Selenoprotein P (SeP) is an extracellular glycoprotein with 8-10 selenocysteines 
per molecule, containing approximately 50% of total selenium in human serum. An 
antioxidant function of SeP has been postulated. In the present study, we show 
that SeP protects low-density lipoproteins (LDL) against oxidation in a 
cell-free in-vitro system. LDL were isolated from human blood plasma and 
oxidized with CuCl2, 2,2'-azobis(2-amidinopropane) (AAPH) or peroxynitrite in 
the presence or absence of SeP, using the formation of conjugated dienes as 
parameter for lipid peroxidation. SeP delayed the CuCl2- and AAPH-induced LDL 
oxidation significantly and more efficiently than bovine serum albumin used as 
control. In contrast, SeP was not capable of inhibiting peroxynitrite-induced 
LDL oxidation. The protection of LDL against CuCl2- and AAPH-induced oxidation 
provides evidence for the antioxidant capacity of SeP. Because SeP associates 
with endothelial membranes, it may act in vivo as a protective factor inhibiting 
the oxidation of LDL by reactive oxygen species.