Kinetic Study of the Radical-scavenging Activity of Vitamin E and Ubiquinone

Abstract

Quantitative in vitro studies of antioxidant activities have been performed under aerobic conditions. However, since the biological system has lower oxygen tension, the effectiveness of antioxidants may be considerably different in vivo. α-Tocopherol, in vivo the most active tocopherol, is a very poor antioxidant in vitro. To clarify these points, the radical-scavenging activities of vitamin E (Toc) (α-, β-, γ- and δ-tocopherols) and ubiquinone were evaluated by the induction period method from the kinetics of polymerization of methyl methacrylate (MMA) initiated by thermal decomposition of 2,2'- azobisisobutyronitrile (AIBN) (alkyl radical, R^.), or benzoyl peroxide (BPO) (peroxyl radical, PhCOO^.) under nearly anaerobic conditions. The ratio of the rate constant of inhibition to that of propagation (k_inh/k_p) for Toc was about 10 in a system with a molar ratio of AIBN to Toc of 100:1, whereas in the corresponding BPO system k_inh/k_p declined in the order α(47)>β(15)>γ(10)>δ(7). In contrast, with AIBN the number of free radicals trapped by the phenolic moiety (n) declined in the order δ(3.0)>γ(2.5)>α(2.2)>β(1.6), whereas with BPO n declined in the order δ(1.9)>γ(1.4)>β(1.0)>α(0.3). A similar tendency was found in systems with a molar ratio of 10:1. Also, ubiquinone-10 showed radical-scavenging activity, although the n (0.02) was much less than that for Toc. The low n value for α-Toc (n=0.3) may be attributed to the formation of stable α-Toc^. during the induction period. With a n=about 1 forβ- and γ-Toc, a dimerization coupling of Tocs is suggested. Thus, the radical-scavenging activity is affected by the number and position of the methyl groups in the benzene nucleus of the various tocopherol compounds.