Methionine sulfoxide, assay

Etienne, F., Resnick, L., Sagher, D., Brot, N., and Weissbach, H. (2003) Reduction of sulindac to its active metabolite, sulindac sulfide assay and role of the methionine sulfoxide reductase system. Biochem. Biophys. Res. Commun. 312, 1005-1010. 

Dye-sensitized photooxidation of methionine occurs selectively in strong formic or acetic acid media (131). All four methionine residues in RNase-A appear to be converted to sulfoxides under these conditions. The product still showed 13% of the initial activity in the Kunitz RNA assay at pH 5 [see, however, Neumann et al. (ISO)]. Apparently an oxygen atom can be accommodated next to each Met sulfur atom in a structure closely resembling that of the native enzyme. The 3 buried tyrosine residues appeared to have been largely normalized. It would be interesting to know if the binding of inhibitors returned these to the buried condition. 

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