Inhibition of rice bran lipase by azadirachtin from Azadirachta indica

Abstract

Background: Chemical constituents of the neem tree are extensively explored; the major component present in the tree is azadirachtin. Azadirachtin, a tetranortriterpenoid, is remarkable for its chemical complexity and for its biological activity. This study, for the first time, has provided information on the interaction and inhibition of rice bran lipase (RBL) by azadirachtin. RBL, an esterase, is a versatile enzyme that catalyzes the hydrolysis of ester linkages, primarily in neutral lipids such as triglycerides. Results: The catalytic activity of RBL in the presence of azadirachtin has been investigated. There was a decrease in enzyme activity relative to the control at all concentrations tested. At 1 mmol L^-1 concentration of azadirachtin the enzyme loses 84% of its activity. The kinetics of inhibition of RBL by azadirachtin are competitive in nature. From analysis of the kinetic data the inhibition constant K_i is found to be 0.14 mmol. Fluorescence measurements showed a gradual decrease in emission intensity; a red shift in emission maximum may be attributed to perturbation of the aromatic residues. Far-ultraviolet circular dichroism results showed minor conformational changes. Conclusion: The results suggest that azadirachtin specifically inhibits and binds to RBL in bringing about inhibition with minor structural alteration. This inactivation of RBL by azadirachtin can be utilized to prevent oxidation of RBL with an ultimate goal of exploiting the potential of the plant source for producing edible-grade oil from rice bran using a natural molecule at very low concentration.