Abstract
Proline (Pro) is an important stress protecting molecule in plants and other cellular systems against oxidative damage. However, the actual sensitivity and selectivity of Pro against specific reactive oxygen species (ROS), especially singlet oxygen and superoxide, is debated. Here we present in vitro studies by using EPR and oxygen uptake detection of ROS showing that Pro can indeed scavenge both singlet oxygen and superoxide. The scavenging activity of Pro is compared to that of ascorbate (Asc), a known quencher of singlet oxygen and superoxide. In contrast to Asc, which a chemical quencher, Pro eliminates singlet oxygen via a physical mechanism with ca. 1% efficiency that of Asc. The superoxide scavenging activity of Pro does not proceed via H2O2 production as in case of superoxide dismutase (SOD), but similar to that of Asc that includes electron transfer, and its efficiency is ca. 6% that of Asc on equal molar basis. Since Pro can accumulate up to 10-fold higher concentrations under stress conditions than Asc, the overall singlet oxygen and superoxide elimination activity of Pro can reach 10 and 60% of the activity of Asc, respectively. In stress exposed plants this could be physiologically relevant in cell compartments with low amount of more efficient scavengers.
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