Glutathione peroxidase belongs to the family of selenoproteins and plays an important role in the defense mechanisms of mammals, birds and fish against oxidative damage by catalyzing the reduction of a variety of hydroperoxides, using glutathione as the reducing substrate. However, the physiological role of human plasma glutathione peroxidase remains unclear due to the low levels of reduced glutathione in human plasma and the low reactivity of this enzyme. The crystal structure of human plasma glutathione peroxidase was determined by Patterson search methods using a polyalanine model modified from the known structure of bovine erythrocyte glutathione peroxidase. The structure was refined to a crystallographic R-factor of 0.228 (R(free) = 0.335) with I > 2sigma(I) reflections in the resolution range of 8 to 2.9 A. The asymmetric unit contains a dimer. Tetramers are built up from dimers by crystallographic symmetry. The subunit structure of the plasma enzyme shows the typical structure motif of the thioredoxin fold consisting of a central beta-sheet and several flanking alpha-helices. The active site selenocysteine residue is situated in the loop between beta1 and alpha1 and is located in a pocket on the protein surface. The overall structure of the human plasma enzyme is similar to that of the bovine erythrocyte enzyme. The main differences in their subunit structures are an extended N terminus and the possible existence of a disulfide bridge in the plasma enzyme. Compared to the bovine erythrocyte enzyme, a number of residues in the active site are mutated or deleted in the plasma enzyme, including all the residues that were previously suggested to be involved in glutathione binding. The observed structural differences between the two enzymes suggest differences in substrate binding and specificity.