J. Agric. Food Chem. 1998, 46, 923−927
923
Comparing Methylene Blue-Photosensitized Oxidation of Methyl-Conjugated Linoleate and Methyl Linoleate Jin Jiang* and Afaf Kamal-Eldin Department of Food Science, Swedish University of Agricultural Sciences, P.O. Box 7051, S-750 07 Uppsala, Sweden
The photooxidations of methyl-conjugated linoleate (MCL) and methyl linoleate (ML) were compared using methylene blue as a sensitizer in ethanol solutions. MCL was lost to a lower extent and yielded less hydroperoxide but bleached methylene blue at a higher rate than ML. The different isomers of MCL were not equally lost, and the photooxidation of MCL was accompanied by interisomerization. The results suggested that different mechanisms were involved in the photooxidations of MCL and ML Keywords: Conjugated linoleic acid; methyl-conjugated linoleate; methyl linoleate; methylene blue; photooxidation INTRODUCTION
Conjugated linoleic acid (CLA) is a generic name for a mixture of isomers of linoleic acid (LA) with conjugated double bonds at positions 9 and 11 or 10 and 12 (Ha et al., 1987). CLA is present mainly in animal foods such as dairy and meat products (Chin et al., 1992; Banni et al., 1994; Jiang et al., 1996). It has also been identified in human serum, bile, and duodenal juices (Gawood et al., 1983; Iversen et al., 1985). CLA has been shown to exhibit a number of desirable physiological properties including anticarcinogenic (Ha et al., 1987, 1990; Shultz et al., 1992; Ip et al., 1991, 1994) and hypocholesterolemic and antiatherogenic effects (Lee et al., 1994; Nicolosi and Laitinen, 1996). The mechanisms responsible for these unique physiological effects are not yet known. In the search for a possible anticarcinogenic mechanism, CLA was tested for its antioxidant effects using autoxidation models (Ha et al., 1990; Ip et al., 1991; van den Berg et al., 1995; Chen et al., 1997). Two studies have suggested CLA to possess antioxidative properties (Ha et al., 1990; Ip et al., 1991), whereas other studies showed that CLA was not an antioxidant and may even act as a prooxidant (van den Berg et al., 1995; Chen et al., 1997). The controversy in this issue warrants further investigations on the oxidation mechanisms of CLA. The conjugated structure of this fatty acid suggests possible interactions with active oxygen species, for example, singlet oxygen generated during photooxidation. Therefore, this investigation was initiated to compare the photooxidation of methyl-conjugated linoleate (MCL) and methyl linoleate (ML) using methylene blue (MB) as a sensitizer in a model system. MATERIALS AND METHODS Chemicals. The CLA standard was a gift from Dr. S. F. Chin (Food Research Institute, University of Wisconsin, Madison) and was a mixture of the following isomers: 9,11 * Author to whom correspondence should be addressed (telephone +46 18 67 2192; fax +46 18 67 2995; e-mail
[email protected]).
(cis,trans and trans,cis) 48.5%; 10,12 (trans,cis) 48.2%; 9,11 and 10,12 (cis,cis) 1.8%; 9,11 and 10,12 (trans,trans) 1.5%. CLA was methylated according to the method of Werner et al. (1992) using 14% boron trifluoride in methanol as reagent (Sigma Chemical Co., St. Louis, MO). MCL produced were used in all experiments. ML (99% pure), MB, and butylated hydroxytoluene (BHT) were also purchased from Sigma. Methyl eicosanoate (20:0), used as internal standard for GC analysis of residual fatty acids, was purchased from Larodan Fine Chemicals AB (Malmo¨, Sweden). Sodium borohydride powder (NaBH4, 98%) was from Aldrich (Gillingham, JL), and the plates used for thin-layer chromatography (TLC, 0.25 mm silica gel 60) were from Merck (Darmstadt, Germany). All other reagents and organic solvents used were of analytical grade. Photooxidation. Photosensitized oxidation was carried out in ethanol solution containing 0.11 × 10-3 M MB. The light source was a Philips HPI-T 400 W fluorescent lamp of wavelength 350-700 nm. Samples were put into clear glass vials (Glaswarenfabrik Karl Hecht, GmbH, Sondheim-rho¨n, Germany) of 14 mL volume (24 mm in diameter), which were tightly closed. The bottles were placed on a glass panel (0.6 cm in thickness), whereby the light of wavelengths
