Selenium belongs to the category of redox nonme-tals. Selenium is included in the same class as sulfur (sometimes referred to as metalloids), which implies that selenium should be able to substitute for sulfur in biological complexes. As a congener of sulfur, selenium becomes part of a protein's structure as selenocysteine and selenomethionine, not as a selenium atom ligated directly to the protein as a prosthetic group. The former are the active cofactors in selenium enzymes.

Reactivity Although a selenium ion is clearly capable of redox reactions, there is still little information available as to how selenium functions as a cofactor. Enzymes such as glutathione peroxidase are soluble enzymes that transfer electrons to and from substrates. Replacing the selenium with sulfur in the enzyme negates the activity. With only a few selenoenzymes available, there is little information as to the precise catalytic role of selenium.

Glutathione peroxidase in the reduced (resting) form is believed to contain an ionized selenol that can react with either organic peroxides or H2O2 according to the reaction (a) below:

(a) Enz-Se- + ROOH ! Enz-SeOH + ROH (b) Enz-Se- + RI4 ! Enz-SeI + RI3

A selenol enzyme is also believed to be an intermediate in the reaction (b) catalyzed by 5' deiodi-nase, the enzyme that catalyzes the removal of iodine from thyroid hormone. The enzyme-selenenic acid complex (Enz-SeOH) is regenerated by reduced glutathione (GSH), which forms a mixed selenide sulfide intermediate (Enz-Se-S-G). This intermediate then reacts with a second GSH to restore Enz-Se-and releases oxidized glutathione (GSSG) as a product. Regeneration of the Enz-SeI of 5' deiodinase also requires a reducing agent whose identity is still uncertain although dithiothreitol can perform the reduction in vitro.

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