Usually it denotes the release of these chemicals from immune cells, e.g., neutrophils and monocytes, as they come into contact with different bacteria or fungi. They are also released from the ovum of higher animals after the ovum has been fertilized. These substances can also be released from plant cells.
NADPH oxidase, an enzyme family in the vasculature (in particular, in vascular disease), produces superoxide, which spontaneously recombines with other molecules to produce reactive free radicals. The superoxide reacts with NO, resulting in the formation of peroxynitrite, reducing the bioactive NO needed to dilate terminal arterioles and feed arteries and resistance arteries. Superoxide anion, peroxynitrite, and other reactive oxygen species also lead to pathology via peroxidation of proteins and lipids, and via activation of redox-sensitive signaling cascades and protein nitrosylation. NADPH oxidase activation has been suggested to depend on prior PKC activation. Myeloperoxidase uses the reactive oxygen species hydrogen peroxide to produce hypochlorous acid. Many vascular stimuli, including all those known to lead to insulin resistance, activate NADPH oxidase via both increased gene expression and complex activation mechanisms.
To combat infections, immune cells use NADPH oxidase to reduce O2 to oxygen free radical and then H2O2. Neutrophils and monocytes utilize myeloperoxidase to further combine H2O2 with Cl− to produce hypochlorite, which plays a role in destroying bacteria. Absence of NADPH oxidase will prevent the formation of reactive oxygen species and will result in chronic granulomatous disease.
Reactive oxygen species (ROS) in plants are important in various signaling cascades and are continuously produced by cells as byproducts of various metabolic pathways. They were mostly shown to be massively produced after the detection of PAMPs by cell-surface located receptors (e.g. FLS2 or EFR). The production of reactive oxygen species is mediated by the NADPH oxidase, and in plant immunity the subunits RbohD and RbohF have overlapping functions but are expressed in different tissues and in different levels. ROS production can be used as a readout for successful pathogen recognition via a luminol-peroxidase based assay.
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