![]() Evidently mitochondria contribute, along with NOX, to the increased production of ROS in immune cells during infection. Superoxide in mitochondria can be further converted into other ROS such as hydrogen peroxide (H 2O 2) in a reaction mediated by mitochondrial superoxide dismutase (Sod). Consequently, electrons from coenzyme Q travel to one of the active sites of complex I, where, in turn, oxygen accepts electrons and forms superoxide. The infection-associated increase in the activity of the mitochondrial complex II likely leads to over-reduction of coenzyme Q, which is one of the electron carriers in the ETC. Upon macrophage activation, mitochondrial conditions favor reverse electron transport in the ETC. Studies on murine macrophages point towards a specific mechanism responsible for the increased mitoROS in infected cells. Remarkably, the levels of mitoROS rise when phagocytes encounter microbes. This may occur because of the escape of electrons from the electron carriers of the ETC to molecular oxygen. Superoxide can be generated at specific sites of the mitochondrial electron transport chain (ETC), for instance, at complex I or complex III. Interestingly, mitochondria produce low amounts of ROS even under normal, pathogen-free conditions. The mitochondrion is another cellular source of ROS in infected immune cells that is often overlooked. Subsequently, other reactive intermediates can arise from NOX-derived superoxide depending on the pH levels, the presence of transitional metals, and other enzyme activities in activated phagocytes. Upon pathogen recognition and engulfment, the NOX complex is formed within the phagosomal membranes, and it converts molecular oxygen into a highly reactive oxygen intermediate-superoxide. ROS production in phagocytic cells is mainly mediated through the activity of the NOX complex. Mitochondria are one of two main sources of ROS in innate immune cells ![]()
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