Cadmium-binding proteins of rat testes. Apparent source of the protein of low molecular mass

Fractionation of rat testicular cytosolic proteins by gel filtration indicates three major metal-binding proteins, or groups of proteins, termed testicular metal-binding protein (TMBP) 1, 2 and 3 by order of elution. The major heat-stable, metal-binding proteins in testes is TMBP-2, which has an Mr of approx. 25000. In most tissues, metallothionein (MT) is the major heat-stable, metal-binding protein, but it has an Mr of 6000. This testicular protein (TMBP-2) is much larger than MT, and since polymeric forms of MT have been previously reported, further characterization of TMBP-2 was performed. TMBP-2 was separated into two forms by DEAE-Sephadex A-25 anion-exchange chromatography. Amino acid analysis of both forms of TMBP-2 revealed that they differed markedly from MT, having particularly low cysteine contents. However, amino acid analysis showed that TBMP-2 was strikingly similar to TMBP-3, with an approximate stoichiometric relationship of 4:1. Therefore, experiments were conducted to determine if TMBP-3 could be a breakdown product of TMBP-2. Heat treatment of testicular cytosol in room air before gel filtration resulted in a marked increase in TMBP-3 and loss of TMBP-2. Storing intact testes at -20 degrees C for 2 weeks before processing for gel filtration also resulted in an increase in TMBP-3 and a loss of TMBP-2. Addition of a reducing agent (dithiothreitol) or proteinase inhibitor (N-ethylmaleimide) in processing of samples before gel filtration inhibited the appearance of TMBP-3. Results suggest that the low-Mr Cd-binding protein (TMBP-3) of rat testes results from either proteolytic or oxidative breakdown of a higher-Mr species, or from a combination of such factors.