Traditional uses, phytochemistry and pharmacology of allium cepa L

Introduction

Garlic-scented bulbous herbs or onion is allium belonging to the family Amaryllidaceae. Various allium vegetables are consumed mainly as spices for their characteristic flavors and their health-improving effects, like leek (Allium ampeloprasum var. porrum) onion (Allium cepa), and garlic (Allium sativum).^[1] Harvest yield of onion (Allium cepa L.), was 93.1 million tons in 2016 and is among the most widely consumed vegetables.^[2] The word “onion” is originated from the Latin word ‘unio’ which means ‘single’ or ‘one’ as it yields a single bulb^[3] It is a vegetable, also called the bulb onion or common onion that is cultivated for greater than 7000 years. The bulb shape is like a flattened globe or a globe, sometimes spindle-like or mostly cylindrical with a flat top.^[4]

Different refines include onion juice concentrate, onion essential oil, dehydrated onion powder, and; are beneficial in flavoring and seasoning of foodstuffs. ^[5] Since ancient times, globally onion is the biggest economic graze and is used in medicine and food.^[6], ^[7], ^[8], ^[9] Geographical sources of onion are India the biggest cultivator, Turkey, China, Japan, USA Spain, Egypt, Poland and Brazil^[10] and for Egypt, the yield is 2.4 million tons.^[11] The plant onion is currently produced in 139 countries and has been cultivated for more than 4000 years.^[12] After Americans (16 kg), Libyans utilize the maximum quantity of onion, for its worldwide consumption, and yield for a mean of 30 kg annually/capita.^[13] Around, 13000 accessions of onion are kept in gene library globally with large gene variability in their physicochemical and morphological bulb traits such as weight, shape, scale color, flesh, dry matter content (DMC), firmness and pungency.^[14] Onion cultivars are of three types: dehydrator types (>18% DMC), storage types (8 to 12% DMC) and fresh market types (5 to 9% DMC).^[15] A characteristic odor of Allium species is because to the liberation of volatile organosulfur compounds on tissue destruction. ^[16] A species-specific pattern of S-alk(en)yl-cysteine sulfoxides (ACSOs) determines the aroma type of various Allium species.^[17] The main predominant aromatic compound in onion is Isoalliin, along with slight amounts of propiin and methiin and traces of alliin. ^[18]

Pharmacological Activities of A cepa

Allium cepa is used as a potent medicinal agent and possesses antiviral, antibacterial, antiparasitic, antihypertensive, hypoglycemic, antifungal, antihyperlipidemic, and antioxidant, anti-inflammatory antiplatelet activity.

Antiplatelet activity

According to recent studies, the onion's pungency is linked to antiplatelet activity. ^[71] Sulfur compounds, chromium, and vitamin B6 all contribute to this. Platelet aggregation is also linked with atherosclerosis, cardiovascular disease, heart attack, and stroke.^[72]

Antidiabetic activity

Onion is used in the cure of diabetes and its associated complications.^[73], ^[74] Onion soup is used to cure diabetes mellitus type 2 and other lifestyle diseases.^[75] Onions have been reported to possess anti-diabetic properties because of the presence of allyl propyl disulfide, which raises free levels of insulin.^[76] In alloxan diabetic rabbits allyl propyl disulfide increased glucose tolerance. Onion contains S methyl cysteine sulfoxide which is responsible for anti-diabetic activity. ^[77] Chromium improves glucose tolerance by decreasing insulin levels, fasting glucose levels, and lowering blood cholesterol and triglycerides (TG) levels.^[78] Quercetin in allium cepa facilitates glucose-mediated secretion of insulin by assisting phosphorylation of extracellular signal-regulated kinase and protecting pancreatic beta-cells and ultimately controls sugar-induced secretion of insulin, elevating the specific activity of several pancreatic islets and liver glucokinase (a carbohydrate enzyme) that facilitates improved glucose tolerance.^[79]

Anticancer Activity

The flavonoid quercetin contained in allium has been shown to have anticancer properties. It can stop cancer cells from growing. Allium cepa L. inhibits cancer cell proliferation and adipocyte proliferation by inhibiting the enzyme fatty acid synthase.^[73] Ethanolic extract of this plant is beneficial for the treatment of breast tumors. ^[80] A flavonoid, Apigenin is found in onions, has shown to possess anticancer properties. ^[81] Sulfur compounds found in onions have been shown in studies to protect cells from mutation and induce apoptosis, preventing tumor growth and cancer production. In vitro studies show that onion extract inhibits mutagenesis/carcinogenesis, ^[82] modulates enzyme and cell signaling pathways, ^[83] scavenges free radicals, ^[73], ^[84], ^[85] induces apoptosis, and has immunomodulatory^[86], ^[87] and aaffects cell differentiation and cancer development. ^[88] The water leek extract of concentration (50 μg/Ml) was the most important anticancer effectwith a viability percentage of 47.16 ± 14.71 after incubation for 3 days. As it contains entadamide A-β-D-glucopyranoside it induces inhibition of MCF-7 cells. These findings predict the leek's potential being a regulator of food sources that can help increase antineoplastic activity. ^[35]

Antimicrobial activity

Red and white varieties of Allium cepa have shown strong antimicrobial and antioxidant activities. Fresh cold water extract of onion has shown antibacterial activity against four isolates include Staphylococcus aureus, E.coli, Streptococcus pneumoniae and Streptococcus pyrogenes and has antimicrobial activity against Vibrio cholerae Pseudomonas aeruginosa, S. pneumoniae, E. coli, Staph. Aureus, and Staph. Aureus Salmonella typhi and E. coli. ^[89] The biochemical analysis revealed that the three onion varieties are rich in therapeutic compounds. Onion extract possesses antimicrobial activity which revealed that they were effective against three Gram-positive bacteria (S.aureus, B.cereus and L. innocua), as well as P. aeruginosa; as a result, they may be a novel source of biological antimicrobial agents. ^[90]

Antioxidant Activity

Onions are high in quercetin, a strong antioxidant. Quercetin has been shown in a variety of studies to be a free radical scavenger and an inhibitor of LDL (low-density lipoprotein) oxidation. The red and white varieties of Allium cepa demonstrated antioxidant activity. ^[91] Allium cepa's quercetin-3'-O-beta-D glycoside exhibits antioxidant properties. ^[22] In aged rats, peel and flesh of allium cepa improve antioxidant level.^[92] In alloxan diabetic rats, SMCS obtained from onion had anti-diabetic and antioxidant effects when compared to standard drugs. ^[93] Flavonols in the diet defend diabetic human lymphocytes from oxidative DNA damage. ^[94]

Hepatoprotective activity

Onion extracts were found to protect rats' livers from cadmium-induced oxidative damage ^[95]. Onion extract is preferred for dose-dependent hepatoprotection, as well as preventing and protecting against cadmium-induced hepatotoxicity. Hepatoprotective action of an aqueous extract of Allium cepa bulb was found against ethanol-induced hepatotoxicity. ^[96]

Anti-inflammatory activity

Due to the flavonoid quercetin, onions have anti-inflammatory properties. Onions have anti-asthmatic properties^[53] and are used to treat arthritis. ^[97] Ajoene is an anti-inflammatory natural product isolated from the Allium genus is known to possess anti-inflammatory activity. ^[98] The mechanism for the attenuation of atherosclerosis by fistula onion stalk extract involves the modulation of the local inflammatory response. ^[99]

Immunomodulatory effects

Various studies have also shown A. cepa constituents in cultured spleen cells stimulated with pokeweed (PWM) from Blomia tropicalis-sensitized BALB/c mice, quercetin (3.5, 7.5, 15 lg/mL) impeded the development of Th2 cytokines such as interleukin-4 (IL-4), IL-5, IL-13, and IgE. ^[100] The reviewed in vitro and in vivo studies reported that A. cepa and its constituents have modulatory effects on the immune system in a variety of immune dysregulatory disorders. The plant and its components, especially quercetin, decreased Th2 cytokines such as IL-4, IL-5, and IL-13, as well as IL-6, IL-8, IL-10, IL-1b, and TNF-a and IgE levels, while increasing CD4 cells, IFN-c levels, and the IFN-c/IL4 ratio (Th1/Th2 balance) in inflammatory disorders like asthma and breast cancer. However, in inflammatory conditions including LPS-induced osteoclastogenesis in RAW264.7 cells, A. cepa and quercetin decreased IL-6 and IL-1a development while increasing IL-3 and IL-4 levels. ^[101]

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