Oxoberberine: a promising natural antioxidant in physiological environments

Oxoberberine (OB, 2,10-dihydroxy-3,9-dimethoxy-8-oxo-protoberberine, artathomsonine), which was isolated from Artabotrys thomsonii, was shown to exhibit potent antioxidant activity in vitro, however that is the only reported evidence of the radical scavenging activity of this compound thus far. In the present study, thermodynamic and kinetic calculations were used to determine the free radical scavenging activity of OB against a range of biologically important species, under physiological conditions. In the first part the activity is calculated against the HOO˙ radical that is both biologically important and a reference radical for comparison. It was found that OB has high antiradical capacity against HOO˙ in both lipid medium and water at physiological pH with k _overall = 1.33 × 10⁵ and 1.73 × 10⁶ M^-1 s^-1, respectively. The formal hydrogen transfer mechanism defined the activity in nonpolar environments, whereas in the aqueous solution the single electron transfer competes with the hydrogen transfer pathway. The results showed that, in lipid medium, the HOO˙ trapping capability of OB is better than typical antioxidants such as Trolox, BHT, resveratrol and ascorbic acid. Similarly, the activity of OB in water at pH 7.4 is roughly 19 and 7 times faster than those of Trolox and BHT, respectively, but slightly lower than the activities of resveratrol or ascorbic acid. In the second part, it was found that OB also exhibits high activity against other typical free radicals such as CH₃O˙, CH₃OO˙, CCl₃OO˙, NO₂, SO₄˙^-, DPPH and ABTS˙⁺ with k _f ranging from 2.03 × 10⁵ to 5.74 × 10⁷ M^-1 s^-1. Hence, it is concluded that OB is a promising radical scavenger in the physiological environment.