Chemical characterization of the selenoprotein component of clostridial glycine reductase: identification of selenocysteine as the organoselenium moiety.

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Abstract
A small, heat-stable selenoprotein, a component of the glycine reductase complex, was labeled with 75Se by growth of Clostridium sticklandii in the presence of Na275SeO3. The Se-containing moiety, which is essential for the biological activity of the protein, was a selenocysteine residue. It was isolated as its Se-carboxymethyl, Se-carboxyethyl and Se-aminoethyl derivatives from digests of the pure 75Se-labeled protein that was reduced and treated with the various alkylating agents prior to hydrolysis. In each instance the 75Se-labeled moiety obtained from an alkylated protein sample and the corresponding alkyl derivative of authentic selenocysteine were indistinguishable. Several studies of the native selenoprotein showed a chromophore (UVmax 238 nm) that appeared upon reduction of the protein with KBH4 and rapidly disappeared upon exposure to O2. This O2-labile chromophore is probably the ionized -SeH group of the selenocysteine residue.