Free radical reactions with proteins and enzymes: the inactivation of pepsin

The reactions of free radicals produced by ionizing radiation with pepsin have been studied by steady-state inactivation measurements and by pulse radiolysis. In de-aerated solutions the hydroxyl radical has been found to be the most efficient of the primary free radicals generated from water in causing inactivation. The reactions of the more selective oxidizing inorganic radical anion Br2- and (SCN)2-, with pepsin have also been examined. In the case of the thiocyanate radical anion (SCN)2-, the inactivation efficiency is found to depend on SCN- concentration, an effect shown to arise from a reversible redox reaction involving the tryptophan and (SCN)2- radicals. The results demonstrate the tryptophan residue plays and essential role in the enzymic activity of pepsin. (author)