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The earliest work on oxidation of a biologically important protein was on the effects of selected radicals generated by radiolysis on the enzyme, lysozyme (Adams et al., 1969). Both the thiocyanate radical (a selective modifier of tryptophan) and the hydroxyl radical (-OH) were found to inactivate the enzyme, implying that tryptophan residues are essential for biological activity, now well established from classical enzymology. Similarly, for o À 1-antitrypsin, modification of a single methionine residue at position 358 rendered the protein inactive (Carp and Janoff, 1979). Subsequent hydrolysis and amino acid analysis revealed the presence of methionine sulphoxide. This was one of the first pieces of evidence linking amino acid oxidation to denaturation of proteins and loss of function. The importance of protein oxidation in respect of altered function is exemplified by oxidative modifications to histidine and lysine in low density lipoproteins (LDL), which cause altered receptor