[5] Diversity of glutathione peroxidases

This chapter focuses on the diversity of glutathione peroxidases. Selenium was identified as a toxic factor for grazing animals in the first half of the twentieth century and since then has been considered hazardous. Only long after the identification of the first selenoenzymes in bacteria and mammals was a Recommended Dietary Allowance gradually established. In fact, the putative biological roles of the selenoenzymes, particularly those of the glutathione peroxidases (GPX), proved instrumental in the understanding of selenium deficiency syndromes in livestock and humans, although the emerging complexity of selenium enzymology still precludes definitive conclusions. The selenium-dependent peroxidases have long been considered a late achievement of evolution, as they were only detected in vertebrates. This view now has to be revised. Whether the common ancester of the GPX superfamily was a selenoprotein or a cysteine-containing homolog cannot be deduced from the available sequences. The only prokaryotic member of the superfamily detected so far, a cobalamine-binding protein of Escherichia coli, does not contain selenocysteine, and despite ongoing efforts, functionally active glutathione peroxidases have not yet been found in prokaryotes.