Flavonols are found in plants as aglycones and as glycosides. Antioxidant activity of flavonols may occur via several mechanisms within the cell, and mitochondria as a target may play an important role. There is a lack of information about the influence of the sugar moiety on biological activity of flavonoid glycosides. The aims of study were to investigate the effects of quercetin and its glycosides on mitochondrial respiration rates at various metabolic states, and to evaluate their antioxidant potential using chemical and biological approaches. Mitochondrial function was measured using an oxygraphic method, cytochrome c reduction spectrophotometrically, H₂O₂ generation in mitochondria fluorimetrically, and antioxidant activity of flavonoids using an HPLC-post column system. Our data revealed that quercetin and its glycosides isoquercitrin, rutin, and hyperoside uncouple kidney mitochondrial respiration (increasing the State 2 respiration rate) and significantly reduce cytochrome c. Moreover, quercetin, and its glycosides decrease the production of mitochondrial H₂O₂ and possess radical scavenging and ferric reducing capacities. The highest activity was characteristic for quercetin, showing that the sugar moiety significantly diminishes its activity. In conclusion, our results show the efficient radical scavenging, ferric and cytochrome c reducing capacities, and uncoupling properties of quercetin and its glycosides, as well as the importance of the sugar residue and its structure in the regulation of kidney mitochondrial function.

This study investigates the phytochemical composition and cultivation strategies for watermelon (Citrullus lanatus (Thunb.)) in Lithuania’s temperate climate, focusing on its biological activity. Employing innovative grafting techniques and clear plastic film mulching, we successfully countered fusarium wilt while promoting growth and bioactive compound accumulation. Our analysis showed significant cultivar-dependent variations in total phenolic content (ranging from 94.34 ± 8.12 to 327.42 ± 9.14 mg GAE/kg fw in pulps and from 120.46 ± 7.52 to 364.27 ± 6.85 mg GAE/kg fw in rinds), lycopene (ranging from 1.15 ± 0.42 to 103.60 ± 1.69 mg/kg fw in pulps), sugar, and nitrate levels, revealing the influence of genetics and environment on the fruit’s phytochemical profile. Moreover, several Lithuanian watermelon cultivars exhibited comparable or superior levels of key bioactive compounds relative to imported varieties. These findings underscore the potential of watermelon rind and pulp as valuable sources of antioxidants and other bioactive phytochemicals, relevant for nutritional enhancement and medicinal applications. The results contribute to a deeper understanding of watermelon cultivation in Lithuania and highlight opportunities for optimizing agricultural practices to enhance the health benefits associated with this important fruit.