There now exists a bewildering array of biological processes in which free radicals have been implicated (1 ), and we assume that enzymes and structural proteins may be attacked whenever free radicals are generated. As a consequence, oxidative modification of proteins may occur in a variety of physiologic and pathologic processes. Although the distinction is sometimes arbitrary, these modifications may be primary or secondary. Primary modifications occur in metal-catalyzed oxidation, in radiation-mediated oxidation, and in oxidation by ozone or oxides of nitrogen. Secondary modifications occur when proteins are modified by molecules generated by oxidation of other molecules. One important example is the covalent modification of proteins by hydroxynonenal produced by oxidation of lipids (2 ,see also, Chapter 3 of this volume).
