ABSTRACT
The stabilizing effect of some osmolytes including betaine, mannitol, proline, sorbitol, and trehalose (each 0.5 M) was investigated on the ultrasound-irradiated (60 kHz and 138 W, for 240 min) lipase by determination of the enzyme half-life time, evaluation of the enzymatic reaction velocity (Vmax), and hydrolysis of coconut oil for production of lauric acid (the main saturated fatty acid of the oil). The enzyme conformational stability was also assessed by circular dichroism (CD) and fluorescence spectroscopy. The average half-life time of mannitol- and sorbitol-treated lipase under the ultrasound irradiation was 511 ± 3 min and 531 ± 2 min, respectively; 3-fold higher than the unirradiated enzyme. The Vmax value of the ultrasound-treated lipase increased from 100 ± 3 nmol min-1 in the absence of osmolyte to 500 ± 7 nmol min-1 and 500 ± 9 nmol min-1 in the presence of mannitol and sorbitol, respectively. CD and fluorescence spectra indicated that mannitol and sorbitol enhanced the rigidity of the lipase molecular conformational structure, increasing the enzyme stability against the ultrasonic field. The ultrasound-irradiated lipase was then used to hydrolyze coconut oil in the absence or presence of the selected osmolytes, which led to liberate 310 ± 6 mg g-1, 413 ± 7 mg g-1, and 420 ± 4 mg g-1 of lauric acid in the absence or presence of sorbitol and mannitol, respectively. In the absence of an ultrasonic field, the non-osmotically-treated lipase was able to liberate only 211 ± 5 mg g-1 of lauric acid. These promising results indicate that sorbitol and mannitol stabilize the structural conformation of lipase under an ultrasonic field which in turn could improve the enzymatic hydrolysis of coconut oil.