Electron spin resonance (ESR) study on free radical transfer in fish lipid-protein interaction

Oxidised lipids interact with proteins causing undesirable changes in the n utritional and functional properties, including a loss of amino acids, cros s-linking and damage to proteins and DNA. Proteins including egg lysozyme, egg ovalbumin, fish myosin and amino acids arginine, lysine and histidine w ere exposed to oxidised lipids methyl linoleate and oil extracted from Atla ntic mackerel (Scomber scombrus). A strong central singlet signal was induc ed in the proteins and amino acids which was detected by ESR spectroscopy a nd assigned to the carbon radical (g value range 2.0021-2.0049); with ovalb umin and fish myosin a downfield shoulder, attributed to the sulphydryl gro up (g value 2.014-2.017), was also observed. The above changes in the prote ins were accompanied by an increase in fluorescence indicating the formatio n of cross-links. Synthetic antioxidants such as butyl hydroxytoluene (BHT) and butyl hydroxyanisole (BHA) as well as natural antioxidants ascorbic ac id and alpha-tocopherol inhibited the development of both the free radical signal and fluorescence when added to the proteins prior to incubation with oxidised lipids; the central singlet signal attributed to the carbon radic al was reduced by 10-50%. This paper clearly indicates direct free radical transfer from oxidised lipids to amino acids and proteins which results in protein denaturation and the formation of insoluble aggregates in fish fles h on storage. (C) 1999 Society of Chemical Industry.