Cysteine Radical Research Articles

Synergistic inhibition of oxidation in dispersed phosphatidylcholine liposomes by a combination of vitamin E and cysteine

Oxidations of soybean phosphatidylcholine liposomes in an aqueous dispersion initiated by free radicals generated initially either in the aqueous phase or in the lipid phase were efficiently suppressed by vitamin E in the membranes. Vitamin E was consumed linearly with time and, when the inhibition period was over the oxidation proceeded rapidly at a rate similar to that in the absence of vitamin E. l-Cysteine was also effective by itself in scavenging radicals in the aqueous region, but it was consumed more rapidly than vitamin E. On the other hand, cysteine could not scavenge the radicals efficiently in a lipid region. Nevertheless, when vitamin E was incorporated into liposomes, the addition of cysteine in the aqueous phase prolonged the inhibition period and it reduced the rate of decay of vitamin E markedly even when the radicals were generated initially in the lipid bilayer. Furthermore, it was found by an electron spin resonance study that chromanoxyl radical disappeared quite rapidly when it was mixed with cysteine and that the spin adduct of cysteine radical was observed in the presence of α-(4-pyridyl- N-oxide)- N-tert-butyl nitrone. It was concluded that l-cysteine located in an aqueous region could regenerate vitamin E by reacting with vitamin E radical formed in a lipid region and show a synergistic antioxidant effect, although its efficiency of vitamin E regeneration was lower than that by vitamin C.

The Hydrogen Yields in the Radiolysis of Cysteine and Biscysteinatocobaltate(III) in 0.8 n Sulfuric Acid

Abstract Radiation-induced oxidation of cysteine in an acid medium was studied by measuring the hydrogen yields at an initiation stage of radiolysis. In the deaerated system G(H2)=4.0 along with the G(–CySH)=7.1 were observed and the results can be interpreted in terms of the yields of primary radicals from the radiolysis of water in 0.8 n sulfuric acid medium. From stoichio-metric view point it is predicted that the radiolysis takes place through cysteine radical as intermediate to form cystine. In the aerated system on the other hand, the radiolysis of cysteine is much complicated. For a possible explanation satisfying all the G values obtained, 10.5 in the disappearance of cysteine, 4.0 in the formation of hydrogen peroxide and 0.4 in the hydrogen, the air oxidation of cysteine caused by the radicals, HO2 and OH which produce H2O2 and O2 by the radical recombination, is proposed as mechanism. The radiolysis of cysteine is greatly reduced by complexation with metallic ion as biscysteinatocobaltate(III) complex. The absence of hydrogen abstraction from cysteine by H atom, the isolation of cysteine from a ligand site and the extraordinary high G values in the disappearance of optical absorption of the complex can be cited as characteristics in the radiolysis of the biscysteinatocobaltate(III) complex. The intramolecular redox of the complex into Co(II) ion and cysteine is a best fit mechanism as over all reaction in the radiolysis of the biscysteinatocobaltate(III) complex.