Tetrahydro-beta-carboline alkaloids that occur in foods and biological systems act as radical scavengers and antioxidants in the ABTS assay

Tetrahydro- β-carboline alkaloids that occur in foods such as wine, seasonings, vinegar and fruit products (juices, jams) acted as good radical scavengers (hydrogen- or electron donating) in the ABTS (2,2'-Azinobis-(3-ethylbenzothiazoline-6-sulfonic acid)) assay, and therefore, they could contribute to the beneficial antioxidant capacity attributed to foods. In contrast, the fully aromatic β-carbolines norharman and harman did not show any radical scavenger activity in the same assay. During the reaction with ABTS^.++ radical cation, tetrahydro- β-carboline-3-carboxylic acid such as 1-methyl-1,2,3,4-tetrahydro- β-carboline-3-carboxylic acid (MTCA) and 1-methyl-1,2,3,4-tetrahydro- β-carboline-1,3-dicarboxylic acid (MTCA-COOH) were converted to harman, whereas 1,2,3,4-tetrahydro- β-carboline-3-carboxylic acid (THCA) and 1,2,3,4-tetrahydro- β-carboline-1,3-dicarboxylic acid (THCA-COOH) afforded norharman. These results suggest that food and naturally-occurring tetrahydro- β-carboline alkaloids if accumulated in tissues, as reported elsewhere, might exhibit antioxidant activity.