The study investigates antioxidant activity of aqueous and ethanolic extracts of Citrus paradisi using different in vitro models such as scavenging of 1, 1-diphenyl-2-picrylhydrazyl (DPPH), Superoxide (ROS) radicals and inhibition of microsomal lipid peroxidation, reducing power and metal chelating properties. Inhibition of microsomal lipid peroxidation and superoxide scavenging by aqueous and ethanolic extracts showed an IC₅₀ of 1.6-1.7 mg/mL and 1.7-1.9 mg/mL respectively. Metal ion chelation showed an IC₅₀ of 1.7 mg/mL of both the extracts and was equipotent to positive control, Ethylenediaminetetraacetic acid (EDTA). The aqueous extract of Citrus paradisi exhibited higher antioxidant activity in DPPH radical and reducing power than that of ethanolic extract. Results obtained suggest that Citrus paradisi have intrinsic antioxidant activity to scavenge various radicals, viz., superoxide and peroxyl radicals, and ameliorate the level of oxidative stress.

The role of free radicals and reactive oxygen species (ROS) attracted increasing attention over the past decade (Gordon, 1990). It was suggested that a chronic imbalance between formation of ROS and antioxidant defenses characterizes many disease states, such as atherosclerosis and premature aging (Lai et al., 2001). ROS can be generated in various ways. Polymorphonuclear leukocytes, macrophages, and peroxisomes cells are the main endogenous sources for free radicals, which are involved in aerobic respiration. Exogenous sources of ROS include tobacco smoke, certain industrial pollutants, organic solvents, and pesticides (Halliwell and Gutteridge, 1989; Davies, 1994; and Robinson et al., 1997). ROS leads to oxidative stress that alters the structure of the biomolecules like nucleic acids, lipids carbohydrates and proteins. ROS generates rancidity in foods leading to deterioration (Miller et al., 1995; and Sasaki, et al., 1996).

Superoxide O₂ is an oxygen-centered radical which is produced enzymatically by autooxidation reactions or by nonenzymatic electron transfer system. This damages the cell membrane by oxidizing fatty acid chain. Under normal physiological conditions, ROS are continuously produced and scavenged by antioxidant defense mechanisms (Halliwell et al., 1992). There is a balance between the generation of ROS and inactivation of ROS by the antioxidant system in organisms. Under abnormal conditions, the ROS are generated continuously which leads to oxidative stress and lipid peroxidation. Cellular modification in membranes or intercellular molecules is caused by the imbalance between ROS and antioxidant defense system (El-Habit et al., 2000). Several molecules isolated from plant sources are identified as free radicals or superoxide scavengers (Yen and Duh, 1994; and Duh, 1998). Recently, much focus is given to the naturally occurring antioxidants basically derived from the plant origin and to be used as food supplements or nutraceuticals to replace the synthetic antioxidants to avoid their side effects such as carcinogenicity (Ito et al., 1983; and Zheng and Wang, 2001). There are several reports that antioxidants protect the human body against chronic diseases by scavenging the free radicals that are generated (Pryor, 1991; Kinsella et al., 1993; and Lai et al., 2001). Therefore, there is a great deal of interest towards the antioxidant molecules from plant sources.

Biotechnology Journal, Antioxidant, Citrus Paradisi, Superoxide Scavenger, Reducing Power, Lipid Peroxidation, Reactive Oxygen Species, ROS, Trichloroacetic Acid, TCA, Superoxide Radicals, Nitro Blue Tetrazolium chloride, NBT, Phenazine Methosulphate , PMS, Nicotinamide Adenine Dinucleotide-Reduced, NADH.