Structural and functional changes in RNAse A originating from tyrosine and histidine cross-linking and oxidation induced by singlet oxygen and peroxyl radicals.

Oxidation can be induced by multiple processes in biological samples, with proteins being important targets due to their high abundance and reactivity. Oxidant reactions with proteins are not comprehensively understood, but it is known that structural and functional changes may be a cause, or a consequence, of disease. The mechanisms of oxidation of the model protein RNAse A by singlet oxygen (1O2) were examined and compared to peroxyl radical (ROO•) oxidation, both common biological oxidants. This protein is a prototypic member of the RNAse family that exhibits antiviral activity by cleaving single-stranded RNA. RNAse A lacks tryptophan and cysteine residues which are major oxidant targets, but contains multiple histidine, tyrosine and methionine residues; these were therefore hypothesized to be the major sites of damage. 1O2 and ROO• induce different patterns and extents of damage; both induce cross-links and side-chain oxidation, and 1O2 exposure modulates enzymatic activity. Multiple products have been characterized including methionine sulfoxide and sulfone, alcohols, DOPA, 2-oxohistidine, histidine-derived ring-opened species and inter- and intra-molecular cross-links (di-tyrosine, histidine-lysine, histidine-arginine, tyrosine-lysine). In addition to methionine modification, which appears not to be causative to activity loss, singlet oxygen also induces alteration to specific histidine, tyrosine and proline residues, including modification and cross-linking of the active site histidine, His12. The high homology among the RNAse family suggests that similar modifications may occur in humans, and be associated with the increased risk of viral infections in people with diabetes, as markers for 1O2 have been found in early stages of this pathology.