Most selenoproteins contain a single selenocysteine residue per polypeptide 
chain, encoded by an in-frame UGA codon. Selenoprotein P is unique in that its 
mRNA encodes 10-12 selenocysteine residues, depending on species. In addition to 
the high number of selenocysteines, the protein is cysteine- and histidine-rich. 
The function of selenoprotein P has remained elusive, in part due to the 
inability to express the recombinant protein. This has been attributed to 
presumed inefficient translation through the selenocysteine/stop codons. Herein, 
we report for the first time the expression of recombinant rat selenoprotein P 
in a transiently transfected human epithelial kidney cell line, as well as the 
endogenously expressed protein from HepG2 and Chinese hamster ovary cells. The 
majority of the expressed protein migrates with the predicted 57-kDa size of 
full-length glycosylated selenoprotein P. Based on the histidine-rich nature of 
selenoprotein P, we have purified the recombinant and endogenously expressed 
proteins using nickel-agarose affinity chromatography. We show that the 
recombinant rat and endogenous human proteins react in Western blotting and 
immunoprecipitation assays with commercial anti-histidine antibodies. The 
ability to express, purify, and immunochemically detect the recombinant protein 
provides a foundation for investigating the functions and efficiency of 
expression of this intriguing protein.