Inhibitory Kinetics of Betaine on β-N-Acetyl-D-glucosaminidase from Prawn (Litopenaeus vannamei)

The effects of betaine on prawn beta-N-acetyl-D-glucosaminidase (NAGase) activity for the hydrolysis of p-nitrophenyl-N-acetyl- beta-D-glucosaminide (pNP-NAG) have been studied. The results showed that appropriate concentrations of betaine could lead to reversible inhibition against NAGase, and the IC50 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.