Antioxidant Effects of Elderberry Anthocyanins in Human Colon Carcinoma Cells: A Study on Structure–Activity Relationships

Abstract

Scope: Glycosylation is a way to increase structure-stability of anthocyanins, yet compromises their bioactivity. The study investigates the antioxidant activity of purified cyanidin (Cy)-based anthocyanins and respective degradation products in Caco-2 clone C2BBe1 aiming to identify structure–activity relationships. Results and Methods: Cyanidin 3-O-glucoside (Cy-3-glc) and cyanidin 3-O-sambubioside (Cy-3-sam) proved to be most potent regarding antioxidant properties and protection against hydrogen peroxide (H2O2)-induced reactive oxygen species (ROS)-levels measured with the dichloro-fluorescein (DCF) assay. Cyanidin 3-O-sambubioside-5-O-glucoside (Cy-3-sam-5-glc) and cyanidin 3-O-rutinoside (Cy-3-rut) were less efficient and not protective, reflecting potential differences in uptake and/or degradation. Following ranking in antioxidant efficiency is suggested: (concentrations ≤10 × 10−6 M) Cy-3-glc ≥ Cy-3-sam > Cy-3-sam-5-glc ≈ Cy-3-rut ≈ Cy; (concentrations ≥50 × 10−6 M) Cy-3-glc ≈ Cy-3-sam ≥ Cy > Cy-3-sam-5-glc ≈ Cy-3-rut. Cy and protocatechuic acid (PCA) reduced ROS-levels as potent as the mono- and di-glycoside, whereas phloroglucinol aldehyde (PGA) displayed pro-oxidant properties. None of the degradation products protected from oxidative stress. Gene transcription analysis of catalase (CAT), superoxide-dismutase (SOD), glutathione-peroxidase (GPx), heme-oxygenase-1 (HO-1), and glutamate-cysteine-ligase (γGCL) suggest no activation of nuclear factor erythroid 2-related factor 2 (Nrf2). Conclusion: More complex residues and numbers of sugar moieties appear to be counterproductive for antioxidant activity. Other mechanisms than Nrf2-activation should be considered for protective effects.