Protease, a proteolytic enzyme, plays an important role in fish’s metabolism and digestive system, making it a good tool for formulating diets. Act to accelerate biochemical reactions using alternative metabolic pathways to form new products. The objective was evaluated the effects of the enzyme in the diet on performance, nutrient deposition, GHR and IGF-1 gene expression, and plasma parameters in Nile tilapia (Oreochromis niloticus).
In the experiment, 300 tilapias were used (initial weight 5.69 ± 0.27 g), fed for 49 days. The randomized design with five treatments [0 (control); 194; 316; 390 or 600 mg/kg protease], and four replications. Liquid protease was included in the practice diet (35% CP; 3000 kcal/kg DE) by manual spraying, after the extrusion process. Twenty polypropylene tanks (220 L) and 15 fish per experimental unit were used, arranged in a water recirculation system with water temperature control (25 to 27ºC). The fish received food until apparent satiety at 08:00 am, 1:30 and 6:00 pm. The bromatological composition in initial and final fish samples was analyzed to determine the body deposition of nutrients. Relative weight gain (RWG), protein efficiency rate (PER), protein (PBD) and lipid (LBD) body deposition were calculated. Blood was collected by caudal puncture and centrifuged (1248×g, 4ºC, 10 min) to obtain plasma. The determined plasma biochemical indicators were glucose (mg/dL), amino acids (AA, µmol/dL), total proteins (g/dL), albumin (g/dL), and globulin (g/dL). Growth hormone receptor (GHR) and Insulin-like growth factor-I (IGF-1) were determined in the liver and muscle using real-time PCR for gene expression analysis. The collected data were subjected to the Shapiro-Wilk normality test, analysis of variance (ANOVA) and regression (p < 0.05). The regression showed a significant quadratic effect of protease levels (Table 1 (p < 0.05); Figure 1). There was a significant linear effect on plasma AA concentration (y = 837.94 - 0.5335x; R2 = 0.82). The treatments did not influence the other parameters. According to statistical models, a greater amount of protease in the diet affects growth and digestion functions, resulting in increased nutrient deposition in fish. On the other hand, GHR improved the efficiency in triggering biochemical processes within the cell through the use of circulating AA for growth.
This indicates that exogenous protease improves the growth, protein utilization, and metabolism of Nile tilapia (optimal level = 440 mg/kg.