Two pathways for toxication of superoxide anion radical (O2•-) via nonradical products (ONOO- and HOOH) to radical products (•NO2, CO3•-, and HO•). In one pathway, conversion of (O2•-) to HOOH is spontaneous or is catalyzed by SOD. Homolytic cleavage of HOOH to hydroxyl radical and hydroxyl ion is called the Fenton reaction and is catalyzed by the transition metal ions shown. Hydroxyl radical formation is the ultimate toxication for xenobiotics that form O2•- or for HOOH, the transition metal ions listed, and some chemicals that form complexes with these transition metal ions. In the other pathway, O2•- reacts avidly with nitric oxide (•NO), the product of •NO synthase (NOS), forming peroxynitrite (ONOO–). Spontaneous reaction of ONOO- with carbon dioxide (CO2) yields nitrosoperoxy carbonate (ONOOCO2–) that is homolytically cleaved to nitrogen dioxide (•NO2) and carbonate anion radical (CO3•-). All three radical products indicated in this figure are oxidants, whereas •NO2 is also a nitrating agent. Source: Chapter 3. Mechanisms of Toxicity, Casarett & Doull's Essentials of Toxicology, 2e Citation: Klaassen CD, Watkins JB, III. Casarett & Doull's Essentials of Toxicology, 2e; 2010 Available at: http://accesspharmacy.mhmedical.com/DownloadImage.aspx?image=/data/books/klaa2/klaa2_c003f003.gif&sec=39911035&BookID=449&ChapterSecID=39910768&imagename= Accessed: November 06, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved