title: Kinetic Study of Aroxyl-Radical-Scavenging and α-Tocopherol-Regeneration Rates of Five Catecholamines in Solution: Synergistic Effect of α-Tocopherol and Catecholamines.
abstract: Detailed kinetic studies have been performed for reactions of aroxyl (ArO(•)) and α-tocopheroxyl (α-Toc(•)) radicals with five catecholamines (CAs) (dopamine (DA), norepinephrine (NE), epinephrine (EN), and 5- and 6-hydroxydopamine (5- and 6-OHDA)) and two catechins (epicatechin (EC) and epigallocatechin gallate (EGCG)) to clarify the free-radical-scavenging activity of CAs. Second-order rate constants (ks and kr) for reactions of ArO(•) and α-Toc(•) radicals with the above antioxidants were measured in 2-propanol/water (5:1, v/v) solution at 25.0 °C, using single- and double-mixing stopped-flow spectrophotometries, respectively. Both the rate constants (ks and kr) increased in the order NE < EN < DA < EC < 5-OHDA < EGCG < 6-OHDA. The ks and kr values of 6-OHDA are large and comparable to the corresponding values of ubiquinol-10 and sodium ascorbate, which show high free-radical-scavenging activity. The ultraviolet-visible absorption of α-Toc(•) (λmax = 428 nm), which was produced by the reaction of α-tocopherol (α-TocH) with ArO(•), disappeared under the coexistence of CAs due to the α-TocH-regeneration reaction. The results suggest that the CAs may contribute to the protection from oxidative damage in nervous systems, by scavenging free radicals (such as lipid peroxyl radical) and regenerating α-TocH from the α-Toc(•) radical.
authors.affiliation: Department of Chemistry, Faculty of Science, Ehime University , Matsuyama 790-8577, Japan.
authors.forename: Kazuo
authors.initials: K
authors.lastname: Mukai
authors.affiliation: Department of Chemistry, Faculty of Science, Ehime University , Matsuyama 790-8577, Japan.
authors.forename: Kanae
authors.initials: K
authors.lastname: Nagai
authors.affiliation: Department of Chemistry, Faculty of Science, Ehime University , Matsuyama 790-8577, Japan.
authors.forename: Yoshifumi
authors.initials: Y
authors.lastname: Egawa
authors.affiliation: Department of Chemistry, Faculty of Science, Ehime University , Matsuyama 790-8577, Japan.
authors.forename: Aya
authors.initials: A
authors.lastname: Ouchi
authors.affiliation: Department of Chemistry, Faculty of Science, Ehime University , Matsuyama 790-8577, Japan.
authors.forename: Shin-Ichi
authors.initials: S
authors.lastname: Nagaoka
mesh_terms: D002392:Catechin / Q000031:analogs & derivatives / Q000737:chemistry; D002395:Catecholamines / Q000737:chemistry*; D016166:Free Radical Scavengers / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry*; D007700:Kinetics; D010084:Oxidation-Reduction; D012996:Solutions / Q000737:chemistry; D013053:Spectrophotometry; D013329:Structure-Activity Relationship; D024502:alpha-Tocopherol / Q000737:chemistry*
chemical_list: D002395:Catecholamines; D016166:Free Radical Scavengers; D005609:Free Radicals; D012996:Solutions; D002392:Catechin; C045651:epigallocatechin gallate; D024502:alpha-Tocopherol