Degradation of cadmium‐thionein in rat liver and kidney

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Abstract
[ 3 H]Cystine and 115mCd were incorporated into hepatic and renal Cd‐thionein in response to sc administration of 4.4 μmol of Cd 2+ containing 115mCd. Cd‐thioneinbound 115mCd reached a plateau by 24 and 72 h after the Cd 2+ injection in liver and kidney, respectively. The half‐life (t ½ ) of 3 H‐labeled hepatic Cd‐thionein was 3.5 d, whereas the average t½ of the soluble proteins was 3.7 d. The t½ of 3 H‐labeled renal Cd‐thionein was 3.7 d, whereas the average t½ of the soluble renal proteins was 3.8 d. In marked contrast, the 115m Cd content of both hepatic and renal Cd‐thionein was virtually unchanged, even 9 d after administration of this radionuclide. These data indicate that the protein moiety of metallothionein is degraded, although there appears to be a concomitant rebinding of Cd 2+ to nascent thionein polypeptide chains. Thus the lack of metallothionein degradation per se does not account for the long‐term retention of Cd 2+ in liver and kidney during chronic exposure.

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