Incorporation of selenium into egg proteins from dietary selenite

1. The deposition of selenium in egg components has been investigated in two experiments in which sodium selenite was added to a conventional cereal-based layer diet. 2. Addition of graded amounts of selenite up to 4 mg Se/kg resulted in linear increases in the selenium content of egg white and yolk, and in protein fractions derived from them. The presence of selenium in yolk phosvitin indicates that deposition is not dependent upon the presence of cysteine. 3. Addition of sodium nitroprusside at 0.l5 and 0.3 g/kg to diets having an addition of selenite at the highest concentration, 4 mg Se/kg, resulted in substantial reductions in the selenium concentration in egg components. 4. Samples from eggs laid by hens receiving a diet containing an additional 8 mg selenite Se/kg were subjected to dialysis against sodium hydroxide or cysteine, or subjected to reduction with hydrochloric acid and zinc under anaerobic conditions. Comparisons were made with similar samples prepared from eggs laid by hens on the control diet. 5. Both sodium hydroxide and cysteine were more effective at extracting additional diet-derived selenium from whole white than from whole yolk. The proportion of selenium that could be extracted from the water-soluble or the high density fractions of yolk by either reagent was similar for both control and high selenium samples. However, neither reagent was effective at removing selenium from the ovalbumin or globin fractions of white from control eggs but substantial amounts were extracted from high selenium samples. 6. Most of the selenium was present in non-reducible forms in all samples. There was significantly more reducible selenium in ovalbumin from control eggs than from all other samples but even so non-reducible selenium accounted for two thirds of the selenium present. 7. The differential responses to chemical treatment suggest that selenium can be deposited in eggs in an unspecified number of different forms. These have still to be characterised but site of formation of egg proteins, liver or oviduct, has a bearing on the forms of selenium deposited.