Selenium Compounds in Milk. Studies of Selenoproteins, Selenium Enrichment and Oxidative Stability of Food

Selenium is an essential nutrient and in animals and humans it can either be specifically incorporated into proteins as selenocysteine or randomly as selenomethionine. It can also occur as low-molecular-weight compounds and the form of selenium in foods may also influence food quality. The aims of the present studies were: to study selenium and other trace element compounds in milk and whey; to elucidate the effect of dietary supplementation on selenium and other trace elements in cow's milk, whey and blood plasma; to study the expression and activity of selenoproteins in mammary tissue; and to investigate the antioxidant role of selenium compounds in animal foods.

Supplementation of the diet of cows with selenium caused an increase in selenium content in milk, whey and blood plasma of 10, 10 and 2 times, respectively, compared with control values. Most of the increase in whey occurred in the ?-lactoglobulin-?-lactalbumin fraction, while the increase in blood plasma was evenly distributed between six major fractions, including albumin, selenoprotein P and low-molecular-weight compounds. The distribution of copper in whey seemed to be shifted to the high-molecular-weight compounds, while the distribution of zinc was unaffected. The distributions of zinc and copper in plasma were unaffected by selenium supplements.

Milk and whey from cows reared on organic regimens were also compared with those from cows managed using conventional methods. Selenium contents in desalted milk and whey obtained using organic farming practices were significantly lower than those resulting from conventional farming, and the proportion of protein-bound selenium in ?organic? whey was lower than that in ?conventional? whey.

The activities of glutathione peroxidase (GSHPx) and thioredoxin reductase (TR), and the gene expression of five selenoproteins were also studied in mammary tissue. The GSHPx activity varied 16-fold among the investigated cows, and the TR activity and selenium content varied 6-fold. Pair-wise positive correlations were found between these three variables. Moreover, mRNA of cellular GSHPx, extracellular GSHPx, phospholipid

hydroperoxide GSHPx, TR and selenoprotein P (SelP) were also detected in mammary tissue, and the activity of GSHPx and the selenium content were found to be positively correlated to the mRNA of cellular GSHPx and SelP, respectively.

To study the antioxidant role of selenium and selenoprotein in foods, selenium-enriched milk was exposed to light, and lipid oxidation was assessed by hexanal formation. Selenium-enriched milk resisted light-induced lipid oxidation better than control milk. In a tissue model, GSHPx activity in chicken and duck meat decreased to below 20% of the control values after being heated to 80ºC, whereas cold storage did not affect GSHPx activity. Lipid oxidation, measured as thiobarbituric acid reactive substances (TBARS) increased after heat treatment and cold storage. In cooked chicken meat, but not duck meat, adding GSHPx reduced the TBARS formation to approx. 55% of the control value.