THE SELENIUM REQUIREMENT FOR GLUTATHIONE-PEROXIDASE MESSENGER-RNA LEVEL IS HALF OF THE SELENIUM REQUIREMENT FOR GLUTATHIONE-PEROXIDASE ACTIVITY IN FEMALE RATS

To determine critically the selenium (Se) requirement for weanling fem ale rats, we used glutathione peroxidase (GSH: H2O2 oxidoreductase, EC 1.11.1.9) (GPX) mRNA and a number of other parameters to assess Se st atus. Rats were fed a basal torula-yeast diet (0.007 mu g Se/g) supple mented with Se as Na2SeO3 in graded levels from 0 to 0.3 mu g Se/g die t for 32 d (3 rats/group). Selenium supplementation had no effect on g rowth, showing that the Se requirement for growth is less than 0.007 m u g Se/g diet, whereas other parameters showed significant increases w ith Se supplementation. In fats fed the Se-deficient basal diet, liver Se concentration was 4 +/- 0%, plasma GPX activity was 8 +/- 1%, eryt hrocyte GPX activity was 40 +/- 3%, liver GPX activity was 2 +/- 1%, a nd liver GPX mRNA levels were 11-17% of the levels in rats fed 0.1 mu g Se/g diet. Liver Se concentration and GPX activity in plasma, erythr ocytes and liver all reached a plateau breakpoint at or near 0.1 mu g Se/g diet, indicating that the dietary Se requirement-for maximal GPX activity in growing female rats is 0.1 mu g Se/g diet. Liver GPX mRNA levels reached the plateau breakpoint at 0.05 mu g Se/g diet, showing that the minimum dietary Se requirement for maximal GPX mRNA levels in female rats is half of the Se requirement for maximal GPX activity. T his experiment demonstrates that GPX mRNA can be used to determine the dietary Se requirement; the gap between the dietary Se necessary for maximal GPX mRNA and that for maximal GPX activity may represent an ev olutionarily derived biological margin of safety.