Figure 3 of
Winkler, Mol Vis 1999;
5:32.Figure 3 of
Winkler, Mol Vis 1999;
5:32.
Figure 3. Cellular sources of oxygen radicals
Listed below are a number of enzyme catalyzed reactions and activities in cells which are known to produce oxidants and oxygen free radicals. Particularly relevant to the photoreceptor cell/RPE complex are the four activities highlighted in red.
* Mitochondrial respiration: source of ·O2- and H2O2 via the electron transport system 1-5% of oxygen consumed undergoes univalent reduction to produce ·O2- by: (1) breakdown of ubisemiquinone; (2) NADH dehydrogenation
* Phagocytosis of rod and cone outer segments by RPE
* Lipofuscin + blue light yields ·O2-, 1O2 and H2O2
* Endogenous porphyric photosensitizers in choroidal blood plus short wavelength light yield 1O2
* Xanthine oxidase: hypoxia/ischemia/reperfusion, forms ·O2-
* NADPH-dependent oxidase system: phagocytosis (macrophages/ neutrophils), respiratory burst, an efficient source of ·O2- and H2O2
* Autoxidation of catecholamines yields ·O2-
* Prostaglandins: Prostaglandin H2 synthase produces ·O2- when NADH or NADPH is available