Glutathione peroxidase (GSH peroxidase) is a selenoenzyme, containing one selenium atom per subunit. This was determined by neutron activation analysis of GSH peroxidase isolated from bovine blood, revealing four selenium atoms per mole of enzyme, suggesting one atom per subunit. The enzyme was purified and crystallized, and its structure was analyzed. The results indicate that selenium is an essential component of GSH peroxidase, which plays a key role in preventing lipid peroxidation by eliminating or reducing hydroperoxides. Selenium deficiency leads to decreased GSH peroxidase activity, highlighting the importance of selenium in maintaining this enzyme's function. The study confirms that selenium is a constituent of GSH peroxidase, supporting the idea that selenium deficiency can cause various pathological conditions related to lipid peroxidation in biological membranes. The findings suggest that dietary selenium contributes to antioxidant defense by functioning as an integral component of GSH peroxidase. The research was supported by the Deutsche Forschungsgemeinschaft. The study provides the first evidence of selenium in a protein in stoichiometric amounts, and the results have implications for understanding the role of selenium in human and animal nutrition. The study also highlights the importance of selenium in maintaining the function of GSH peroxidase, which is crucial for preventing oxidative damage in biological systems.Glutathione peroxidase (GSH peroxidase) is a selenoenzyme, containing one selenium atom per subunit. This was determined by neutron activation analysis of GSH peroxidase isolated from bovine blood, revealing four selenium atoms per mole of enzyme, suggesting one atom per subunit. The enzyme was purified and crystallized, and its structure was analyzed. The results indicate that selenium is an essential component of GSH peroxidase, which plays a key role in preventing lipid peroxidation by eliminating or reducing hydroperoxides. Selenium deficiency leads to decreased GSH peroxidase activity, highlighting the importance of selenium in maintaining this enzyme's function. The study confirms that selenium is a constituent of GSH peroxidase, supporting the idea that selenium deficiency can cause various pathological conditions related to lipid peroxidation in biological membranes. The findings suggest that dietary selenium contributes to antioxidant defense by functioning as an integral component of GSH peroxidase. The research was supported by the Deutsche Forschungsgemeinschaft. The study provides the first evidence of selenium in a protein in stoichiometric amounts, and the results have implications for understanding the role of selenium in human and animal nutrition. The study also highlights the importance of selenium in maintaining the function of GSH peroxidase, which is crucial for preventing oxidative damage in biological systems.