Absence of both glutathione peroxidase activity and glutathione in bovine milk

Glutathione peroxidases are selenium-dependent, antioxidant enzymes that catalyse the removal of hydroperoxides using the tripeptide glutathione as reducing substrate. The presence and importance of glutathione peroxidase for the oxidative stability of bovine milk is unclear. We failed to detect any specific activity in bovine milk using careful and comprehensive assay conditions. The apparent activity in milk that has been ascribed previously to glutathione peroxidase was shown to be independent of added hydroperoxide, completely inhibited by >6 mM EDTA, and could not be immunoprecipitated with a specific antiserum against the extracellular form of glutathione peroxidase. These results are incompatible with activity of glutathione peroxidase in bovine milk as shown by parallel assays of glutathione peroxidase activity in bovine plasma and in contrast to reports using human milk. Further, there is very little indigenous glutathione in bovine milk to be used as a reducing substrate for glutathione peroxidase. In fact, glutathione added to milk is metabolised rapidly, probably by the action of sulphydryl oxidase.