Effects of β β-carotene and retinal on the formation of methyl linoleate hydroperoxides
In order to clarify the prooxidative role of carotenoids on the oxidation of unsaturated lipids this study examined the effects of β-carotene and its oxidative breakdown product, retinal, on primary oxidation products of linoleic acid methyl ester. Formation as well as isomer distribution of methyl linoleate hydroperoxides were followed by highperformance liquid chromatography. Oxidation of methyl linoleate without or with added β-carotene (5, 20, 200 µg/g) or retinal (7, 18, 180, 360 µg/g) was carried out in the dark under air at 40 °C. Both β-carotene and retinal promoted the formation of hydroperoxides and thus acted as prooxidants in a concentration-dependent way. Moreover, carotenoids also had an effect on the isomeric distribution of primary oxidation products as high contents of retinal increased the portion (%) of trans,trans-hydroperoxides. Being thermodynamically more stable isomers than cis,trans-isomers of hydroperoxides they are known to accumulate during later phases of oxidation or during hydroperoxide decomposition. The results showed that β-carotene and retinal were not effective hydrogen donors. These findings raise the question that carotenoids and their oxidative breakdown products enhance the decomposition of lipid hydroperoxides and this effect partially explains the prooxidative effect of carotenoids.