Cadmium-binding proteins in rat testes: characterization and apparent source of the low-molecular-weight protein

Three major groups of testicular metal-binding proteins (TMBP), with approximate Mr of greater than 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 Mr 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.(ABSTRACT TRUNCATED AT 250 WORDS)