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.

A simulation of free radicals induced oxidation of dopamine in aqueous solution

“…It is found that neurodegenerative diseases are followed by the changes in catecholamines concentrations [1], which supports the conclusion that these species might respond to the oxidative stress related to neurodegenerative diseases. Various attempts to resolve the antioxidant activity of catecholamines and other phenols by employing diverse quantum-chemical and experimental methods provided different reaction mechanisms [3,4,5,6,7,8,9,10,11,12,13,14].Álvarez-Diduk and Galano [8] showed that adrenaline and noradrenaline are efficient in reducing oxidative stress through SPLET in aqueous solution at physiological pH values. Dimić et al [12] discussed the most probable reaction mechanism towards DPPH radical abstraction by naturally occurring catecholamines combining static density functional calculations with UV/VIS spectroscopy.…”