Investigating the antidiabetic and antioxidative potential of peels from selected South African fruits

Abstract

Background There are many complications of diabetes. Oxidative stress has been reported as a major underlying mediator. Medicinal plants, including fruits and vegetables are known to contain dietary antioxidant phytoconstituents, which influences their pharmacological potentials, including mitigating the progression of degenerative diseases, such as diabetes and associated complications. Fruits like avocado (Persea americana Mill) and litchi (Litchi chinensis Sonn.) are widely cultivated for their commercial value, nutritional value and health benefits. The antioxidant and glycaemic control potentials of these fruits have been documented, which has been linked to some bioactive phenolic and flavonoid compounds. Despite being medicinally underutilized, the wastes, including the peel from these fruits have also been shown to contain these bioactive phenolic and flavonoid compounds, which are key in potentiating the documented oxidative stress and diabetes related pharmacological potentials of the peel from these fruits. However, antioxidant and glycaemic control assay-guided assay studies to better understand the antioxidant and glycaemic control phytochemistry of the peel from these fruits are scarce, which discourages their effective therapeutic applications. Therefore, this study was undertaken to investigate the antidiabetic and antioxidative phytochemistry of the peel of avocado and litchi fruit using a bioassay-guided approach. Methodology Litchi (McLean’s Red cultivar) and avocado (Var. Fuerte) fruits were purchased from a local fruit and vegetable store in Bloemfontein, South Africa. The peel of the fruits was dried, pulverized, defatted using hexane and sequentially extracted with dichloromethane, ethyl acetate, methanol, and water, in order of increasing polarity. In vitro, phytochemical models, namely antioxidant (Fe3+ reducing, radical scavenging, and anti-linoleic acid peroxidative) and glycaemic control (α-glucosidase and α-amylase inhibitory and glucose uptake modulatory), were employed for the bioassay-guided isolation, while the potent fractions or isolated compound(s) was characterized using mass spectrometry and mass spectrometry and assessed for dose-dependent inhibition of α-glucosidase and lipopolysaccharide (LPS)-induced cellular ROS production, as well as modulation of cellular glucose uptake. The radical scavenging mechanism of principal compound(s) was investigated using Density Functional Theory (DFT). Also, the ameliorative potential and possible underlying mechanisms of litchi peel ethyl acetate extract on type 2 diabetes related pathologies in a fructose/streptozotocin (STZ) model of diabetic rats was investigated. Results and discussion The methanol extract of avocado peel had the highest phenol content and radical scavenging activity. Fractionation of this extract yield 9 fractions (fractions 2 to 10). Fraction 8 (eluted with hexane:chloroform:methanol volume ratio of 3:6.5:0.5, respectively) had high phenolic content, demonstrated the most potent radical scavenging and showed the predominant presence of Chlorogenic acid (103.5 mg/mL) and 1-O-caffeoylquinic acid (102.3 mg/mL). Fraction 8 and chlorogenic acid dose-dependently inhibited in vitro (IC50 = 5.73 and 6.17 μg/mL) and cellular (IC50 = 15.9 and 9.34 μg/mL) FeSO4-induced lipid peroxidation, as well as LPS-induced ROS (IC50 = 39.6 and 28.2 μg/mL) and NO (IC50 = 63.5 and 107 μg/mL) production, while modulating antioxidant enzyme activity. DFT analysis suggest that an electron transfer, followed by proton transfer at carbons 3′OH and 4′OH positions may be the radical scavenging mechanism of chlorogenic acid. For litchi peel extracts, the ethyl acetate extract had the highest flavonoid contents, which perhaps influenced its potent anti-lipid peroxidative (65.0%) and α-glucosidase inhibitory (52.4%) effects. LC-MS showed the predominant presence of bioactive flavonoids, including epicatechin, procyanidin B5 and proanthocyanidin A2. Using a bioassay-guided approach, epicatechin was isolated from a potent fraction (DCM:MeOH:water; 7:3:0.281 v/v/v solvent system) of the ethyl acetate extract, since the fraction (fraction 5) and its sub-fraction (sub-fraction 4) had appreciable flavonoid contents, radical scavenging, anti-lipid peroxidative, α-glucosidase inhibitory and glucose uptake modulatory effects. Moreover, the isolated epicatechin inhibited α-glucosidase (IC50 = 35.3 μM), modulated cellular glucose uptake (EC50 = 78.5 μM) and inhibited LPS-induced ROS production in RAW 264.7 macrophages in a dose-dependent fashion [IC50 = 18.9 μM; statistically comparable (p ˃ 0.05) to ascorbic acid, IC50 = 9.57 μM]. In a fructose/STZ model of diabetic rats, the ethyl acetate extract exerted antioxidant and glycaemic control effects by improving pancreatic histology and antioxidant status, improving insulin secretion, promoting glycogen storage and modulating cellular glucose uptake and utilization. Conclusion Considering this study is bioassay-guided, it is logical to conclude that chlorogenic acid strongly influences the antioxidant capacity of avocado fruit peel, while flavonoids including catechins, proanthocyanidins and procyanidins are some of the predominant constituents influencing the glycaemic control and antioxidant properties of litchi peel.