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

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.
The glycosylation pattern of elderberry anthocyanins affects their antioxidant activity. Complex residues and number of sugar moieties appear counterproductive. Only cyanidin 3‐O‐glucoside and cyanidin 3‐O‐sambubioside are protective against hydrogen peroxide‐induced reactive oxygen species (ROS) production in Caco2‐C2BBe1 colon tumor cells. Of note, degradation products are equipotent in antioxidant activity, but none are protective. Phloroglucinol aldehyde even displays pro‐oxidant properties. The mechanism of protection remains still ambiguous.