Partial sequence of human plasma glutathione peroxidase and immunologic identification of milk glutathione peroxidase as the plasma enzyme.

Plasma glutathione peroxidase (p-GSHPx) is a unique selenoglycoprotein. A hepatic cell line synthesizes both this extracellular form for secretion and the cellular form that remains within the cells. Because the two forms could be a result of post-translational modifications of a product of a single gene, we partially sequenced p-GSHPx. Purified p-GSHPx was trypsin digested, and three of the peptides were sequenced. Only one of the peptide sequences was partially homologous to a sequence found in human cellular glutathione peroxidase. Because p-GSHPx is a secreted enzyme, we determined whether GSHPx in milk (another extracellular fluid) is due to this form of the enzyme. Ninety percent of human milk GSHPx activity could be precipitated by anti-p-GSHPx-immunoglobulin G. Thus, most, if not all, GSHPx activity in milk is due to the plasma selenoprotein form of the enzyme. In milk of two North American women, 3.6% and 14.3% of selenium was associated with GSHPx.