Thiyl radicals in biological systems: significant or trivial?

Thiyl radicals are formed from one-electron oxidation of thiols. Thiyl radicals participate in a number of reactions including electron transfer, hydrogen abstraction and addition reactions with several biological constituents and xenobiotics. Thiyl radicals can be detected by optical spectroscopy or by electron spin resonance (ESR) spectroscopy. Thiyl radicals appear to play a role in the nitrosylation of thiols and protein thiols. The exact mechanism of thiol-induced enhancement of oxidative modification of low-density lipoprotein remains questionable. The proposed role of thiyl radicals in lipid peroxidation needs to be re-examined. It has been proposed that thiyl radicals are detoxified by superoxide dismutase in mammalian cells and by a thiol-specific enzyme in bacterial systems. We propose that thiols or protein thiols act as potent antioxidants in radical-induced damage via formation of thiyl radicals.