| Abstract
| - Rat selenoprotein P is an extracellular glycoprotein of 366 amino acid residues that is rich incysteine and selenocysteine. Plasma contains four isoforms that differ principally by length at the C-terminalend. Mass spectrometry was used to identify sites of glycosylation on the full-length protein. Of thepotential N-glycosylation sites, three located at residues 64, 155, and 169 were occupied, while the twoat residues 351 and 356 were not occupied. Threonine 346 was variably O-glycosylated. Thus, full-lengthselenoprotein P is both N- and O-glycosylated. The shortest isoform of selenoprotein P, which terminatesat residue 244, was analyzed for selenide−sulfide and disulfide linkages. In this isoform, a singleselenocysteine and seven cysteines are present. Mass spectrometric analysis indicated that a selenide−sulfide bond exists between Sec40 and Cys43. Two disulfides were also detected as Cys149−Cys167and Cys153−Cys156. The finding of a selenide−sulfide bond in the shortest isoform is compatible witha redox function of this pair that might be analogous to the selenol−thiol pair near the C terminus ofanimal thioredoxin reductase. The disulfide formed by Cys153−Cys156 also has some characteristics ofa redox active pair.
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