Photooxidation methionine residues

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. 

The involvement of methionine sulfonium salts in these chemical cleavage reactions may stimulate experiments to show the significance of special methionine residues in the active centers and the catalytic sites of enzymes. Such an important role for a particular methionine has been postulated for the enzymes phosphoglucomutase and chymotrypsin on the basis of photooxidation studies (Ray et al., 1960). 

Dye-sensitized photooxidation with visible light to identify residues critically involved in the biological activity of enzymes has been introduced by Weil et al. (425). The procedure is mild and moderately selective for some amino acid side chains in addition to tryptophan, tyrosine, histidine, methionine, cysteine and cystine are also photooxidizable. It has been found possible, by an appropriate choice of dye, to bring about specific modification of a particular residue. There is, however, considerable variation in the effects of different dyes and of particular experimental conditions [see ref. 112, 255, 319, 426) for reviews]. 

Despite the extensive use to which photochemical oxidations of proteins have been put very little attention has been given to identifying the end products of the photoreactions. Apart from methionine which is converted to methionine sulfoxide and more slowly to its sulfone, the fate of the other amino acid residues of irradiated proteins remains largely unknown. On the basis of limited chemical studies, it is clear, however, that photoreactions usually lead to a mixture of products. The multiplicity of products formed by photooxidation of enzymes will undoubtedly limit the utility of the technique. There are in fact numerous examples from work dealing with the chemical modification of enzymes which indicate that modification of a particular residue with different chemical reagents leads to enzyme derivatives with different biological and physico-chemical properties. 

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