Cytotoxic reactions of free radical species of oxygen.

— We report on autooxidation reactions related to selective cell killing in vivo and present oxygen uptake and EPR data on the molecular nature of these changes, with a note on a reaction used to test for singlet oxygen. Selective obliteration by 6-hydroxydopamine, 6-aminodopamine and their congeners of beta-adrenergic cell receptors and of adrenergic and dopaminergic neurons in vivo and in vitro have been reported recently. These effects devolve from autoxidation of the compounds in situ after appropriate biological concentration, but it is unclear whether cytotoxicity is mediated by the reactive oxygen species themselves or by nucleophilic reactions of the product quinones. Because of the possibility of harnessing these reactions for a generalized “chemical surgery” if the former mechanism is operative, and because many other oxidative damage reactions involving unsaturated lipids of biomembranes seem to share similar autoxidative initiating and chain-carrying steps, we have studied the autoxidation of ascorbate and other reductants catalyzed by polyhydroxy quinols related to 6-hydroxydopamine. We find that superoxide anion mediation may or may not be important, depending on redox potentials and reaction kinetics of particular compounds. To the extent that the cellular toxicity of these compounds is oxidative in nature, it is facilitated by—and may depend upon—hydroxyl radical production due to Fenton-type reactions. Likely Fenton reactants in vivo are hydrogen peroxide produced from the quinol-catalyzed autoxidations and non-heme iron reduced cyclically by superoxide anions from the same autoxidations. Studies using flow systems adapted for EPR at 35 GHz have provided support for some of the reaction mechanisms proposed. In nonaqueous solvents eerie oxidation produced transient free radicals from hydrophobic diphenyl and isobenzo furans, which are often cited as “specific” probes for singlet oxygen detection in biochemistry.