Utilization of quercetin and quercetin glycosides from onion (Allium cepa L.) solid waste as an antioxidant, urease and xanthine oxidase inhibitors
Graphical abstract
Introduction
In the present era, there is a significant loss of valuable nutritional and non-nutritional food components, such as carbohydrates, proteins, fibers, and important secondary metabolites through industrial processing of various foods and food products (Mirabella, Castellani, & Sala, 2014). However, there are different methods and practices for the recovery and valorization of these valuable nutritional and non-nutritional components from the residual materials produced during industrial processing of fruit and vegetables (Manousaki, Jancheva, Grigorakis, & Makris, 2016). Onion (Allium cepa L.) is a vegetable crop belonging to the family Alliaceae and has considerable economic importance as it is one of the most valuable vegetable crop grown and utilized globally for domestic food preparations with important medicinal, nutritional, and functional properties (FAO, 2009, Nile and Park, 2013a). Onions are largely consumed as a vegetable and utilized by the processed food industry for commercialized and mechanized chopping that leads to the production of huge amount of onion solid waste (OSW) (Manousaki et al., 2016). An enormous amount of onion waste were produced in several countries like the 100,000 tons of onion waste annually produced in California, USA and about 500,000 tons of onion waste were produced every year in the European Union, mainly in Spain, Holland, and UK (Sharma et al., 2016, Waldron, 2001). The onion waste primarily consists of outer fleshy scales, roots top and bottom bulb part; onion skins generated during mechanical peeling; and undersized, malformed, diseased, or damaged onion bulbs (Khiari et al., 2009, Sharma et al., 2016). The onion waste also contains outer papery brown, yellow, or red skins with colored tops and bottoms of the onion bulbs. The external and internal scales of onions are white and fleshy with a high amount of phenolics and organosulfur compounds (Khiari et al., 2009). The waste products produced from industrially processed onions consist of significant amount of dietary and phenolic compounds. These produced OSW and their bioactive phytochemicals have various biological activities including, antioxidants, anti-inflammatory, antidiabetic, anti-allergic, and enzyme inhibitory potential (Benítez et al., 2011, Sharma et al., 2016).
Onion bulbs and onion waste consist of numerous dietary compounds and bioactive phytonutrients, including phenolics and flavonoids (e.g., sugars, fibers, vitamins, anthocyanins, and quercetin glucosides). They also contains a significant amount of metal ions mostly chromium, manganese, molybdenum, folates, potassium, and copper (Ly et al., 2005, Sharma et al., 2016). Most of the nutrients and bioactive compounds concentrated in outer layers of onions which were wasted during industrial processing or domestic cooking. These bioactive compounds and nutrients can be recovered through bio-refining or employing various extractions methods for industrial and domestic onion solid waste (Sharma, Assefa, Ko, Lee, & Park, 2015). In onions, the quercetin, quercetin 3,4′-diglucoside, quercetin 4′-glucoside, quercetin 3-glucoside, isorhamnetin 3,4′-diglucoside, and isorhamnetin 4′-glucoside were reported as the primary flavonols, comprising about the 90% overall composition of different onions species including inner and outer scale content of onion (Bonaccorsi et al., 2008, Benítez et al., 2011). The bioactive compounds such as phenolics, flavonoids, and flavonol glycosides from different fruits and vegetables are known for their biological effects, including anticancer, antimutagenic, antioxidant, antiproliferative, and antimicrobial effects (Ko et al., 2015, Sharma et al., 2016). The various pharmacological and therapeutic properties of these extracted phenolics, flavonoids, and flavonol glycosides from onions have been mainly evaluated for their antioxidant, anti-inflammatory, and enzyme inhibitory activities (Nile & Park, 2013a).
The antioxidants are nothing but the natural or synthetic chemical compounds having radical scavenging potential for preventing or slowing down of oxidation of free radicals or reactive oxygen species that occurs via oxidative stress which was triggered during various metabolic processes in a human beings (Nile and Khobragade, 2011, Nile et al., 2013). These antioxidant compounds are imperative for removal and prevention of free radicals or reactive oxygen species before their action to damage the essential cellular components in the human body including cells and tissues throughout biochemical pathways catalyzed by various metabolic enzymes (Nile, Patil, & Park, 2015). During purine metabolism, the initial reaction occurred was oxidation of hypoxanthine to uric acid via xanthine intermediate catalyzed by the xanthine oxidase along with the production of superoxide anions and hydrogen peroxides as byproducts leading to increased oxidative stress in various cells and tissues (Nile & Khobragade, 2011). The excess formation and deposition of uric acid during this purine catabolism in the form of urate monohydrate crystals get deposited in human synovial joints leading to the development of disease called gout (Nile et., 2013). These aspect highlights the need for the discovery of efficient and safe xanthine oxidase and urease inhibitors as important area of research due to the association of urease and xanthine oxidaase with several pathological conditions for the development of various diseases including gout and urolithiasis (Khan et al., 2010). The activities of these enzymes (xanthine oxidase and urease) during the catabolic processes in humans leads to the development and production of several free radicals or reactive oxygen species (superoxide anions and hydrogen peroxides). These free radicals or reactive oxygen species may be significantly involved in numerous pathological and physiological processes, including inflammation, tissue damage, apoptosis, cell death, cancer, atherosclerosis, and aging (Nijveldt et al., 2001, Uddin et al., 2016). The naturally occurring phytochemicals including various phenolics and flavonoids occurred in a great variety of fruits, vegetables, and medicinal plants were found to be promising potent inhibitors against urease and xanthine oxidase (Nile and Park, 2013a, Uddin et al., 2016). However, to the best of our knowledge, there are no specific reports on the utilization of onion solid waste (OSW) and their bioactive compounds against xanthine oxidase and urease in the literature studied previously. We reported the high amount of quercetin and quercetin glycosides in OSW, with significant antioxidant and enzyme inhibitory potential against urease and xanthine oxidase using the relevant data from the literature.
Section snippets
Chemicals
All chemicals and solvents used in this study were of high performance liquid chromatography (HPLC) analytical grades. 2,2-Diphenyl-1-picrylhydrazine (DPPH) was purchased from Sigma-Aldrich Co. (St. Louis MO, USA); 2,4,6-tripyridyl-s-triazine (FeCl 6H2O TPTZ) was sourced from Fluka Chemicals, Switzerland; and bovine milk xanthine oxidase (Grade I), allopurinol, thiourea, xanthine, sodium acetate trihydrate, ferric chloride, trichloroacetic acid, (TCA), 2-thioharbituric acid (TBA), and gallic
Total phenolic contents
Considering the pharmacological and medicinal importance of phenolic compounds for total antioxidant capacity and other biological activities, the amount of total phenolic compounds were estimated by the Folin-Ciocalteu (FC) method for OSW extracts. The FC reagent shows a significant reaction with phenolics as well as with other reducing agents like ascorbic acid; hence, releavant precautions were taken to avoid deflection of the results obtained by this test for the total reducing capacity of
Conclusions
Several studies have been performed to gain knowledge about the chemical composition of onion solid waste (OSW) as a source of dietary fiber, nutritive compounds, mineral elements, fatty acids, and sulfur components extracted from the skin and top-bottom of OSW (Benítez et al., 2011, Roldan et al., 2008). The data obtained from this study using OSW, provides a new baseline for the utilization of OSW and their flavonol glycosides as valuable phytochemicals for pharmaceutical, food, and cosmetic
Acknowledgements
This research was supported by KU research professor program-2017 of Konkuk University, Seoul, South Korea.
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