Selenium and selenoproteins in milk and mammary tissue

Abstract

Deficiency of selenium has been associated with various disorders such as cancer, heart disease and Keshan disease. Milk is an important source of this nutrient, since it supplies about 20% of the daily intake selenium in Scandinavian countries. Thus, it may be possible to increase selenium intake by improving the content of selenium in milk. The aims of this study were to investigate the effect of selenium supplementation in cow feed on the properties of bovine milk and to study the relationship between selenium and different selenoproteins in mammary tissue. Six cows were divided into two groups, which were fed with selenium fortified feed (25 mg organic Se/day) or control feed. Selenium contents in bovine serum and milk were measured with graphite furnace atomic absorption spectrometry (GF-AAS) and inductively coupled plasma mass spectrometry (ICP-MS), respectively. Selenium content in serum was increased more than three fold after two weeks of supplementation in both groups, whereas the concentration of selenium in milk was increased approximately six fold during the same period. The rate at which selenium increased in milk was significantly higher than that in serum. The increased incorporation of selenomethionine into milk proteins conferred novel antioxidant properties and enhanced the oxidative stability of milk. In another sub-project mammary tissue obtained at slaughter of nine cows with different age and lactation stage, was analysed for selenium content (ICP-MS) and enzyme activities of thioredoxin reductase (TR) and glutathione peroxidase (GSHPx). The results showed that the activities of TR and GSHPx varied six and fifteen fold between mammary tissues, respectively. There were positive correlations between the following variables: Se and TR (p<0.01), TR and GSHPx (p<0.01) and Se and protein content (p<0.01). This indicated that selenium status regulated selenoprotein activities in bovine mammary gland but also other variables seemed to be of importance.