Annual Report of Activities CNC 2011 - Center for Neuroscience and ...

explain the formation of all products from linoleic acid autoxidation hitherto detected in experiments in vitro under mild conditions, except for pyranosic and furanosic products, which we are currently addressing. Applying graph theoretical algorithms we could then trace minimal pathways to each product. Some of these pathways are more plausible than those proposed in the literature. Finally, we selected potential markers for the occurrence of specific scission mechanisms in vivo [Saravanan, Piñeiro & Salvador, manuscript in preparation]. Why do human erythrocytes use both catalase (Cat) and peroxiredoxin (Prx2) to defend against H 2 O 2 ? Our kinetic modeling [Benfeitas, Antunes, Salvador, in preparation] indicates that Cat and Prx2 complement each other in the defense against H 2 O 2 as follows. Under low/mild oxidative stress Prx2 is the main H 2 O 2 scavenger. However, the flux capacity of the Prx2 system is limited to ~0.23 µM H 2 O 2 /s. For higher H 2 O 2 influx Cat gradually becomes the main H 2 O 2 scavenger. Thus, Cat is necessary as a backup defense at high oxidative loads. But why don’t erythrocytes rely solely on Cat for H 2 O 2 elimination, thus sparing reducing equivalents? Fully replacing Prx2 such that the same [H 2 O 2 ] as in normal erythrocytes would be retained would require a Cat amount corresponding to ~32% of the hemoglobin contents. In turn, Prx2, with its higher specific second-­‐order rate constant, accomplishes the same at an amount corresponding to just 1.7 % of hemoglobin contents. Thus, reliance on both Cat and Prx2 resolves a trade-­‐off between space and operation costs. Namely, by combining both defenses erythrocytes achieve effective protection over the whole physiological range of H 2 O 2 loads at both modest protein allocation and limited NADPH expenditure. 40