Identification and quantification of flavonoids from two Southern Italy cultivars of Allium caepa L. Var. Tropea (red onion) and Montoro (copper onion) and their capacity to protect human erythrocytes from oxidative stress

Onions (Allium cepa) are consumed worldwide and represent an important source of dietary phytochemicals with proven antioxidant properties, such as phenolic acids, flavonoids, thiosulfinates, and anthocyanins. Epidemiological and experimental data suggest that regular consumption of onions is associated with a reduced risk of degenerative disorders. Therefore, it is of interest to investigate the biological properties of different varieties of onions. Here, we characterized for the first time a variety of onion, called Ramata di Montoro (coppery onion from Montoro), grown in a niche area in southern Italy, and compared its phenolic profile and antioxidant properties to a commercial ecotype of red onion, Tropea, also present in southern Italy. An analytical method based on high-performance liquid chromatography coupled with UV detection and mass spectrometry was used to separate and characterize the phenolic fraction (anthocyanins and flavonols) extracted from both coppery and red types. The main compounds detected in the two ecotypes were quercetin and quercetin glucosides, isorhamnetin glucosides, kaempferol glucoside, and, among anthocyanins, cyanidin glucosides. Tropea ecotype onion showed a higher content of flavonols (632.82 mg/kg fresh weight) than Montoro type onion (252.91 mg/kg fresh weight). Accordingly, the antioxidant activity of the former was 2.8-fold higher compared to the latter. More pronounced were the differences existing between the four anthocyanins detected in the two ecotypes, with those in the Tropea ecotype onion present at concentrations 20-230-fold higher than in the Montoro type onion. Both extracts reduced LDL oxidation about 6-fold and protected human erythrocytes from oxidative damage induced by HClO by about 40%. In addition, as a consequence of HClO treatment, glutathione concentration in erythrocytes was reduced about 50% and pretreatment with onion extracts induced a recovery of glutathione level by about 15-22%. Qualitative differences highlighted in the chemical composition of the two phenolic extracts, especially the total content of anthocyanins, which was 30-fold higher in Montoro type onion compared to Tropea ecotype, can be associated with the protective effects measured against oxidative damage induced in human erythrocytes.