Effect of dietary methionine on the biopotency of selenite and selenomethionine in the rat.

The effect of dietary methionine (Met) on the Biopotency of selenium (Se) from selenite and selenomethionine (Se-Met) was studied in rats fed a 30% torula yeast-based diet containing 0.24% Met and less than 0.02 ppm Se. Biopotency was quantitated by assaying liver, plasma and heart for the glutathione peroxidase (GSH-Px) increase elicited by a given level of Se fed for 1 week to Se-deficient rats. At dietary Se levels up to 0.5 ppm Se, the level of dietary Met supplementation did not alter selenite biopotency. In contrast, dietary Met supplementation increased the biopotency of Se-Met. With basal Met intakes, the biopotency of Se-Met was 25% that of selenite below 0.5 ppm Se. The addition of 0.4% Met to the diet made Se-Met biopotency equivalent to selenite biopotency in one experiment, but in a second experiment with younger, faster growing rats, 0.4% Met did not completely restore the biopotency of Se-Met. These results indicate that low dietary Met decreases the biopotency of Se-Met but not of selenite. Altered Se metabolism at suboptimal dietary Met may occur because more Se-Met is incorporated into protein and thus less Se is available for GSH-Px synthesis. These results suggest that adequate dietary Met is required for optimal utilization of the Se in feedstuffs of plant origin, as Se-Met is presumably a major form of plant selenium.