Kinetic study of the prooxidant effect of alpha-tocopherol. Hydrogen abstraction from lipids by alpha-tocopheroxyl radical.

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.