivo and in vitro.