Year: 1994
Type: article
Abstract: Cytotoxic quinones such as 6-hydroxydopamine (6-OHDA), antitumour quinones, or phenolic metabolites of benzene exert toxic effects through cyclic reduction and reoxidation, producing active oxygen (02.-,H202, 'OH).Mitochondria1 respiration also produces active oxygen through autoxidation of reduced quinone intermediates.This thesis investigates the mechanisms of reaction of hydroquinones with oxygen.The thesis has three main parts: a) an evaluation of mitochondrial respiratory impairments on autoxidation of reduced quinone intermediates, b) investigations into roles of trace metals and radical propagators in the autoxidations of 6-hydroxydopamine and ascorbate, and c) a systematic comparison of the autoxidation mechanisms of hydroquinones differing in reduction potentials, aromaticity, and metal affinity.Respiring submitochondrial particles did not measurably increase 0 2 ' -generation as reduced intermediates accumulate with a decline in oxygen tension to anoxia.Partial removal of cytochrome c however, as occurs in ischaen~ia-reperfusion injury, increased normoxic 02'generation relative to oxygen consumption by 60-100%.A hypothesis is presented whereby, with cellular age, mitochondrial mutations which disrupt normal electron flow to oxygen, as with respiratory inhibitors, may increase oxidative stress through autoxidation of accumulating reduced intermediates.The effects of metal chelators and superoxide dismutase on the reaction of 6-OHDA with oxygen revealed the importance of trace metals as initiators, and of 0 2 ' -as a reaction propagator.Superoxide dismutase inhibited autoxidation by up to 96%, but control experiments with albumin and denatured superoxide dismutase revealed synergistic coordination of trace metals by the apoprotein.Coordination of trace metals with desferrioxamine, histidine, or albumin inhibited autoxidation 30-40% and enhanced inhibition by catalytic amounts of superoxide dismutase from less than 25% to over 95%.Metal chelation thus precludes formation of a ternary 6-0HDA:metal:oxygen complex, and forces the autoxidation to occur by an outer sphere, 02'--propagated pathway.The superoxide dismutase mimic, Mn-desferrioxamine (green complex), did not inhibit 6-OHDA autoxidation in the presence of excess desferrioxamine, instead catalysing autoxidation.With added transition metals, EDTA enhanced catalysis by Fe, but inhibited catalysis by Cu, Mn, or V. Desferrioxamine and histidine inhibited catalysis by each metal.A plot of catalytic activity versus reduction potential of meta1:ligand complexes revealed a potential midway between reductant and oxygen as optimal for facile reduction and reoxidation.In ascorbate autoxidation, synergism between metal chelators and superoxide dismutase was not evident due to over 98% inhibition by chelators alone.Stronger inhibition by heat-denatured superoxide dismutase than native enzyme indicated a greater affinity of the unfolded protein for catalytic metals.The plasma antioxidant, uric acid, inhibited 6-OHDA autoxidation synergistically with ascorbate or superoxide dismutase, indicating metal-binding and cooperation with ascorbate and superoxide dismutase in its antioxidant activities.The autoxidations of selected benzenediols, benzenetriols, and naphthalenediols differed in their responses to superoxide dismutase and metal chelators.In the autoxidation of benzenetriols (gallate; 2,3,4-trihydroxybenzoate; pyrogallol; 1,2,4-benzenetriol), 5 Ulml superoxide dismutase inhibited 17-60%, and metal chelators sensitized to further ~nhibition.With benzenediols (catechol and 1,4-hydroquinone), superoxide dismutase accelerated autoxidation (70% and 300%), and metal chelators did not produce inhibition.Superoxide dismutase evidently accelerates autoxidation when thermodynamic limitations to reduction of oxygen outweigh propagating actions of 02.-.With the naphthalenediols (12-and 1,4naphthalenediol), superoxide dismutase inhibited briefly but then accelerated autoxidation.More facile quinhydrone formation with naphthoquinones allows comproportionation to quickly supercede 02.-as a propagator .Whether an autoxidation proceeds by comproportionation or 02'--propagation, biologically important effects of superoxide dismutase are to limit formation of semiquinones and to facilitate redox pathways for excretion of cytotoxic quinones.I much appreciate the contributions of the members of my graduate committee, Dr.
Source: Summit (Simon Fraser University)
Author: Brian Bandy
Language: English
Subfield: Toxicology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W171761601