1. Selenium compounds, i.e., selenite and selenium dioxide, can react with glutathione (GSH) and other thiols to form selenotrisulfides that will ultimately react to produce superoxide and hydrogen peroxide, and are toxic.
2. Diselenides, i.e., selenocystine and selenocystamine in the presence of GSH and other thiols, are reduced to selenols (RSeH), which are catalytic, produce superoxide and hydrogen peroxide, and are toxic.
3. Selenium compounds that do not react with thiols,i.e., selenate and all tested selenoethers (RSeR), do not produce superoxide or hydrogen peroxide in vitro and are not toxic per se.
4. Selenate and selenoethers are toxic in tissue culture or in vivo only after being reduced to selenite or a selenol.
5. Selenium toxicity manifests itself acutely or chronically when oxidative damage exceeds antioxidant defenses or the ability of either plants or animals to form seleno -
proteins, selenoethers, or elemental selenium (Se0) (Spallholz, 1994).