Comparative evaluation of the antioxidant activity of melatonin and related indoles

Recent scientific evidence suggests that melatonin is present in foods and beverages. Therefore, its antioxidant activity needs to be evaluated and compared with that of other bioactive compounds such as polyphenols. This paper presents a systematic study of the antioxidant activity of melatonin and other compounds involved in the melatonin biosynthetic pathway and chemically related substances. The methods selected were the ferric reducing antioxidant power (FRAP) assay, the oxygen radical absorbance capacity (ORAC) assay and the 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay. The reactions depend on the mechanisms involved (electron transfer or hydrogen atom transfer). The kinetic parameters (stoichiometric factors, rate constants) and antioxidant activity show considerable differences within the indole compounds: some compounds do not react at all, melatonin presents a discrete in vitro antioxidant activity while the highest values are shown by 5-hydroxy compounds. These compounds have a clear relation between structure and activity when an electron transfer mechanism is involved (FRAP and DPPH assays). Melatonin and indole-3-acetic acid react better through the hydrogen atom transfer mechanism. The radical scavenging ability towards the DPPH was studied and EC50, TEC50TEC50 and antioxidant efficacy (AE) were calculated: the 5-hydroxy-indoles present values that are similar to those of ascorbic acid or kaempferol.