Tese de mestrado em Bioquímica, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2011Peroxiredoxin 2 (Prx2) is a hydrogen peroxide reducing protein abundant in human erythrocytes. Due to its large concentration (3.7×10-4 M) and high reactivity with hydrogen peroxide (k~108 M-1s-1) it is a potentially very relevant antioxidant for the defense of erythrocytes against hydrogen peroxide. However, its contribution for H2O2 defense is not clear. The antioxidant activity of Prx2 is affected by the limited capacity that erythrocytes have for reducing Prx2 to its active form after it is oxidized by hydrogen peroxide. Further, human erythrocytes also possess catalase, an H2O2 scavenger whose antioxidant activity entails minimal NADPH consumption. In contrast, Prx2-mediated H2O2 reduction is stoichiometrically coupled to NADPH oxidation. Why do then erythrocytes need Prx2? Why do they not rely only on catalase, the H2O2 scavenger whose activity requires less NADPH? Is Prx2 redundant for H2O2 defense in human erythrocytes? We used kinetic analyses and mathematical modeling to clarify these issues. We considered the main processes protecting human erythrocytes against H2O2: glutathione peroxidase (GPx1), catalase and Prx2. The analysis of the dynamic behavior of the Prx2 system under various oxidative conditions indicates the following. Basal oxidative conditions, assuming a sustained extracellular H2O2 (eH2O2) concentration <10-7 M or intracellular H2O2 (iH2O2) concentration <2×10-11 M allow Prx2 to remain fully reduced. Prx2 then consumes more than 99% of the iH2O2. On the other hand, catalase becomes the main defense against H2O2 in erythrocytes facing sustained exposure to loads that fully oxidize Prx2 (eH2O2> 1.4×10-6 M). Catalase and GPx1 then contribute for 84% and 16% of the H2O2 consumption, respectively. Even when Prx2 is fully oxidized, eH2O2 consumption by erythrocytes is limited by membrane permeation. Thus, although erythrocytes are quite likely the main sink of plasma H2O2, Prx2 is dispensable for this role. Despite the slow Prx2 regeneration under strong oxidative loads, our analyses also suggest that erythrocytes could not rely solely on catalase and Gpx1 for defense against H2O2. To fully replace Prx2, maintaining the same H2O2 concentration and response time as in normal erythrocytes, ~11×10-12 g catalase (6×10-12 g GPx1) would be required. This would amount to 32% (18 %) of the hemoglobin in human erythrocytes. Such a solution could therefore impair oxygen transport either by making erythrocytes bigger, and thus less able to cross capillaries, or by reducing the space available for hemoglobin. On the other hand, a <1 % increase in Prx2 concentration would suffice to compensate for the effect of catalase elimination on H2O2 concentration and response time at basal oxidative loads. These differences are due to the much higher specific activity of Prx2. Our analyses also suggest that crossing of inflammation sites may lead to substantial but brief Prx2 oxidation. The residence time of a circulating erythrocyte in an inflammation site is determined by the time (~2 s) to cross the capillaries, where these cells are in most intimate contact with the vasculature walls and circulate most slowly. Full Prx2 oxidation occurs if those inflammation foci lead to > 25×10-6 M H2O2 in the plasma, which is in the scale of the strongest loads experimentally determined for inflammatory foci in other organisms. Once all Prx2 is oxidized, erythrocytes can regenerate the reduced form in minutes if the oxidative load returns to basal values. However, a stepwise increase in Prx2 oxidation through repeated crossing of inflammation sites is very unlikely. This because such crossings are expected to occur many hours in between, time enough for Prx2 to be fully reduced. Altogether, our results show that Prx2 is the main defense of human erythrocytes against H2O2 under most physiological circumstances. Extensive Prx2 oxidation may be attained in inflammation sites, affecting its contribution for H2O2 consumption. Yet, this does not impair the erythrocytes’ ability to scavenge plasma H2O2 because they can still rely on catalase for the defense against H2O2. Our results have important implications for the study of antioxidant protection and reactive oxygen species-associated diseases in mammalian cells, where Prx2 and catalase are found
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