Strain dependence in mice of resistance and susceptibility to the testicular effects of cadmium: assessment of the role of testicular cadmium-binding proteins.

The nature of the cadmium (Cd)-binding proteins in the mouse testes is unknown, although some studies have implied metallothionein (MT) is responsible for the marked strain dependence of Cd-induced testicular necrosis in mice. This study attempted to define the role of MT in strain-dependent Cd resistance using NFS (susceptible) and BALB/c (resistant) mice. In all cases, testicular proteins were compared to hepatic MT isolated after treatment with zinc (Zn). A low-molecular-weight (Mr) Zn-, Cd-binding protein was detected in testicular and hepatic cytosol from both strains after gel filtration. These proteins were extractable by heat treatment and sequential acetone precipitation. When such extracts were further purified with reverse-phase HPLC, two forms of authentic MT were shown by amino acid analysis from both NFS and BALB/c livers. However, of two testicular forms separated by HPLC from NFS and BALB/c mice, neither could be classified as MT based on amino acid composition. Methylation of the MT-I gene was also studied in testicular and hepatic DNAs isolated from control mice or from mice made resistant to Cd-induced testicular necrosis by Zn treatment. Major differences in methylation between tissues were seen, as the testicular gene of both NFS and BALB/c mice was highly methylated, a condition often linked to genetic quiescence. Zn treatment had no effect on MT-I gene methylation in testes of either strain, although rendering the NFS strain resistant to Cd. Zn treatment did not alter levels of this testicular protein in either strain while causing a marked induction of hepatic MT in both. These results indicate the low-Mr Cd-, Zn-binding proteins present in the testes of both resistant and susceptible mice are not MTs and further that the MT gene may not be expressed in either strain.