Draba cemileae (Karaer): Phytochemical composition, antioxidant and enzyme inhibitory activity

Highlights

• Phytochemistry and biological activity of D. cemileae was studied.

• Leaf and seed extracts were potent antioxidant agents.

• Leaf and seed extracts exhibited enzyme inhibitory activity.

Abstract

Plant polyphenols have always attracted the attention of researchers with their excellent biological activity potentials. In this study, roots, leaves and seeds of Draba cemileae (Karaer) were evaluated for their chemical compositions and biological activities. Spectrophotometric analysis showed that phenolics and flavonoids were present in high quantities in leaf and seed extracts (22.84 mg GAEs/g and 23.32 mg REs/g, respectively). Chromatographic analysis showed that the p-hydroxybenzoic acid and chlorogenic acid contents of the extracts were significantly high. The seed extract also contained 550 mg/g of rosmarinic acid. While the leaf extract showed high activity in phosphomolybdenum, CUPRAC, FRAP and ferrous ion chelating activity tests (0.88, 2.23, 1.73 and 3.68 mg/ml, respectively), DPPH and ABTS scavenging activity tests resulted in the superiority of the seed extract (3.75 and 2.53 mg/ml, respectively). The leaf extract also showed the highest activity in α-amylase and tyrosinase inhibitory activity tests (1.78 and 4.99 mg/ml, respectively). On the other hand, the seed extract exhibited higher activity than the others in α-glucosidase, AChE and BChE inhibitory activity tests (7.14, 1.13 and 5.11 mg/ml, respectively). Correlation coefficients between the composition and the biological activities were over 0.9. It was concluded that D. cemileae could be a new and effective source of antioxidant and enzyme inhibitory phytochemicals in medicine, food and cosmetics industries.

Introduction

Plant polyphenols are compounds containing hydroxyl groups attached to aromatic rings in their structure (Zhou et al., 2019). It is estimated that the number of plant polyphenols identified so far, including those that can be consumed, is several thousand (Stagos, 2020). Plants have been the richest source of exogenous antioxidant compounds for many years. Authorities claim that almost two-thirds of the plant species distributed on the world have various biological/pharmacological activities and almost all of them contain strong antioxidant compounds (Krishnaiah et al., 2011). After the discovery of ascorbic acid, the first exogenous antioxidant compound, plants were relied upon for the treatment of many diseases caused by increased oxidative stress (Boo, 2019; Burgos-Morón et al., 2019; Pawlowska et al., 2019). Many researchers agree that polyphenols can be used as antioxidants in the treatment of diseases caused by oxidative stress (Jin et al., 2018). Therefore, plant-derived antioxidants are a promising source of reference to combat the problems caused by oxidative stress (Jin et al., 2018; Kasote et al., 2015).

Diabetes is a chronic disease that occurs due to inadequate carbohydrate metabolism or impairment of cell surface receptors. The high level of glucose in the blood causes many health problems (Mathers and Loncar, 2006; Shaw et al., 2010). The enzymes responsible for metabolizing polysaccharides or disaccharides in the body are α-amylase and α-glucosidase. Inhibiting these enzymes to slow down carbohydrate metabolism is one of the important strategies in the treatment of diabetes (Azad et al., 2017; Kim et al., 2000; Ramasubbu et al., 1996; Zhen et al., 2017). Today, some synthetic enzyme inhibitors are used to reduce the blood glucose level. However, these substances cause some side effects such as edema, hypoglycemia, excessive weight gain, anemia, gastrointestinal disorders and lactic acidosis (Magaji et al., 2020). Therefore, researchers also focused on plants to discover new and alternative α-amylase and α-glucosidase inhibitors to treat diabetes.

In addition to the therapeutic properties mentioned above, phytochemicals also exhibit promising activities for the treatment of disorders in the cholinergic system. It has been determined that the metabolic activity of cholinesterases (ChEs) is closely related to the pathology of various neurological diseases, especially Alzheimer's disease (AD). Especially in AD, high ChE activity contributes to the increase of amyloid plaque aggregation as well as slowing down neural conduction. Symptoms such as memory loss, irreversible neurological tissue degeneration, inability to perform daily vital activities are frequently seen in AD (Choubdar et al., 2019; Vickers, 2017). Today, the most effective approach in the treatment of AD is the use of cholinesterase inhibitors (donepezil, galantamine, rivastigmine) (Ahmad et al., 2019; Yiannopoulou and Papageorgiou, 2020). Despite being clinically effective, these compounds have been reported to have common side effects such as diarrhea, nausea, and vomiting. Therefore, researchers have focused on the development or isolation of new phytochemicals (Cheenpracha et al., 2016; García et al., 2015; Kiełczewska et al., 2021; Liu et al., 2017; Mohebbi et al., 2018; Richmond et al., 2013).

Tyrosinase is a critical enzyme that converts monophenols to diphenols and then to melanin through o-quinone oxidation (Tian et al., 2019; Yu et al., 2019). Melanin is a pigment found in the skin of organisms that acts as a filter against UV rays. However, if it is synthesized excessively, freckles and age spots occur on the skin. Overexpression of melanin synthesis has also been reported to be associated with melanoma (Chang, 2009). Polyphenol oxidases, which are structurally similar to tyrosinase, cause browning in fruits and vegetables. Browning leads to deterioration of quality and taste in these foods (Brotzman et al., 2019; Chang, 2009). Inhibition of tyrosinase is one of the most rational solutions in the treatment of skin diseases due to abnormal melanin synthesis in medicine, in the preparation of skin whitening preparations in the cosmetics industry and in the prevention of browning in the food industry. Today, there are some synthetic substances used as tyrosinase inhibitors. However, natural tyrosinase inhibitors are more preferred on the grounds that their biocompatibility capacity is higher and they are sustainable. For this reason, researchers are investigating plant species for the discovery of new and effective tyrosinase inhibitors (Wang et al., 2020; Zolghadri et al., 2019).

In this study, roots, leaves and seeds of Draba cemileae (Karaer) were evaluated for their chemical compositions and biological activities. In addition to spectrophotometric and chromatographic analysis, total antioxidant activities of the extracts based on their chlorogenic acid equivalents (CAEs) of total phenolic and flavonoid contents were determined by performing square wave stripping voltammetry (SWSV) on multi-walled carbon nanotube paste electrode (MWCNTPE).