**1. Introduction**

Metabolic activity in the human body produces several reactive chemical species which are known as free radicals. Though some of these free radicals like superoxide (O<sup>2</sup>−), hydroxyl radical (OH), nitric oxide radical (NO), and hydrogen peroxide (H2O2) play beneficial roles in the maintenance of homeostasis, their presence in excessive amounts is often considered detrimental to the body system [1,2]. High levels of free radicals in the body are associated with increased risk of developing metabolic disorders like diabetes mellitus, and cardiovascular diseases [3–5]. Free radicals are frequently involved in the activation of signal transduction systems such as mitogen-activated protein kinases-dependent pathways which interfere with different gene expressions, homeostasis, and the development of diseases [5]. Similarly, increased activity of xanthine oxidase has been linked to the

development of oxidative stress and metabolic diseases. There is significant correlation between increased xanthine oxidase activity and some metabolic syndrome markers viz. superoxide dismutase, abdominal circumference, high-sensitivity C-reactive protein content, and body mass indices [6]. Increased xanthine oxidase activity has also been implicated in the development of obesity which is considered a key factor in the pathogenesis of diabetes and cardiovascular diseases [7].

Plants have the inherent ability to biosynthesize several compounds with antioxidant properties which could be explored in the search for a better antioxidant agen<sup>t</sup> for the enhancement of health and prevention of diseases [8,9]. *Averrhoa bilimbi* (family Oxalidaceae) is an Asian medicinal plant commonly known in the Malay language as "belimbing buluh". *A. bilimbi* has been reported to possess significant antioxidant and antidiabetic properties [10]. Topical administration of *A. bilimbi* crude leaves extract has been reported to impede ultraviolet light-induced oxidative damage in albino mice [11]. Oral administration of the methanolic extract (250 and 500 mg/kg,) has been reported to prevent CCl4-induced hepatic damage in experimental animal model [12]. Crude ethanol and water extracts, as well as the semi-purified butanol fraction of *A. bilimbi* leaves have been reported to display significant (*p* < 0.05) antihyperglycemic activity, i.e., comparable to metformin in streptozotocin-induced diabetic rats [13–15]. However, there is a paucity of information on the phytoconstituents of this plant that could be responsible for its antioxidant properties. Free radicals and xanthine oxidase are believed to be associated with the development of oxidative stress and related metabolic disorders [3–7]. Hence, this study investigates the in vitro antiradical and xanthine oxidase inhibitory effects of *A. bilimbi* leaves. An attempt has also been made to identify the active constituents of *A. bilimbi* leaves through LC-QTOF-MS/MS analysis and the molecular docking approach.

#### **2. Materials and Methods**
