ROLE OF METALLOTHIONEIN IN THE LIVER IN PROTECTION AGAINST CADMIUM TOXICITY

Effect of pre-synthesized metallothionein in livers of rats on cadmium distri-bution in the soluble fractions was investigated in vitro and in vivo.The liver of a mail rat injected with cadmium (1 mg/kg, intraperitoneally) was removed at 24 hr after the injection and soluble fraction was prepared from the tissue. Cadmium was concentrated in a low molecular weight portion corresponding to metallothionein in the soluble fraction, and zinc was also included in this portion. When this soluble fraction was added with cadmium in vitro, most of cadmium was found in this portion, and zinc was replaced by cadmium. The proportion of these metals was 1 : 1 in molar ratio.Male rats were pretreated with a small amount of cadmium (0.3 mg/kg, intraper-itoneally) at 24 hr before injection with a challenge dose of cadmium (3 mg/kg, in-traperitoneally). Cadmium intake in the livers was significantly promoted, and cadmium concentration in the soluble fraction was higher in comparison with non-pretreated rats. A large amount of cadmium was also found in the low molecular weight portion, whereas zinc in this portion disappeared at 2 hr after the challenge injectien. It was probable that this promotion of cadmium intake was brought predominantly by the replacement of zinc in zinc-thionein by cadmium; the proportion being estimated to be the same as in vitro. Thus, cadmium may be more rapidly immobilized in the livers of cadmium pretreated rats than that of non-pretreated rats.On the basis of these findings, the protective effect of the pretreatment against cadmium toxicity was discussed in relation with metallothionein induced by the pre-treatment.