Thiol-induced hydroxyl radical formation and scavenger effect of thiocarbamides on hydroxyl radicals

The effects of thiols and thiocarbamides on hydroxyl radical (·OH) formation by the hypoxanthine(HYP)-xanthine oxidase(XOD)-Fe 3+ ·EDTA system were investigated in the range of 0.5–5 mM by colorimetrically measuring salicylate hydroxylation. Thiocarbamides powerfully inhibited the hydroxylation while thiols showed a paradoxical effect, enhancing it at low concentrations, but inhibiting it at high ones. Thiols in the presence of Fe 3+ ·EDTA generated superoxide anions O 2 −· and ·OH during the oxidation, but thiocarbamides did not. A study of the effect of ergothioneine, a thiocarbamide present in mammals, on the ·OH spin adduct of 5, 5-dimethyl-1-pyrroline- N-oxide(DMPO) by EPR spectrometry showed that it effectively decreased the ·OH spin adduct without causing the appearance of other signals. Reaction mechanisms are proposed for the O 2 −· evolution and ·OH formation by the thiols themselves in the presence of Fe 3+ ·EDTA and ·OH with thiols and thiocarbamides.