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Purpose: To study the effect of Ocimum sanctum (OS) on selenite-induced morphological and biochemical changes in isolated rat lenses as well as on cataract incidence in rat pups. Methods: Transparent rat lenses were divided into normal, selenite-only, and four treated groups. Selenite-only and treated group lenses were subjected to oxidative stress in vitro by incorporating sodium selenite (100 µ M) in the culture medium. The effect of OS (70, 140, 280, and 560 µ g/ml) was studied on the levels of reduced glutathione (GSH) and thiobarbituric acid reacting substances (TBARS) in selenite-challenged lenses. The lowest concentration of OS offering significant modulation on these two parameters was determined. Subsequently, the effect of prior and cotreatment with the lowest effective concentration of OS was studied on TBARS, GSH, and on lens antioxidant enzymes such as superoxide dismutase (SOD), glutathione peroxidase (GSHPx), catalase (CAT), and glutathione-S-transferase (GST). Changes in lens protein profiles under different incubation conditions were analyzed by SDS gel-electrophoresis. In vivo , cataract was induced by a single subcutaneous injection of sodium selenite (25 µ mole/kg b.w.) to 9-day-old rat pups. The anticataract effect of OS (5 and 10 mg/kg b.w.) injected intraperitoneally 4 hr prior to selenite challenge was evaluated by the presence of lens nuclear opacity in rat pups on the 16th postnatal day. Insolubilization of lens proteins post–selenite injection was monitored for 4 days. Results: The lenses in the selenite-only group developed cortical opacities in 24 hr. OS showed different degrees of positive modulation in selenite-induced morphological as well as biochemical changes. The lowest effective dose of OS that significantly modulated glutathione and thiobarbituric acid reacting substances was found to be 140 µ g/ml. At this dose, a significant increase in antioxidant enzyme levels and preservation of normal lens protein profile was observed. OS at the dose of 70 µ g/ml did not show any significant protection with respect to either morphology or biochemistry of lenses. In vivo , 5 and 10 mg/kg of OS reduced the incidence of selenite cataract by 20% and 60%, respectively, and prevented protein insolubilization as well. Conclusions : Aqueous extract of OS possesses potential anticataract activity against selenite-induced experimental cataractogenesis. The protective effect was supported by restoration of the antioxidant defense system and inhibition of protein insolubilization of rat lenses as well. [ABSTRACT FROM AUTHOR]