Selective Free-radical Reactions with Proteins and Enzymes: Pulse Radiolysis and Inactivation Studies on Papain

SummaryThe effect of the selective free radicals (CNS)2− and Br2− on the activity of papain has been studied in neutral and alkaline solution. These radicals are more efficient than the OH radical for inactivating papain in neutral solution. In alkaline solution, OH and Br2− are equally effective, whereas (CNS)2− is less efficient than OH.The absolute rate constants for the reactions of these radicals with papain have been measured as a function of pH for both active and inactive preparations of the enzyme. The transient absorption spectra of the reaction products have also been determined.The results show that tryptophan residues as well as cysteine-25 are essential to the activity of papain.