Characterization of biochemical properties of a selenium-independent glutathione peroxidase ofCryptosporidium parvum

Abstract

Glutathione peroxidase (GPx; EC 1.11.1.9) is an important antioxidant enzyme that catalyses the reduction of organic and inorganic hydroperoxides to water in oxygen-consuming organisms, using glutathione as an electron donor. Here, we report the characterization of a GPx of Cryptosporidium parvum (CpGPx). CpGPx contained a standard UGU codon for cysteine instead of a UGA opal codon for seleno-cysteine (SeCys) at the active site, and no SeCys insertion sequence (SECIS) motif was identified within the 3'-untranslated region (UTR) of CpGPx, which suggested its selenium-independent nature. In silico and biochemical analyses indicated that CpGPx is a cytosolic protein with a monomeric structure. Recombinant CpGPx was active over a wide pH range and was stable under physiological conditions. It showed a substrate preference against organic hydroperoxides, such as cumene hydroperoxide and t-butyl hydroperoxide, but it also showed activity against inorganic hydroperoxide, hydrogen peroxide. Recombinant CpGPx was not inhibited by potassium cyanide or by sodium azide. The enzyme effectively protected DNA and protein from oxidative damage induced by hydrogen peroxide, and was functionally expressed in various developmental stages of C. parvum. These results collectively suggest the essential role of CpGPx for the parasite's antioxidant defence system.

Selenium and protozoan parasitic infections: selenocompounds and selenoproteins potential

“…Moreover, Se-independent homologues of these proteins have been characterized in protozoan parasites (except in C. parvum). For instance, C. parvum parasite completely lacks selenoproteins, but the glutathione peroxidase (with cysteine instead of selenocysteine in its active site) had been described (Kang et al 2014). So, this enzyme is not inhibited by potassium cyanide, a known selenoprotein inhibitor which exerts its activity by releasing the Se atom from the enzyme active site.…”