A kinetic Study of the aroxyl (ArO.) radical-scavenging reaction of alpha-tocopherol (alpha-Toffl) has been performed in the presence of six kinds of alkali and alkaline earth metal salts (Lil, LiClO4, Nal, NaClO4, KI, and Mg(CIO4)(2)) in methanol solution, using stopped-flow spectrophotometry. The decay rate of tile ArO. for tile reaction of alpha-TocH with ArO. increased linearly with increasing concentration of metal salts. The second-order rate constants (K-s) for the reaction of a-TocH with ArO. increased in the order of no metal salt < KI similar to NaClO4 similar to Nal < LiClO4 < Mg(ClO4)(2) < LiI at the same concentration of metal salts. For example, the k(s) values in methanol Solution including 4.00 x 10(-1) M of Lil and Mg(ClO4)(2) were 3.04 and 1.30 times larger than that in the absence of metal salts, respectively. The alkali and alkaline earth metal salts having smaller ionic radius of cation and anion and larger charge of cation gave larger rate constants (Q. Effects of metal cations oil the UV-vis absorption spectra of the alpha-Toc center dot (and ArO center dot) radical were negligible in methanol solution, suggesting that the complex formation between the alpha-Toc center dot (and ArO center dot) radical molecule and metal cations is hindered by the hydrogen bond between radical and methanol molecules. The results indicate that the hydrogen transfer reaction of alpha-TocH proceeds via all electron transfer intermediate from a-TocH to ArO center dot radicals followed by proton transfer. Both the coordinations of metal cations to the one-electron reduced anions of ArO center dot (ArO: (-)) and of counteranions to the one-electron oxidized cations of alpha-TocH (alpha-TocH(+center dot)) may stabilize the intermediate, resulting in tile acceleration of electron transfer. On the other hand, the effect of metal salts on the rate of bimolecular self-reaction (2k(d)) of the alpha-Toc center dot radical was not observed. The result suggests that the hydrogen transfer reaction between two alpha-Toc center dot radical molecules proceeds via a one-step hydrogen atom transfer mechanism rather than via an electron-transfer intermediate. High concentrations of alkali and alkaline earth metal salts coexist with alpha-TocH in plasma, blood, and many tissues, suggesting the contribution of tile metal salts to the antioxidant actions of alpha-TocH.