Lipid Peroxidation by “Free” Iron Ions and Myoglobin as Affected by Dietary Antioxidants in Simulated Gastric Fluids TAIR LAPIDOT,RINA GRANIT, AND JOSEPH KANNER* Department of Food Science, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel Grilled red turkey muscle (Doner Kabab) is a real “fast food” containing ∼200 µM hydroperoxides, homogenized in simulated gastric fluid and oxidized more rapidly at pH 3.0 than at pH 5.0, after 180 min, producing 1200 and 600 µM hydroperoxides, respectively. The effects of “free” iron ions and metmyoglobin, two potential catalyzers of lipid peroxidation in muscle foods, were evaluated for linoleic acid peroxidation at pH 3.0 of simulated gastric fluid. The prooxidant effects of free iron ions on linoleic acid peroxidation in simulated gastric fluid was evaluated in the presence of ascorbic acid. At low concentrations of ascorbic acid, the effects were prooxidative, which was reversed at high concentrations. In the presence of metmyoglobin, ascorbic acid with or without free iron enhanced the antioxidative effect. Lipid peroxidation by an iron-ascorbic acid system was inhibited totally by 250-500 µM catechin at pH 3.0. The catechin antioxidant effect was determined also in the iron- ascorbic acid system containing metmyoglobin. In this system, catechin totally inhibited lipid peroxidation at a concentration 20-fold lower than without metmyoglobin. The ability of catechin to inhibit lipid peroxidation was also determined at a low pH with -carotene as a sensitive target molecule for oxidation. The results show that a significant protection was achieved only with almost 100-fold higher antioxidant concentration. Polyphenols from different groups were determined for the antioxidant activity at pH 3.0. The results show a high antioxidant activity of polyphenols with orthodihydroxylated groups at the B ring, unsaturation, and the presence of a 4-oxo group in the heterocyclic ring, as demonstrated by quercetin. KEYWORDS: Lipid peroxidation; iron ions; myoglobin; dietary antioxidants; gastric fluid; red meat; frying oil; foods INTRODUCTION Peroxidation in foods is one of the major degrative processes responsible for losses in food quality. In addition to the potential implication of lipid peroxidation for changes in flavor, color, and texture, the oxidation of unsaturated lipids results in a significant generation of cytotoxic and genotoxic compounds (1-10). Furthermore, the free radicals generated by the process of lipid peroxidation not only generate cytotoxic compounds but also cooxidize vitamins such as vitamin A and carotenoids, vitamin E, and vitamin C, affecting the nutritional quality of the food. Transition metals such “free” iron or copper ions, with their labile electrons, are well-suited to catalyze redox reactions. Hemeproteins, in general, and myoglobin and hemoglobin, in particular, are implicated in lipid peroxidation of foods and in vivo biological systems (2, 4). Foods contain not only endog- enous catalysts but also dietary antioxidants; the overall results of oxidation and byproducts formation in the systems depend on the concentrations and activities of all of these constituents in the medium. Most recently, we found that human gastric fluid (HGF) may be an excellent medium for enhancing the oxidation of lipids and other dietary constituents (5). We believe that the stomach acts as a bioreactor in which many molecules interact and that the oxidation of high fat and cholesterol-rich foods could be enhanced by endogenous metal catalysts. The overall results of these reactions, however, will depend not only on the activity of endogenous catalysts and oxidizing compounds but also on the presence of dietary antioxidants, mostly polyphenolic compounds, which could generally affect lipid peroxidation. Several studies found that oxidized lipids in the diet are a source of plasma hydroperoxides and oxidized lipids in chylo- microns are a source of animal and human serum (6, 7). Chylomicrons containing dietary oxidized lipids may be a metabolic product involved in the injury to the arterial wall and may constitute a potential link between postprandial lipaemia and atherogenesis (8-10). The aim of this study was to better understand the interactions between free metal catalysts, myoglobin (hemeproteins), and dietary antioxidants affecting lipid peroxidation in simulated gastric fluid (SGF) at pH 3.0. We hypothesize that these reactions in the stomach would have an impact on our health. * To whom correspondence should be addressed. Tel: 972-3-9683761. Fax: 972-3-9683692. E-mail: vtkanner@volcani.agri.gov.il. J. Agric. Food Chem. 2005, 53, 3383-3390 3383 10.1021/jf040402g CCC: $30.25 © 2005 American Chemical Society Published on Web 04/06/2005