Kinetic Study of the Prooxidant Effect of α‐Tocopherol. Hydrogen Abstraction from Lipids by α‐Tocopheroxyl Radical

Abstract

A kinetic study of the prooxidant effect of alpha-tocopherol was performed. The rates of allylic hydrogen abstraction from various unsaturated fatty acid esters (ethyl stearate 1, ethyl oleate 2, ethyl linoleate 3, ethyl linolenate 4, and ethyl arachidonate 5) by alpha-tocopheroxyl radical in toluene were determined, using a double-mixing stopped-flow spectrophotometer. The second-order rate constants (k (p)) obtained are <1 x 10(-2) M(-1 )s(-1) for 1, 1.90 x 10(-2) M(-1 )s(-1) for 2, 8.33 x 10(-2 )M(-1 )s(-1) for 3, 1.92 x 10(-1) M(-1 )s(-1) for 4, and 2.43 x 10(-1 )M(-1 )s(-1) for 5 at 25.0 degrees C. Fatty acid esters 3, 4, and 5 contain two, four, and six -CH(2)- hydrogen atoms activated by two pi-electron systems (-C=C-CH(2)-C=C-). On the other hand, fatty acid ester 2 has four -CH(2)- hydrogen atoms activated by a single pi-electron system (-CH(2)-C=C-CH(2)-). Thus, the rate constants, k (abstr)/H, given on an available hydrogen basis are k (p)/4 = 4.75 x 10(-3 )M(-1 )s(-1) for 2, k (p)/2 = 4.16 x 10(-2) M(-1 )s(-1) for 3, k (p)/4 = 4.79 x 10(-2 )M(-1 )s(-1) for 4, and k (p)/6 = 4.05 x 10(-2 )M(-1 )s(-1) for 5. The k (abstr)/H values obtained for 3, 4, and 5 are similar to each other, and are by about one order of magnitude higher than that for 2. From these results, it is suggested that the prooxidant effect of alpha-tocopherol in edible oils, fats, and low-density lipoproteins may be induced by the above hydrogen abstraction reaction.

Aroxyl-Radical-Scavenging Rate Increases Remarkably under the Coexistence of α-Tocopherol and Ubiquinol-10 (or Vitamin C): Finding of Synergistic Effect on the Reaction Rate

“…In order to investigate the effect of the concentration of UQ 10 H 2 on the reaction rate (k s α-TocH (+UQ 10 H 2 )), a similar measurement was performed for the mixture including higher constant [UQ 10 H 2 ] (see measurement 2 in Table 2A). As shown in Figure 6b, the k s α-TocH (+UQ 10 …”

“…k r values obtained for UQ 10 H 2 and Na + AsH − are 2.01 × 10 5 and 1.19 × 10 6 M −1 s −1 , respectively, as listed in Table 1. The rate constant (k r ) of Na + AsH − is 5.92 times larger than that of UQ 10 Measurements of the k s α-TocH (+UQ 10 H 2 ) and k s UQ10H2 (+α-TocH) values were performed for the solution including α-TocH and UQ 10 H 2 . "k s α-TocH (+UQ 10 H 2 ) and k s UQ10H2 (+α-TocH)" indicate the rate constants obtained in the solution Table 3B and text).…”

“…The k s UQ10H2 (+α-TocH) values obtained at low and high [UQ 10 Table 2B). The k s UQ10H2 (+α-TocH) values of the former and the latter are 2.47 and 1.61 times larger than the k s UQ10H2 (alone) value, respectively.…”

Concurrent supplementation of arginine and antioxidant vitamins E and C reduces oxidative stress in broiler chickens after a challenge withEimeriaspp.

“…Increased production of these reactive species could cause oxidation of biomolecules and cell injury (Guney et al 2007). Vitamin E breaks the lipid peroxidation chain, but this process results in the formation of oxidized forms of the vitamin, which could in turn act as prooxidant agents (Ouchi et al 2009). Ascorbic acid can also form a radical (ascorbate radical), thus acting as an antioxidant by itself, or it could also participate in the a-tocopherol antioxidant system, converting its oxidized forms back to a-tocopherol (Guney et al 2007).…”

Review: In vivo and postmortem effects of feed antioxidants in livestock: a review of the implications on authorization of antioxidant feed additives

“…Thus, prooxidants could exert detrimental effects on animal health and product quality in contrast to antioxidants. There are available data that showed the potential prooxidant effect of vitamin E (Pearson et al, 2006;Ouchi et al, 2009), vitamin C (Podmore et al, 1998), carotenoids (Palozza, 1998), synthetic antioxidants (Kahl et al, 1990) and phenolic compounds (Fukumoto and Mazza, 2000;Simić et al, 2007). It is noteworthy that the in vivo prooxidant effect of carotenoids and vitamins C and E has triggered much criticisms (Bland, 1998;Young and Lowe, 2001;Hathcock et al, 2005) and their prooxidant activity is yet to be convincingly proven due to sparse and conflicting data.…”