The ability of unsaturated fatty acid methyl esters to modify amino acid residues in bovine serum albumin (BSA), glutamine syn- thetase, and insulin in the presence of a metal-catalyzed oxidation system [ascorbate/Fe(Ill)/O2] depends on the degree of unsatura- tion of the fatty acid. The fatty acid-dependent generation of carbonyl groups and loss of lysine residues increased in the order methyl linoleate < methyl linolenate < methyl arachidonate. The amounts of alkyl hydroperoxides. malondialdehyde. and a number of other aldehydes that accumulated when polyunsaturated fatty acids were oxidized in the presence of BSA were significantly lower than that observed in the absence of BSA. Direct treatment of proteins with various lipid hydroperoxides led to a slight increase in the formation of protein carbonyl derivatives, whereas treat- ment with the hydroperoxides together with Fe(ll) led to a sub- stantial increase in the formation of protein carbonyls. These results are consistent with the proposition that metal-catalyzed oxidation of polyunsaturated fatty acids can contribute to the generation of protein carbonyls by direct interaction of lipid oxidation products (αβ-unsaturated aldehydes) with lysine resi- dues (Michael addition reactions) and also by interactions with aIkoxyl radicals obtained by Fe(ll) cleavage of lipid hydroperoxides that are formed. In addition, saturated aldehydes derived from the polyunsaturated fatty acids likely react with lysine residues to form Schiff base adducts.
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