De-oiled rice bran (DORB) is a potentially useful by-product of the rice bran oil industry. DORB may prove to be an important protein source, and also contains many other micronutrients. This study has the principal aim of optimizing the process of DORB protein hydrolysate preparation, and then testing the hydrolysate to determine its lipase inhibitory activity. DORB underwent hydrolysis using Alcalase® and response surface methodology (RSM). The resulting degree of hydrolysis (DH) was then monitored along with the extent of any lipase inhibitory activity. The optimum levels of lipase inhibition were obtained at a temperature of 49.88 °C, a duration of 150.43 minutes, and 1.53% Alcalase® used for the sample 5% (w/v) solution. In these conditions, the DH value was 35.65%, and the IC₅₀ value for lipase inhibitory activity was 2.84 μg mL^-1. Five ranges of different molecular weights were obtained via fractionation, whereupon it was determined that the highest level of inhibitory activity was achieved by the <0.65 kDa fraction. This fraction was then further purified via RP-HPLC, and the resulting peak had a retention time of 21.75 minutes (F ₂ sub-fraction) and exhibited high lipase inhibitory activity. Mass spectrometry was used to determine the amino acid sequence for this peak, identified as FYLGYCDY. This particular peptide is categorized as bitter, with a non-toxic profile, and having poor water solubility. The synthesized form of this peptide showed lipase inhibitory activity measured by an IC₅₀ value of 0.47 ± 0.02 μM. The Lineweaver-Burk plot revealed that FYLGYCDY is a non-competitive inhibitor, while analysis of the docking results provided details of the FYLGYCDY peptide binding site with the porcine pancreatic lipase (PPL) complex, which is a competitive type. It can be inferred from these findings that DORB may prove a useful raw material source for the production of anti-obesity peptides which might enhance the therapeutic and commercial performance of functional foods and healthcare products.

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