Pathways of Selenium Metabolism Including Respiratory Excretory Products

Selenium occurs in a variety of different chemical forms in plants and animals. Plants take up inorganic forms of selenium and make numerous selenium analogs of sulfur compounds. Exclusion of selenium from proteins is characteristic of plant species able to tolerate high concentrations of selenium. Animals metabolize both inorganic and organic forms of selenium into biologically active compounds, including selenocysteine in glutathione peroxidase. Intermediary metabolism of selenium in animals characteristically involves reduction and methylation, resulting in the excretion of urinary or respiratory metabolites that include the trimethylselenonium ion and dimethylselenide. Some mechanisms for reduction and methylation of selenium in animals are discussed, as well as the effects of methylation on toxicity.