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During the normal course of metabolism, oxygen is partly
reduced as electrons leak out of the electron transport
chain during respiration. These partially reduced oxygen
species (ROS) can react with organic substances through
non-catalytic means. Furthermore, ROS can be generated via
endogenous enzyme systems like plasma NADPH oxidase,
cytoplasmic xanthine oxidase and organelle sources e.g.,
cytochrome P-450. ROS have been implicated in regulating
diverse cellular functions including proliferation, defense
against pathogens, intra-cellular signaling, transcriptional
activation and apoptosis. Elevation of ROS beyond the
buffering capacity of the cell can lead to oxidative stress.
Elevated ROS levels can lead to damage of DNA/RNA, proteins
and lipids which may lead to apoptosis. Cells have developed
several mechanisms to counter act elevated ROS levels such
as a thiol reducing buffer composed of cellular thiol levels
(glutathione and thioredoxion) for the maintenance of the
reduction-oxidation (redox) state of the cell, and enzymes
to remove ROS (catalase, superoxide dismutase and
glutathione peroxidase) (1-2).
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