Characterization of potential selenium-binding proteins in the selenophosphate synthetase system

Selenophosphate, an activated form of selenium that can serve as a selenium donor, is generated by the selD gene product, selenophosphate synthetase (SPS). Selenophosphate is required by several bacteria and by mammals for the specific synthesis of Secys-tRNA, the precursor of selenocysteine in selenoenzymes. Although free selenide can be used in vitro for synthesis of selenophosphate, the physiological system that donates selenium to SPS is incompletely characterized. To detect potential selenium-delivery proteins, two known sulfurtransferases and glyceraldehyde-3-phosphate dehydrogenase (GAPDH; EC 1.2.1.12) were analyzed for ability to bind and transfer selenium. Rhodanese (EC 2.8.1.1) was shown to bind selenium tightly, with only part of the selenium being available as substrate for SPS in the presence of added reductant. 3-Mercaptopyruvate sulfurtransferase (3-MST; EC 2.8.1.2) and GAPDH also bound selenium supplied as selenodiglutathione formed from and glutathione. Selenium bound to 3-MST and GAPDH was released more readily than that from rhodanese and also was more available as a substrate for SPS. Although rhodanese retained tightly bound selenium under aerobic conditions, the protein gradually became insoluble, whereas GAPDH containing bound selenium was stable at neutral pH for a long period. These results indicate that 3-MST and GAPDH have more suitable potentials as a physiological selenium-delivery protein than rhodanese. In the presence of a selenium-binding protein, a low level of selenodiglutathione formed from and glutathione could effectively replace the high concentrations of selenide routinely used as substrate in the SPS in vitro assays.