Degradation of cadmium‐thionein in rat liver and kidney

3H-cystine and 115mCd were incorporated into hepatic and renal cadmium-thionein in response to a subcutaneous administration of 4.4 micronmol of Cd2+ containing 115mCd. Cadmium-thionein bound 115mCd reached a plateau by 24 hrs. and 72 hrs. after the Cd2+ injection in liver and kidney, respectively. The half-life (t 1/2) of 3-H-labeled hepatic cadmium-thionein was 3.5 days, whereas the average t 1/2 of the soluble proteins was 3.7 days. The t 1/2 of the soluble renal proteins was 3.8 days. In marked contrast, the 115mCd content of both hepatic and renal cadmium-thionein was virtually unchanged even 9 days 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 Cd2+ to nascent thionein polypeptide chains. Thus the lack of metallothionein degradation, per se, does not account for the long-term retention of Cd2+ in liver and kidney during chronic exposure.