Glutathione peroxidase and selenium in meat - Tissue and species variation, thermal stability and selenium speciation

Abstract: Lipid oxidation is a major problem in muscle foods, leading to a loss in quality and nutritional value. It occurs in raw meat but is usually more pronounced in cooked meat. Since the demands of consumers for ready-to-eat foods have increased, the control of off-flavour in cooked meat is an important industrial problem. One way of minimising the development of lipid oxidation is to optimise the action of endogenous antioxidative enzymes in the meat. In the thesis the activity of selenium-containing glutathione peroxidase (GSHPx) in tissues used for meat production was studied in relation to the concentration and distribution of selenium compounds. In comparing seven different species the GSHPx activity in the muscles was found to vary tenfold, the activity being highest in duck muscle (3.9 U/g), followed by cattle and lamb (1.6 U/g), chicken, turkey and ostrich (0.8 U/g) and finally pig muscle (0.4 U/g). For several of the species, significantly higher GSHPx activity was found in the oxidative muscles than in the glycolytic ones. Regarding tissues other than muscle, GSHPx activity was found to be significantly higher in kidney, liver and spleen of pigs than in the corresponding bovine organs, whereas for the heart and diaphragm the difference was in the opposite direction. No effect of meat ageing of the pork and beef for 4-14 days or of the pig RN phenotype on GSHPx activity was observed. GSHPx activity generally was found to decrease somewhat in raw ground meat during cold storage of several days. Heating led to a decrease in GSHPx activity, and an increase in lipid oxidation products. The muscle selenium content varied only twofold across species and was found to be highest in duck. In lamb and turkey the selenium content of the oxidative muscles was found to be significantly higher than that of the corresponding glycolytic ones. The rank order of five of the pig and cattle organs with respect to selenium content was also found to be the same: kidney > liver > spleen > heart > diaphragm. The proportion of the total selenium found as soluble selenium in the muscle varied from 44% in lamb to 70% in chicken. Speciation of soluble selenium compounds was performed by size-exclusion chromatography coupled to ICP-MS. Four peaks of soluble selenium were found in most of the muscles. The second peak corresponded to a molecular weight of 65-90 kDa and the third peak to a calculated molecular weight of 6.3-4.4 kDa, corresponding approximately to the chromatographic mobility of glutathione peroxidase and of selenoprotein W, respectively. The percentage of selenium was found to be higher in peak 2 than in peak 3 in bird muscles, the opposite being the case for cattle, lamb and pig. Further study of the nature of these selenocompounds and other selenoproteins is needed for evaluating their role in meat quality.