The beneficial effects of exogenous protease K originated from Parengyodontium album on growth performance of grass carp (Ctenopharyngodon idella) in relation to the enhanced intestinal digestion and absorption capacities

The study was primarily designed to analyze the effect of exogenous protease K on growth performance, amino acid metabolism, digestion and absorption capacities, expression of amino acid transporters related genes and proteins in intestine of sub-adult grass carp (Ctenopharyngodon idella). Six experimental diets with protease K (0, 3000, 6000, 9000, 12,000 and 15,000 U/kg) were fed to 450 grass carp (619.51 ± 0.54 g) for 60 days. The results indicated that the highest value of final body weight, percent weight gain (PWG), specific growth rate, protein efficiency ratio, activity of hepatopancreas glutamate-oxaloacetate transaminase and glutamate-pyruvate transaminase, and the lowest concentration of plasma ammonia were observed in groups with protease K of 6000 and 9000 U/kg. Meanwhile, the groups with 6000 and 9000 U/kg protease K exhibited the highest activities of digestive and brush border membrane enzymes in the three intestinal segments, as well as the highest value of intestinal somatic index and intestinal length index. The highest concentration of free amino acid in the serum and the lowest in intestine were observed in groups with supplementation of protease K 6000 and 9000 U/kg, which might be related to the up-regulated mRNA level and protein expression of intestinal amino acid transporters (SLC1A5, SLC7A6, SLC1A3 et al) in these groups. Furthermore, optimal levels of protease K up-regulated the mRNA levels of target of rapamycin (TOR), S6 kinase 1 (S6K1), and protein expression of TOR and phosphorylation TORSer2448, but reduced the mRNA level of 4E binding protein 1 (4EBP1). In summary, optimal levels of exogenous protease K was found to be conducive to facilitate the improvement of growth performance probably via enhancing digestion and absorption capacity, and utilization of amino acid in sub-adult grass carp. In addition, using quadratic regression analysis of PWG and alkaline phosphatase in the midgut, the optimum addition of protease K for the sub-adult grass carp were noted to be 7380.82 and 7406.44 U/kg, respectively.