Does lysine drive the conversion of fatty acid hydroperoxides to aldehydes and alkyl-furans?

During the oxidation of lipids in food or in vivo, fatty acids are initially converted to hydroperoxides, which undergo decomposition to various secondary decomposition products, including aldehydes and alkyl-furans. Aldehydes and alkyl-furans reduce the sensory quality of food by contributing to rancidity, and aldehydes reduce nutritional value by reacting with some essential nutrients such as lysine and thiamine. In vivo, reactions of aldehyde with proteins and DNA contribute to the pathogenesis of physiological disorders. Conversion of fatty acid hydroperoxides to aldehydes is generally believed to involve free radical reactions. However, it was recently hypothesized that lysine can catalyze the non-radical conversion of hydroperoxides to aldehydes. Thus the aim of the present study was to determine such lysine-catalysed conversion of linoleic acid hydroperoxides to aldehydic products. Linoleic acid hydroperoxides were prepared by the photosensitized oxidation of linoleic acid. The mixture of hydroperoxides was reacted with lysine, in the presence of a radical scavenger, and the organic fraction analysed by gas chromatography-mass spectrometry (GC-MS). Hexanal was detected as a major aldehydic product. The alkylfuran, 2-pentylfuran was also surprisingly detected under these conditions, and a pathway for its lysine-catalysed formation via the highly cytotoxic aldehyde, 4-hydroxy-2-nonenal proposed. The results of this study imply that, in the prevention of lipid oxidation-associated food deterioration and development of physiological disorders, more attention should be paid to pathways not involving free radical reactions, and which cannot be prevented by radical scavenging antioxidants.