Cadmium-Binding Proteins in Rat Testes: Characterization and Apparent Source of the Low-Molecular-Weight Protein

Abstract

Three major groups of testicular metal-binding proteins (TMBP), with approximate M_r of > 80,000 (TMBP-1), 25,000 (TMBP-2) and 8,000 (TMBP-3) were separated by gel filtration chromatography. Two forms of TMBP-3 were detected following anion-exchange chromatography; however, the cysteine content of these proteins was much lower (5–7%) than MT (27–28%). Thus, this major metal-binding protein in testes, assumed to be MT, is a different protein. However, most assays for MT quantitation detect high basal levels in this organ. Further experiments indicated that the Cd-hemoglobin assay for MT, which detects heat-stable, metal-binding proteins, primarily detects TMBP-2 in rat testes. In most tissues, MT is the major heat-stable, metal-binding protein, but it has an M_r of 6,000. Because TMBP-2 is much larger than MT and polymeric forms of MT have been reported, further characterization of TMBP-2 was performed to determine whether TMBP-2 was polymerized MT. Anion-exchange chromatography separated TMBP-2 into two forms but their amino acid content was much different than MT. However, the amino acid content of TMBP-2 and TMBP-3 were quite similar. Freezing and thawing of whole testes or heat-treatment of testicular cytosol resulted in disappearance of TMBP-2 and concomitant increase of TMBP-3. Conversely, addition of a reducing agent or protease inhibitor in sample preparation inhibited the appearance of TMBP-3. Results suggest that the low-M_r cadmium-binding protein (TMBP-3) of rat testes occurs as a consequence of the breakdown of a higher M_r species (TMBP-2) and that neither TMBP-2 nor TMBP-3 are monomeric or polymeric forms of MT. Because MT is thought to play an important role in the detoxification of cadmium, the apparent absence of MT in rat testes may account for the unique sensitivity of this organ to acute necrotizing and chronic carcinogenic effects of cadmium.