EPR Spectra and Antioxidant Activity of Gamma-Irradiated Papain

The structure and number of paramagnetic centers (PC) trapped in papain γ-irradiated with a dose of 50 to 2300 kGy have been studied using EPR spectroscopy. The radiation-chemical yield G ≈ 6 PC/100 eV during radiolysis at 77 K is six times greater than that in the samples irradiated at 300 K. In the case of radiolysis at 300 K, the maximum PC concentration is achieved at doses of 200 kGy at еру level of 8 × 1018 PC/g, whereas paramagnetic centers in papain radiolyzed at 77 K accumulate up to 2300 kGy and reach 2.2 × 1020 PC/g. During papain radiolysis at 77 K, the cleavage of the peptide bonds prevail over the scission of bonds in molecular groups of amino acid residues, including the sulfur-containing ones. As a result, $$ - {\text{C}}( {=} {\text{O}}){-} \mathop {\text{C}}\limits^\bullet {\text{H}}{-} {\text{C}}{{{\text{H}}}_{{\text{2}}}}{\text{R}}$$ radicals are mainly recorded in the EPR spectra. In the multicomponent spectrum of papain irradiated at 300 K, a doublet with splittings of 1.77 mT is distinguished, which is attributed to the $$ - {\text{C}}( {=} {\text{O}}){-} \mathop {\text{C}}\limits^\bullet {\text{H}}{-} {\text{NH}}{-} $$ radical formed by hydrogen abstraction from the glycine residue. Peroxide radicals formed during radiation-induced oxidation at 300 K are not retained as trapped radicals in the matrix of irradiated papain and, most likely, participate in secondary radiation-chemical processes with the formation of oxygen-containing products. A tendency for the antiradical and antioxidant activity of papain to increase with increasing radiation dose is noted as a result of radiation-induced destruction of the peptide bond with the formation of amino acid fragments that are hydrogen atom donors.