Inhibitory Kinetics of Betaine on β-M-Acetyl-D-giucosaminidase from Prawn (Litopenaeus vannamei)

The effects of betaine on prawn β-N-acetyl-D-glucosaminidase (NAGase) activity for the hydrolysis of p-nitrophenyl-N-acetyl- β-D-glucosaminide (pNP-NAG) have been studied. The results showed that appropriate concentrations of betaine could lead to reversible inhibition against NAGase, and the IC 50 value was estimated to be 15.00 ± 0.30 mM. The inhibitory kinetics assay showed that betaine was a mixed type inhibitor with a K I value of 9.17 ± 0.85 mM and a K IS value of 45.58 ± 2.52 mM. The inhibitory model was set, and the microscopic rate constants were determined using the kinetic method of the substrate reaction. The time course of the hydrolysis of pNP-NAG catalyzed by NAGase in the presence of different betaine concentrations showed that at each betaine concentration, the rate decreased with an increase in time until a straight line was approached, indicating that the inhibition of NAGase by betaine is a slow, reversible reaction with fractional residual activity. The fact that k +0 is much larger than k +0 indicated that the free enzyme molecule is more fragile than the enzyme—substrate complex against betaine. It is suggested that the presence of the substrate offers marked protection of NAGase against inhibition by betaine.