Comparison of relative antioxidant activities of British medicinal plant species in vitro.

We have determined the relative levels of endogenous antioxidant activity in a range of British medicinal plant species (representative of a variety of plant families, selected on the basis of their widespread use in traditional herbal medicine), via competitive scavenging of the ABTS(z.rad)(+) or O(2)(-)(z.rad) radicals in vitro. A number of plant species with appreciable levels (i.e. greater than or comparable with the activity in corresponding extracts of Ginkgo biloba or Panax ginseng, as recognised phytological sources of antioxidant activity) of antioxidant activity against the ABTS(z.rad)(+) radical were identified as potentially novel sources of free radical scavenging compounds; however none of these extracts showed measurable antioxidant activity when assayed against the O(2)(-)(z.rad) radical. It, therefore, follows that any comparative determination of antioxidant activity should clearly define the radical species and assay method employed. For those extracts showing activity against the ABTS(z.rad)(+) radical, in general, it was not possible to predict the potential antioxidant capacity of a given plant species (or the most active part of the plant) on a taxonomical basis, or from its use in traditional or modern medicine. In the longer term further characterisation of the active compounds from plant species with appreciable antioxidant activity identified in the present investigation may prove of value for treatment of disorders such as Alzheimer's disease, in which free radical induced tissue damage has been implicated.