Kinetic Study of Aroxyl-Radical-Scavenging and α‑Tocopherol-Regeneration Rates of Five Catecholamines in Solution: Synergistic Effect of α‑Tocopherol and Catecholamines

• Published in: The journal of physical chemistry. B Vol. 120; no. 29; pp. 7088 - 7097
• Main Authors: Mukai, Kazuo, Nagai, Kanae, Egawa, Yoshifumi, Ouchi, Aya, Nagaoka, Shin-ichi
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 28.07.2016
• Subjects: alpha-Tocopherol - chemistry; Catechin - analogs & derivatives; Catechin - chemistry; Catecholamines - chemistry; Free Radical Scavengers - chemistry; Free Radicals - chemistry; Kinetics; Oxidation-Reduction; Solutions - chemistry; Spectrophotometry; Structure-Activity Relationship
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/acs.jpcb.6b04285

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 (k s and k r) 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 (k s and k r) increased in the order NE < EN < DA < EC < 5-OHDA < EGCG < 6-OHDA. The k s and k r 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.