Comparative analysis of in vitro antioxidant capacities of mycosporine-like amino acids (MAAs)

The most common Mycosporine-like Amino Acids (MAAs) in red algae (asterina-330, shinorine, palythine, palythinol, and porphyra-334) were evaluated regarding their in vitro antioxidant capacities, comparing them with synthetic (BHT and Trolox) and natural (ascorbic acid, gallic acid, p-cumaric acid, quercetin, and rutin) antioxidants. Folin-Ciocalteu, ABTS+, and FRAP assays have the same reaction mechanism of electron transfer and usually show a positive correlation. However, the antioxidant capacities for the MAAs were discrepant between these assays. While porphyra-334, shinorine, and palythine were as active as, or more active than synthetic phenolic antioxidants (Trolox and BHT) in Folin-Ciocalteu assay, the antioxidant potentials of MAAs were lower than those of the standard compounds in ABTS+ and FRAP assays. The pH-dependent antioxidant power was hypothesized as an explanation for this distinct behavior and was evaluated through ABTS+ assay. Alkaline pH allows antioxidant capacities similar to Trolox in ABTS+ assay, supporting our hypothesis. Comparing to standard compounds, MAAs showed low activity using ORAC assay and were pro-oxidant in β-carotene/linoleic acid assay. Therefore, imino-MAAs present relevant in vitro antioxidant capacity under specific pH conditions in assays based on electron transfer.