Digestion of dietary protein is an important driving force in animal nutrition, as protein and amino acids are one of the most expensive nutrients. Besides endogenous protease playing critical roles in all metabolic processes in animal body, exogenous protease have two important purposes in animal nutrition: firstly, for food or feed raw materials processing, such as protein hydrolysates, and secondly, direct use as feed additives or for feed manufacture.
Accurate knowledge of the digestibility of amino acids in feed ingredients and the nutritional requirements of animals is important for formulating diets supplemented with protease . Giving animals a diet that meets their nutritional needs is crucial for optimum production performance. However, high levels of protein in the diet may affect intestinal health of animals and the quality of poultry house bedding. Another important issue is that nitrogen excreted in large quantities in animal feces can cause environmental pollution. Adding exogenous protease to feed can be an effective measure to lower protein levels in diets to ensure animals have higher production performance.
Protein digestion in feed is accomplished by the endogenous proteolytic enzyme system in the digestive tract, including gastric proteases, pancreatic proteases, chymotrypsin, elastase, aminopeptidase, and carboxypeptidase. Animals cannot fully digest and absorb protein and amino acids in the diet. The undigested components can serve as fermentation substrates for the microbiota in the hindgut of monogastric animals, but also lead to digestive disorders. Differences between the ileal digestibility and fecal excretion rates of amino acids in broilers suggest that the metabolic role of hindgut microbiota on amino acids may be significant.
Adding exogenous protease enzymes as feed additives can further increase the solubility and hydrolysis of dietary protein, and reduce the substrates needed for poor hindgut fermentation. In vitro studies have investigated the effects of Bacillus subtilis protease pretreatment of soybean meal on soluble crude protein and soybean trypsin inhibitor, showing a significant increase in soluble crude protein and a decrease in soybean trypsin inhibitor under certain conditions.
Generally, animal bodies finely regulate the activity of endogenous proteases because these enzymes, if activated in the wrong place, may digest their own tissues or activate inflammatory pathways. The pancreas can regulate the secretion of its enzymes based on the composition of the diet. After animals adapt to a diet rich in a particular nutrient, the level of this nutrient in the animal's gut increases, followed by a rapid increase in the level of the corresponding enzymes in the pancreas. These adaptive measures are purely intuitive, indicating that the digestion process is finely regulated to avoid excessive production of digestive fluids. Exogenous protease enzymes can compensate for the gap between feed composition and animal digestive enzyme supplementation. Adding exogenous protease enzymes to feed can save energy by reducing the secretion of endogenous enzymes, rather than improving production performance by increasing digestibility.
Further evaluation of the optimal conditions for protease in animal nutrition can help in studying their efficacy in different animal species, different protease enzymes, and translating results into practical feed formulations, proposing an elementary setting. The addition of exogenous protease has positive effects on animal gastrointestinal health, intestinal morphology or additive effects under challenging conditions.
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