Pulse Radiolysis of Aqueous Papain

The pulse radiolysis of the proteolytic enzyme papain has been studied in nitrogen-purged and N₂O-purged aqueous solutions at pH 6.4. The rate constant for reaction of OH with papain is 4.7 × 10¹⁰ M⁻¹ s⁻¹. The initial spectrum of the intermediate formed by this radical is consistent with a major fraction of attack at tyrosine residues with smaller proportions reacting at tryptophan and other sites. Subsequent spectral changes are also consistent with the preferential attack of OH on tyrosine. The overall rate constant for reaction of [Formula: see text] with papain is 4.1 × 10¹⁰ M⁻¹ s⁻¹ and the spectrum of the electron adduct closely resembles that of the disulfide anion [Formula: see text] However, there is evidence that not all reactions of [Formula: see text] with papain lead to the formation of this species. The results are discussed in the light of deactivation yields for Co⁶⁰γ radiolysis measured by biochemical techniques.