Dynamics of (Cd,Zn)-metallothioneins in gills, liver and kidney of common carp Cyprinus carpio during cadmium exposure

Cadmium concentrations, (Cd,Zn)-metallothionein (MT) concentrations, MT syn thesis and the relative amounts of cadmium bound to (Cd,Zn)-MTs were determ ined in gills, liver and kidney of common carp Cyprinus carpio exposed to 0 , 0.5 muM (0.06 mg.l(-1)), 2.5 muM (0.25 mg.l(-1)) and 7 muM (0.79 mg.l(-1) ) Cd for up to 29 days. Cadmium accumulation was in the order kidney > live r > gills. Control levels of hepatic (Cd,Zn)-MT were four times higher comp ared to these of gills and kidney. No increases in (Cd,Zn)-MT concentration s were observed in liver during the exposure period. In comparison with con trol carp, (Cd,Zn)-MT concentrations increased up to 4.5 times in kidney an d two times in gills. In both these organs, (Cd,Zn)-MT concentrations were linearly related with cadmium tissue levels and with the de novo synthesis of MTs. Hepatic cadmium was almost completely bound to (Cd,Zn)-MT, while pe rcentages of non-MT-bound cadmium were at least 40% in gills and 25% in kid ney. This corresponded with a total saturation of (Cd,Zn)-MT by cadmium in kidney and a saturation of approximately 50 and 60% in gills and liver, res pectively. The final order of non-MT-bound cadmium was kidney > gills > liv er. Our results indicate that cadmium exposure causes toxic effects, which cannot be correlated with the accumulated levels of the metal in tissues. A lthough cadmium clearly leads to the de novo synthesis of MT and higher (Cd ,Zn)-MT concentrations, the role of this protein in the detoxification proc ess is clearly organ-specific and its synthesis does not keep track with ca dmium accumulation. (C) 2001 Elsevier Science B.V. All rights reserved.