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Selective free radical reactions with proteins and enzymes: the inactivation of alpha-chymotrypsin.

SummaryThe reactions of the selective free radicals (CNS)2− and Br2− with α-chymotrypsin have been studied by pulse radiolysis and steady-state inactivation measurements.Rate-constants for the reaction have been measured over the pH range 7 to 12. The reactivity increases at pH 11 where the tyrosine residues deprotonate. Transient product spectra also indicate a significant increase in attack at tyrosine at this pH. A small increase in rate was observed by lowering the salt concentration from 10−1 M to 4 × 10−2 M.The inactivation data show (CNS)2− to be protective relative to OH over the pH range 2 to 11, whereas Br2− sensitizes between the pH range 4 and 9.From the pulse radiolysis and inactivation data, it was concluded that a histidine residue was essential to the activity of α-chymotrypsin.