Inhibitory kinetics ofβ-N-acetyl-D-glucosaminidase from prawn (Litopenaeus vannamei) by zinc ion

Toxicity of chronic waterborne zinc exposure in the hepatopancreas of white shrimp Litopenaeus vannamei

Zinc (Zn) is necessary for the survival of aquatic organisms; nevertheless, the accumulation of Zn in excessive amounts may have toxic consequences. Few studies focusing on the biochemical, morphological, and transcriptional effects of aqueous Zn in Litopenaeus vannamei have been reported, and the underlying toxic mechanism remains largely unknown. The present study was performed to investigate the growth performance, morphological alterations, physiological changes, and transcriptional responses after Zn exposure at 0 (control), 0.01, 0.1, and 1 mg/L concentrations for 30 days in white shrimp L. vannamei hepatopancreas. The results found that survival rate (SR) and growth performance were significantly reduced in 1 mg/L Zn group. Significant structural damage and significant Zn accumulation in hepatopancreas were observed. The activities of trypsin and amylase (AMS), and the total antioxidant capacity (T-AOC) were attenuated, while the production of reactive oxygen species (ROS) and malondialdehyde (MDA) content were significantly increased after Zn exposure. Many differentially expressed genes (DEGs) were obtained after Zn exposure, and the majority of these DEGs were downregulated. Ten DEGs involved in oxidative stress, immunological response, apoptosis, and other processes were selected for qRT-PCR validation and the expression profiles of these DEGs kept well consistent with the transcriptome data, which confirmed the accuracy and reliability of the transcriptome results. Subsequently, we screened 12 genes to examine the changes of expression in different concentrations in more detail. All the results implying that Zn exposure caused severe histopathological changes and increased Zn accumulation in hepatopancreas, altered immune, antioxidant and detoxifying response by regulating the gene expressions of related genes, and eventually might trigger apoptosis. These findings provide valuable information and a new perspective on the molecular toxicity of crustaceans in response to environmental heavy metal exposure.

Cadmium inhibits molting of the freshwater crab Sinopotamon henanense by reducing the hemolymph ecdysteroid content and the activities of chitinase and N-acetyl-β-glucosaminidase in the epidermis

Molting is an essential process during the growth of crustaceans, which is coordinated by ecdysteroids secreted by the Y-organ, molting inhibiting hormone secreted by the X-organ sinus-gland complex, as well as chitinase and N-acetyl-β-glucosaminidase synthesized by the epidermis. Cadmium is one of the toxic metals in the aquatic environment. However, the endocrine effects of cadmium on the molting of freshwater crabs and the underlying mechanisms are unknown. To investigate these, freshwater crabs (Sinopotamon henanense) were acutely exposed to 0, 7.25, 14.5 and 29 mg/l Cd for 3, 4, 5 days or in some experiments for 4 days after eyestalk-ablation. The concentration of hemolymph ecdysone and the activities of the molting enzymes chitinase and NAG were measured. Histological changes in the epidermal tissues were documented. Our results showed that eyestalk ablation increased the ecdysteroid content as well as the activities of chitinase and NAG, which were inhibited by cadmium in a concentration-dependent manner; histological examinations demonstrated that eyestalk ablation produced storage particles in the epidermal tissues, which was also reduced by cadmium in a concentration-dependent manner. Our data suggest that cadmium disrupts endocrine function through inhibiting the secretion of ecdysteroids by the Y-organ and altering with the regulation of chitinase and NAG activity in the epidermis. This work provides new insights into the mechanisms underlying the molting inhibition effect of cadmium on the crabs.

Irreversible inhibitory kinetics of mercuric ion on N-acetyl-β-D-glucosaminidase from Nile tilapia (Oreochromis niloticus)

N-acetyl-β-D-glucosaminidase (EC 3.2.1.52, NAGase), hydrolyzes dimers or trimers of N-acetyl-β-D-glucosamine (NAG) into monomers and is shown to be important for the reproduction of male animals. NAGase is purified from the spermary of Nile tilapia, and its enzyme activity can be strongly inhibited by mercuric chloride (HgCl2). In this paper, we determined the kinetics of HgCl2-mediated inhibition of NAGase, and our results showed that it was irreversible inhibition with an IC50 value at 2.70±0.02 μM. Moreover, Hg(2+) reduced the thermal and pH stability of the enzyme. We determined the inhibition kinetics of Hg(2+) by using the kinetic method of substrate reaction. With this inhibition model, the microscopic rate constants for the reaction of Hg(2+) with free enzyme (k1) and the enzyme-substrate complex ( [Formula: see text] ) were determined to be 4.42×10(-4) mM(-1) s(-1) and 7.06×10(-5) mM(-1) s(-1), respectively, indicating that the presence of substrate can protect NAGase from Hg(2+) inhibition.

Inhibitory kinetics of betaine on beta-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 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.