Selenophosphate (SeP), the energy-rich Se compound required for synthesis of many selenoenzymes is formed by selenophosphate synthetase (SPS) from ATP and an inorganic form of Se. The purified enzyme contains a bound UV absorbing chromophore characterized by Dr. Matt Wolfe. The participation of specialized selenium delivery proteins to furnish Se specifically to SPS serves to maintain intracellular levels of selenium below toxic levels. Certain classes of delivery proteins that can utilize selenocysteine as substrate, selenocysteine lyases, include three NifS related proteins from E. coli and one from Methanococcus vannielii, an anaerobic organism that is particularly rich in selenoenzymes and factors required for selenoprotein biosynthesis. A selenium-binding protein that reacts with inorganic selenium was isolated previously from M. vannielii and the gene encoding this protein was isolated, cloned, and expressed in E. coli. Antibodies produced in rabbits to the isolated recombinant protein were purified and used by Renee Pierce to prepare antibody columns. The native selenium binding protein from M. vannielii and the recombinant protein from E. coli were purified effectively using the antibody column technique. Detailed studies on the physical properties of the highly hydrophobic selenium-binding protein by Kemberly Patteson helped to explain the ability of the native and recombinant forms to retain bound selenium. The protein which consists of 8.8 kDa subunits that contain 81 amino acids is very resistant to dissociation, especially by heat treatment. There is evidence that the single cysteine residue present in each subunit participates in selenium-binding. A potential role of the protein in the delivery of selenium to selenophosphate synthetase is under investigation. Attempts to obtain selenophosphate synthetase in crystalline form are continuing as a means of locating the site on the protein that undergoes phosphorylation by ATP. The identity of the UV absorbing chromophore and its role are continuing to be investigated by Matt Wolfe. Selenium metabolism in a single celled eukaryotic organism, Dictyostellium, was investigated using 75Se by Thressa Stadtman. A prominent radioactive protein that migrated on gels as a dimer has been partially purified and attempts are under way to determine its identity. The ease of culture of this amoeba in ordinary types of liquid media make it an attractive organism for study of selenoenzyme biosynthesis in eukaryotes.
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