Conceptual Overview Toshihiko Kitajimaa and Yasunori Chiba * Selenomethionine metabolism and its toxicity in yeast

The importance of selenium for organisms can be explained by its existence as selenocysteine in the catalytic centers of glutathione peroxidase and thioredoxin reductase. Another selenoamino acid, selenomethionine, is the major form of selenium in foods, and organisms that require selenium as a nutrient directly metabolize selenomethionine to a reactive form of selenium or store it in general proteins. Selenium is recognized as an essential nutrient for human and animal health; however, its excessive uptake harms mammals and the cytotoxic mechanism of selenium remains unclear. Recent progress in the development of selenium-enriched yeast and selenomethionine-resistant mutant to produce selenomethionine-containing proteins for X-ray crystallography has provided new insights into the molecular mechanism of selenomethionine toxicity. In this review, we describe the metabolism of seleno-compounds in yeast and discuss the cytotoxicity caused by selenomethionine against yeast from a metabolic viewpoint.