Research Progress on digestion and absorption of protein in Ruminants

In animal nutrition, protein in feed usually refers to crude protein (CP), which is defined as nitrogen in feed material multiplied by 6.25.It includes true protein, non-protein nitrogen (NPN) and insoluble nitrogen. True proteins (proteins in biochemical sense) are macromolecules with different size, shape, function, solubility and amino acid composition, which can be classified according to their three-dimensional structure and solubility, such as albumin, globulin, glutenin, gliadin, histone and fibrin (Orten and Neuhaus,1975; Rodwell,1985; Van Soest, 1994). NPN usually refers to the residual nitrogenous substance (Licitra et al.,1996) in the filtrate after the true protein is extracted with tungstic acid or trichloroacetic acid, including low molecular weight compounds such as peptides, free amino acids, nucleic acids, amide, amines and ammonia nitrogen. Common feed true protein and NPN account for only 65% of feed total nitrogen on average (Blethen et al.,1990). The rest is insoluble nitrogen, which includes proteins that bind to intact starch granules in grains, most cell wall binding proteins and some chloroplast proteins, and heat denatured proteins that bind to neutral detergent fiber (NDF) (Van Soest, 1994). 1 digestion of protein 1.1 the degradation of dietary protein in the rumen after the dietary protein entered the rumen of ruminants, it was mixed with endogenous protein (protein contained in animal saliva, exfoliated epithelial cells and rumen microbial residues) for fermentation. Oligopeptides, amino acids and ammonia were released under the action of protein-degrading enzymes of rumen microorganisms (bacteria, protozoa and fungi). Different microorganisms have different functions: ① bacteria participate in the process of feed protein degradation and synthesize bacterial protein, which is the main body of microbial protein. The degradation of amino acids and the production of ammonia exceed the need for bacterial protein synthesis, which will lead to the waste of dietary protein. For many years, it has been thought that deamination is limited to a large number of bacteria that can produce ammonia from proteins and protein hydrolysates. However, the latest research shows that the deamination of amino acids is the result of a large number of bacteria with low deaminase activity and a small number of bacteria with high deaminase activity (Wallace,1996). ② protozoa also have activity in the process of rumen protein degradation and play an important role. Protozoa only engulf particles (bacteria, fungi and small feed particles). Therefore, the effect of protozoa on the degradation of insoluble feed protein (such as soybean meal and fish meal) is stronger than that of soluble protein (Jouany and Ushida,1999). At the same time, this phagocytosis increases the turnover and consumption of nitrogen in the rumen. In addition, protozoa have strong deamination as bacteria, but protozoa can not synthesize amino acids (Nolan,1993) from ammonia, so protozoa are net ammonia-producing microorganisms. Therefore, although protozoa make a great contribution to rumen microbial biomass, their contribution to the amount of protein entering the duodenum is not proportional to their contribution to rumen microbial biomass. ③ due to the low content of fungi in rumen fluid, the role of fungi in rumen protein degradation is generally considered to be negligible. The degree of dietary protein degradation in the rumen depends on the difficulty of fermentation and the retention time in the rumen. The retention time of roughage is relatively long, and there is little roughage protein passing through the rumen without being degraded (P > rskov,1992). There is very little rapid degradation of animal protein (such as fishmeal), so most of the protein is not degraded and passes through the rumen to the true stomach and small intestine. Compared with the original rumen protein, the amino acid composition of the feed fermented by rumen was changed. Ganev (1979) found that the amino acid composition of non-degradable protein in roughage changed greatly compared with the original. Hvelplund and Hesselholt (1987) studied the composition of rumen protein and amino acids in seven kinds of concentrate feeds such as soybean cake. The results showed that the contents of most amino acids were not significantly different from those in the original feed, but the contents of Ile, Leu, Phe, Try, Thr and Val in the residue tended to increase compared with the feed raw materials. Specific to individual amino acids, the degradation of various amino acids in the rumen is different, such as branched chain amino acids (Leu, Ile) can resist rumen microbial degradation (Erasmus et al.,1994a), methionine has different anti-degradation ability in different feeds (Tamminga,1979). 1.2 the synthesis of rumen microbial protein the rumen microorganism not only degrades feed protein, but also synthesizes microbial protein using volatile fatty acids (VFA), energy (ATP) and some oligopeptides, amino acids and ammonia (including endogenous ammonia) produced during feed fermentation. Finally, these microbial proteins, together with rumen proteins (including rumen intact proteins and some small peptides) in feed, flow into the true stomach and small intestine with chyme. The synthesis and decomposition system of this microbial protein in the rumen not only enables ruminants to make extensive use of NPN, but also plays an important role in protein metabolism of ruminants. There are many factors affecting rumen microbial protein synthesis: ① rumen microbial protein synthesis is affected by energy and nitrogen balance and synchronous release. The synchronization of protein and carbohydrate digestion has a great influence on microbial protein synthesis. Experiments in vivo and in vitro show that the digestion rate of carbohydrates is the main factor controlling the energy needed for microbial growth (Hoover and Stokes,1991). The synthesis of rumen microbial protein of ② was also affected by rumen chyme outflow rate. Dietary level, roughage ratio (Owens and Goersch,1986), increased drinking water caused by salt intake (Harrison et al., 1975 / Hadjipanayiotou et al.,1982) and saliva secretion (Froetschel et al.,1989) can indirectly affect the amount of microbial protein entering the duodenum by affecting rumen chyme outflow rate. Different nitrogen sources (peptides, amino acids and ammonia) have different effects on microbial protein synthesis by ③. Russell et al. (1992) pointed out that the fermentation of structural carbohydrates only needs ammonia as a nitrogen source, while microorganisms that degrade non-structural carbohydrates grow faster when amino acids are provided. According to many reports, peptides are preferred by rumen microorganisms, followed by amino acids (Cooper and Ling, 1985, et al.,1979). But in fact, different strains have different priorities for the utilization of peptides and amino acids (Ling and Armstead,1995). ④ branched chain amino acids and ectopic organic acids can also regulate the growth of microorganisms. The amino acid composition of rumen microbial protein has been considered to be constant by most researchers since the 1960s. However, a large number of experiments by French Rulquin and Verite (1990) showed that the amino acid composition of rumen microbial protein was different, and then in 1992, scholars represented by American Clark put forward the view that the amino acid composition of rumen microorganism is variable. Erasmus et al. (1994b) showed some differences in the amino acid composition of rumen bacteria in different diets, which may be due to the different types of microorganisms entering the small intestine under different dietary conditions. Domestic scholars have also drawn similar conclusions (Wang Hongrong, 1998). 1.3 digestion of protein in the small intestine the digestion process of protein in the small intestine of ruminants is similar to that of monogastric livestock, but the source of protein is different. The proteins entering the small intestine of ruminants include microbial proteins synthesized in the rumen, undegraded rumen proteins in feed and endogenous proteins in the digestive tract. They have different meanings and functions in nutrition: the microbial proteins synthesized by the rumen of ① account for most of the protein flowing into the small intestine (Titgemeyer and Merchen,1990), which is the main source of intestinal protein. Increasing the production of microbial protein is the main measure to improve the small intestinal protein. ② rumen protein is an important part of small intestinal protein, which is of great significance in production. Adjusting feed rumen protein is an important means to regulate the composition of small intestinal protein in ruminants. For example, when most or all of the forage is high-quality grasses or legumes, it is extremely important for high-yield dairy cows to provide high digestible rumen protein. The endogenous proteins in digestive tract of ③ are mainly digestive enzymes, mucin, exfoliated epithelial cells of digestive tract, blood protein and so on. Normally, about 90% of this endogenous protein is reabsorbed by the body (Lu Dexun,1986) before it is excreted in the feces. This turnover of endogenous protein (released into the intestine and reabsorbed by the intestine) also plays an important role in the nutritional metabolism of ruminants. 2 protein degradation products have been absorbed for many years, it has been thought that proteins must be hydrolyzed into free amino acids (FAA) before they can be absorbed, but now many studies have shown that proteins can be absorbed in two forms in the gastrointestinal tract after degradation, namely free amino acids and small peptides. 2.1 absorption of protein degradation products in the forestomach because in the past, people thought that proteins had to be hydrolyzed into free amino acids before they could be absorbed, so it was considered that the forestomach had no effect on the absorption of protein degradation products. However, recent studies have found that whether the forestomach can absorb amino acids has not been determined, but it can absorb small peptides (mainly dipeptides and tripeptides). It was found that rumen and valvular stomach had the ability to absorb small peptides by isotope labeling method, and the absorptive capacity of rumen was stronger than that of rumen (Matthews,1991). MRNA (Webb et al.,1992) and peptide transporter (Walker and Hirst,1997) were found in rumen and valvular stomach. For ruminants, rumen and valve are the main sites to absorb amino acids in the form of peptides (Matthews et al., 1991, 1996; Cheng Maoji, 2000), almost no free amino acids are absorbed. 2.2 absorption of protein degradation products in the small intestine with the deepening of the study of peptide nutrition physiology, it has been recognized that in addition to free amino acids, the absorption of small peptides in the intestinal tract is another important form of intestinal amino acid absorption. Studies have shown that there are two independent transport mechanisms of small peptides (mainly dipeptides and tripeptides) and free amino acids in the intestinal tract of animals (Rubino et al., 1971 Hellier et al., 1972). The small peptide transport system has the characteristics of fast transport speed, low energy consumption and not easy to be saturated (Ganapathy and Leiback,1985; Hara et al.,1984;Matthews,1987), so the absorption of small peptides in the small intestine is faster than that of free amino acids (Webb,1990). In addition, the absorption of peptides can avoid competitive absorption among amino acids (Rubino et al.,1971; Wang Gang, 1999). The intestinal tract of animals has two mechanisms of absorbing small peptides and free amino acids at the same time, which is a self-nutritional regulation mechanism (Webb,1990) for animals to adapt to the environment and the changes of nutritional status. It is reported that under long-term nutritional restriction (low-protein diet), the absorption of amino acids in the small intestine decreased, while the absorption of peptides was relatively enhanced. The N digestibility of sheep and goats is about 68% (Wang Hongrong, 1998; Peng Yulin, 2000), of which more than half of the N in sheep and dairy cows is absorbed in the form of amino acids (Lapierre et al., 2000a), and the proportion of N retained in beef cattle in the form of amino acids is 42.46% (Huntington and Prior,1985;Lapierre et al., 2000b). This shows that the free amino acids absorbed by the intestines of ruminants are still the main source of nitrogen nutrients, and the peptides absorbed in the stomach are useful supplements (Seal et al., 1991). Therefore, it is generally believed that the intestinal tract mainly absorbs free amino acids and absorbs less small peptides, and jejunum and ileum are the main sites of free amino acid absorption. Conclusion with the application of molecular biology and isotope tracer in animal nutrition, new breakthroughs have been made in the research on protein digestion and absorption of ruminants. At present, the research on the composition and function of rumen microflora and the quantitative study of peptide absorption in different digestive sites have become the focus of research in this field. The progress made in these studies will provide a solid theoretical basis for putting forward new nutrition assessment indicators and nutrition regulation techniques in the future. -- very good materials. Thanks for your hard work, Dr. Zhao. -- Yes, save the study, thank you for providing-- very good! Height determines the field of view-the plane angle determines the direction-the two sides of the vertical intersect a straight line-read, thank you. -- saved the study, thank you for providing! Brother, the author of this post is Dr. Zhao of our school. -- study. There is too much to learn. 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