Free Radicals in Medicine. I. Chemical Nature and Biologic Reactions*

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Free Radicals in Medicine. I. Chemical Nature and Biologic Reactions* PETER A. SOUTHORN, M.B., B.S., GARTH POWIS, D. Phil. Mayo Clinic Proceedings Volume 63, Issue 4, Pages 381-389 (April 1988) DOI: 10.1016/S0025-6196(12)64861-7 Copyright © 1988 Mayo Foundation for Medical Education and Research Terms and Conditions

Fig. 1 Diagram of heterolytic and homolytic covalent bond fission. Each component in compound AB donates an electron (×) to create a double electron “covalent” bond. In upper equation, heterolytic asymmetric cleavage of this bond produces negatively charged A ion and positively charged B ion. In lower equation, symmetric homolytic cleavage of bond produces free radicals A· and B·, each of these components possessing one unpaired electron. Mayo Clinic Proceedings 1988 63, 381-389DOI: (10.1016/S0025-6196(12)64861-7) Copyright © 1988 Mayo Foundation for Medical Education and Research Terms and Conditions

Fig. 2 Sequential univalent reduction of molecular oxygen. (See text for discussion.) Mayo Clinic Proceedings 1988 63, 381-389DOI: (10.1016/S0025-6196(12)64861-7) Copyright © 1988 Mayo Foundation for Medical Education and Research Terms and Conditions

Fig. 3 Arrangement of electrons in outermost occupied orbitals of molecular oxygen and delta and sigma singlet oxygen. Arrows indicate direction of electron spin. (Data from Halliwell and Gutteridge.8) Mayo Clinic Proceedings 1988 63, 381-389DOI: (10.1016/S0025-6196(12)64861-7) Copyright © 1988 Mayo Foundation for Medical Education and Research Terms and Conditions

Fig. 4 Formation of superoxide anion radical by oxidation-reduction cycling of paraquat pyridinyl cation. The following reactions are involved in paraquat toxicity. A, Pentose phosphate pathway provides reducing equivalents in the form of reduced nicotinamide-adenine dinucleotide phosphate (NADPH). B, Paraquat pyridinyl cation (PQ++) is reduced to cation radical (PQ ). This reaction proceeds continuously, being catalyzed by NADPH-cytochrome P450 reductase. C, Oxidation-reduction cycling of pyridinyl cation radical reacting with molecular oxygen forms superoxide anion radical (O2 ) and pyridinyl cation. Mayo Clinic Proceedings 1988 63, 381-389DOI: (10.1016/S0025-6196(12)64861-7) Copyright © 1988 Mayo Foundation for Medical Education and Research Terms and Conditions

Fig. 5 Lipid peroxidation initiated by hydroxyl radical. PUFA = polyunsaturated fatty acids. Mayo Clinic Proceedings 1988 63, 381-389DOI: (10.1016/S0025-6196(12)64861-7) Copyright © 1988 Mayo Foundation for Medical Education and Research Terms and Conditions

Fig. 6 Diagram of enzyme systems to bypass reactive intermediates being released during sequential univalent reduction of molecular oxygen. Mayo Clinic Proceedings 1988 63, 381-389DOI: (10.1016/S0025-6196(12)64861-7) Copyright © 1988 Mayo Foundation for Medical Education and Research Terms and Conditions